The post Filter and shunt capacitor overvoltage analysis  appeared first on Xuansn Capacitor.

The post Capacitor filter protection and fault handling strategies appeared first on Xuansn Capacitor.

1 Small group AC filter differential protection

1.1 Purpose and scope of protection

The purpose of this protection is to detect the internal grounding and phase-to-phase short-circuit faults of the AC filter group and the shunt capacitor group.

The protection range is the area between the head end TA and the tail end TA of the AC filter group and the shunt capacitor group. This article introduces a commonly used ratio brake differential protection. The ratio differential protection can reflect the internal phase-to-phase short-circuit fault and ground short-circuit fault of the AC filter, capacitor, and reactor (some protection equipment manufacturers also configure differential current speed cut protection, differential current over-limit protection, etc., and differential current speed cut protection can be used to quickly cut off serious internal faults of the filter). Shunt capacitor device protection can also effectively prevent electrical faults of the shunt capacitor group.

1.2 Protection principle

Measure the current on the high-voltage side and low-voltage side of the filter group and the parallel capacitor group, and compare the differential current phase by phase. The protection is only sensitive to the power frequency. The protection principle is shown in Figure 1.

Figure 1 Schematic diagram of differential protection of small group AC filter

1.3 Protection criterion and logic

Calculate the maximum current error and total current error based on the error of TA at the rated current. The differential current setting should avoid the total current error.

The ratio differential action equation is shown in formula (1):

                     (1)

I_op is the differential current, I_op.0 is the differential minimum action current setting value, I_res is the braking current, I_res.0 is the minimum braking current setting value, S is the ratio braking coefficient setting value, and the direction of the current on each side is the positive direction pointing to the filter.

(1) For three-side differential (such as HP3 high-pass filter, lightning arrester grounding current transformer), I_op is as shown in formula (2)

                                                               (2)

The braking current is the grounding current.

İ₁, İ₂, İ₃ are the currents on the secondary side of the current transformers TA1 on the bus side of the filter, TA3 on the grounding side, and TA2 on the lightning arrester side.

(2) For two-side differential (the case where the F2 lightning arrester has no grounding current transformer), I_op is as shown in formula (3):

                                                            (3)

The braking current is the grounding current.

İ₁ and İ₂ are the currents of the secondary side of the current transformers TA1 on the bus side and TA3 on the ground side of the filter respectively.

The action characteristics of the ratio differential protection are shown in Figure 2.

Figure 2 Ratio differential protection characteristic diagram

The logic of the ratio differential protection is shown in Figure 3.

Figure 3 Ratio differential protection logic diagram

Note: This differential has a delay function, and the specific delay is directly included in the action of the ratio differential element.

1.4 Consequences of protection action

Stage I: Delayed tripping, starting circuit breaker failure protection, and locking the AC circuit breaker.

Stage II: Immediate tripping, starting circuit breaker failure protection, and locking the AC circuit breaker.

2 Group filter capacitor unbalance protection (taking ratio unbalance as an example)

2.1 Purpose and scope of protection

The purpose of this protection is to protect the high-voltage end capacitor of the H-bridge structure to avoid capacitor avalanche damage due to component failure.

The protection range is the entire H-bridge arm capacitor.

2.2 Protection principle and strategy

(1) Capacitor count unbalance protection

(2) Capacitor ratio unbalance protection (the most basic configuration of the converter station, some converter stations are equipped with capacitor bridge arm current imbalance protection).

(3) Capacitor bridge differential overcurrent protection.

If the fuse of a capacitor unit is blown, the capacitance of the bridge arm changes, resulting in an unbalanced current flowing through the bridge. The increase of the unbalanced current is detected and the protection is activated. The protection principle is shown in Figure 4.

Figure 4 Schematic diagram of group filter unbalance protection

Due to the manufacturing deviation of the capacitor, the total current of the filter is needed to balance the initial unbalanced current. The current correction is only performed when the filter is charged for the first time or after the capacitor is replaced. Protection coordination: The selection of protection settings should avoid capacitor avalanche damage. The protection of the Shunt capacitor device protection that the capacitor group responds in time to an unbalanced fault and avoids equipment damage due to current imbalance.

2.3 Protection criteria and setting principles

Usually, there are three levels of protection in the converter station: alarm, delayed tripping and immediate tripping.

(1) Alarm. It can maintain operation for at least 2 weeks without damaging the equipment and affecting the system.

(2) Delayed tripping. Delayed tripping for 2 hours.

(3) Tripping. Immediate tripping.

2.4 Protection action sequence

(1) Alarm.

(2) Tripping stage I. Delayed tripping, breaker failure protection started, AC circuit breaker locked.

(3) Tripping stage II. Immediate tripping, breaker failure protection started, AC circuit breaker locked.

3 Group filter resistance circuit/reactance circuit harmonic overload protection

3.1 Purpose and scope of protection

The purpose of this protection is to protect the reactor or resistor from thermal damage.

The protection scope is the resistor and reactor in the filter, and the protection of the shunt capacitor device protection is also included that these devices are not damaged by overload during operation.

3.2 Protection principle and strategy

The protection calculates the power loss of each component and integrates the power loss according to the thermal time constant of the component to determine the thermal stress on the component. The protection simulates the skin effect of the reactance and corrects the power loss. If the calculated temperature value exceeds the thermal rating of the component, the protection is activated and the AC circuit breaker is tripped. The principle is shown in Figure 5.

Figure 5 Schematic diagram of harmonic overload of resistors/reactors

Generally, capacitors can withstand 1.3 times the rated current. This protection is used to prevent damage to the capacitor caused by heating when the current is higher than this. The protection of Shunt capacitor device protection that when the current is overloaded, the capacitor and other related devices are protected in time to prevent damage to the equipment caused by heating. The purpose of its protection is to eliminate thermal overload faults, not short-circuit faults. The alarm of stage I has delayed tripping, and stage II has delayed tripping.

3.3 Protection judgment criteria and setting principles

The selection of protection settings should avoid component damage.

3.4 Protection action sequence

(1) Alarm.

(2) Trip stage I. Delay tripping, start circuit breaker failure protection, and lock the AC circuit breaker.

(3) Trip stage II. Immediate tripping, start circuit breaker failure protection, and lock the AC circuit breaker.

4 Small group filter overcurrent protection and shunt capacitor device protection

4.1 Purpose of protection

The purpose of this protection is to protect the filter group or parallel capacitor group to prevent damage caused by overcurrent.

4.2 Protection principle and strategy

Measure the current on the filter group or parallel capacitor group. The protection is only sensitive to the power frequency. Protect the filter group, capacitor group, and reactor group to prevent damage caused by overcurrent. The action formula is: I > I_set; where I is the secondary current of TA on the high-voltage side, and I_set is the set value. The protection can be set to multiple setting values as needed, which can act on alarm and tripping respectively. Alarm in stage I, delayed tripping in stages II and III.

4.3 Protection judgment and setting principle

(1) Alarm. The setting of the alarm level ensures that it will not be activated when the system voltage fluctuates normally. The typical value of the alarm level is 1.1 (per unit value).

(2) Delayed tripping. Delayed tripping at 1.3 times the rated load current, the typical value of the delay is equal to 1.5s.

(3) Immediate tripping. This part is used as a backup protection for differential protection. The trip level is higher than the highest current (inrush current) that may occur in the filter group, but lower than the lowest short-circuit current level. The protection of the shunt capacitor device protection will be adjusted according to the specific situation to avoid damage.

4.4 Protection action sequence

(1) Alarm.

(2) Section I. Delayed tripping, starting circuit breaker failure protection, locking the AC circuit breaker.

(3) Section II. Immediate tripping, starting circuit breaker failure protection, locking the AC circuit breaker.

5 Zero-sequence current protection of small group filters and shunt capacitor device  protection

5.1 Purpose and scope of protection

The purpose of this protection is to protect the AC filter from short-circuit faults. The most important thing is to detect faults at the low-voltage end, because the fault current is small and the differential protection may not detect it. This protection also detects open-circuit faults in each phase, such as unsuccessful tripping or closing of a single phase.

5.2 Protection principle and strategy

Measure the three-phase currents at the low-voltage end of the filter and add them to obtain the zero-sequence current. In order to reduce the impact of the difference between the 11th and 13th harmonics, the protection is only sensitive to the power frequency and has a time-limited characteristic.

5.3 Protection criteria and setting principles

(1) Alarm. The current setting is higher than the normal unbalanced current (3I₀) and the zero-sequence current caused by the measurement error. The typical value is 15% of the power frequency load current.

(2) Delay tripping. ① The typical value of the delay is equal to 10s. According to the zero-sequence current caused by the internal fault of the filter, this protection is used to detect faults that are difficult to detect by the differential protection. The setting value is as low as possible. However, the tripping level is higher than the normal unbalanced current (3I₀) and the zero-sequence current caused by the measurement error. A reasonable choice is 20% of the rated power frequency load current. ② The typical value of the delay is equal to 4s. The delay can avoid out-of-zone faults and protection malfunctions. The shunt capacitor device protection must also ensure that the normal operation of the equipment will not be affected when a fault current occurs.

5.4 Protection action sequence

(1) Alarm.

(2) Tripping. Start the circuit breaker failure protection and lock the AC circuit breaker.

6 Group filter detuning monitoring protection and shunt capacitor device protection

6.1 Purpose and scope of protection

The purpose of this protection is to detect small changes in the early values ​​of the filter components.

The protection scope is the group AC filter.

6.2 Protection principle and strategy

The TA at the low-voltage end of each filter detects the zero-sequence harmonic current and obtains the zero-sequence current through the vector sum of the three-phase current. The protection is sensitive to harmonic zero sequence and detects the change of the single-phase impedance of the filter. If one phase is detuned, the harmonic cannot pass through the filter, and at the same time, it causes overstress to the normal filter due to asymmetry. The detuning monitoring function cannot distinguish whether the zero-sequence harmonic component is caused by a filter fault or an AC system fault.

6.3 Protection judgment criteria and setting principles

The total harmonic current is calculated based on the total rated current and rated power frequency current during continuous operation. The judgment criteria should ensure that even if there is no maximum harmonic component in normal operation, an alarm should be issued once the detuning occurs. The total harmonic current is calculated as shown in formula (4):

                                                                           (4)

Where I_t-total rated current, A;

I_f-rated power frequency current, A;

I_ht-total harmonic current, A.

The typical value of zero-sequence current is set to 35% of the total harmonic current, with a delay of 2min. The shunt capacitor device protection works together in this process to ensure that the capacitor will not suffer unnecessary damage due to detuning.

6.4 Protection action results

Alarm.

The post AC filter and shunt capacitor device protection appeared first on Xuansn Capacitor.

The post DC filter capacitor protection and fault diagnosis strategy appeared first on Xuansn Capacitor.

The post Filter and parallel capacitor fault and protection configuration appeared first on Xuansn Capacitor.

1 Principles of condition maintenance of filter and shunt capacitor devices

For any equipment to carry out condition maintenance, the principle of “must be repaired if necessary, and must be repaired well” should be followed. Filters and shunt capacitors are no exception. According to the results of equipment status evaluation, risk assessment is carried out considering equipment risk factors, and maintenance plans for equipment are dynamically formulated. The plan and content of condition maintenance are reasonably arranged, which is specific to the content of condition maintenance work of AC and DC filter and shunt capacitor devices, including power outage, power outage testing and experiment, and power outage and non-power outage maintenance and maintenance work.

In the process of carrying out the condition inspection and maintenance of the filter and shunt capacitor device, dynamic condition evaluation management should be implemented. A condition evaluation should be carried out after each equipment inspection or test. At the same time, regular equipment evaluation should be carried out according to local requirements to grasp the equipment status in real time. Generally, when the filter shunt capacitor device equipment is newly put into operation, a condition evaluation should be carried out on the equipment within one month. In the early stage of equipment operation, according to the provisions of DL’T 393-2010 “State Inspection and Test Procedures for Transmission and Transformation Equipment”, routine tests should be arranged for the filter and shunt capacitor equipment within one year after commissioning. At the same time, a comprehensive inspection should be carried out on the equipment and its accessories (including electrical circuits and equipment appearance parts), various state quantities should be collected, and a condition evaluation should be carried out again. After this evaluation, the various equipment state quantity information collected will be comprehensively compared with the equipment state quantity information collected during the equipment installation handover. The equipment operation and maintenance personnel should have a preliminary understanding of the quality of the equipment and the problems that may arise in the future operation.

For filter and shunt capacitor equipment that have been in operation for more than 10 years, the probability of failure or failure has increased significantly. It is advisable to adjust the maintenance plan and content according to the equipment operation and evaluation results.

2 Classification and status inspection and maintenance items of filter and parallel capacitors

2.1 Maintenance classification

According to the nature and scope of work, the maintenance of AC and DC filters and shunt capacitor devices is divided into 4 categories: A-type maintenance, B-type maintenance, C-type maintenance, and D-type maintenance. Among them, A, B, and C are power-off maintenance, and D is non-power-off maintenance.

Class A maintenance

Class A maintenance refers to the overall inspection, maintenance, replacement and testing of AC and DC filters and shunt capacitor device.

In order to better define the Class A maintenance of filter and shunt capacitor device, based on the domestic self-flow converter station’s many years of operation experience, this type of maintenance generally includes: ① The capacitor tower avalanche breakdown is replaced as a whole, or the faulty capacitors on a single tower exceed 30% of the total number of capacitors in the tower and need to be replaced; ② The resistors, reactors, current transformers, and lightning arresters in the device have serious faults and cannot be repaired on site and need to be replaced as a whole; ③ Various routine tests and diagnostic test items before and after equipment processing

Class B maintenance

Class B maintenance refers to the local maintenance of AC and DC filters and shunt capacitor devices, disassembly inspection, maintenance, replacement and testing of components.

Class B maintenance usually includes: ① Replacement of a small number of capacitors in the capacitor tower, generally considered to be less than 20%; ② The resistors, reactors, current transformers, lightning arresters, etc. in the filter device do not need to be replaced as a whole, and the local disassembly inspection and repair of the equipment or the replacement and maintenance of a part of the equipment can be completed on site; ③ Various routine tests and diagnostic test items before and after equipment processing; ④ Complex diagnostic test items to verify the quality of the equipment.

Class C maintenance

Class C maintenance is the routine inspection, maintenance and testing of AC/DC filters and shunt capacitor device.

Class C maintenance is: ① After the power outage, the appearance of the equipment is inspected and cleaned, and the equipment parts are repaired and repaired, which does not involve any replacement work; ② Routine testing of the equipment, generally does not include diagnostic testing of the equipment.

Class D maintenance

Class D maintenance is the live testing, appearance inspection and maintenance of AC/DC filters and shunt capacitor device without power outage.

Class D maintenance of equipment is relatively easy to define in reality, that is, any work on the equipment without power outage can be considered as Class D maintenance.

2.2 Maintenance items

The status classification and maintenance items of the maintenance of AC/DC filters and shunt capacitor devices are shown in Table 1.

Table 1 Classification of equipment status and maintenance items

Note With the continuous improvement of maintenance methods and the continuous application of new technologies, the original Class C maintenance may be partially converted into Class D maintenance. These require everyone to continuously apply new methods and technologies in future work to gradually reduce the workload. At the same time, on the other hand, the equipment has changed from power outage processing to the same processing work that can be completed without power outage, which also improves the equipment availability.

3 Maintenance strategies for filtering and shunt capacitor installations

The filter and shunt capacitor device maintenance strategy is how to carry out equipment maintenance work. Only after the equipment has undergone status evaluation and risk assessment, can its maintenance strategy be obtained. The maintenance strategy includes maintenance equipment, maintenance cycle, maintenance test items, etc. After the formulation of the maintenance strategy is completed, the on-site maintenance personnel will perform the corresponding maintenance work according to the strategy.

3.1 Requirements for filter and shunt capacitor device maintenance strategy

The status maintenance strategy for equipment reactors, capacitors, resistors, current transformers and lightning arresters in filter and shunt capacitor devices includes the formulation of annual maintenance plans, as well as defect handling, maintenance and inspection without power outages during testing. The maintenance strategy should be adjusted dynamically according to the results of the equipment status evaluation.

The annual maintenance plan should be revised at least once a year. According to the evaluation results of the most recent equipment status, consider the equipment risk assessment factors, and refer to the manufacturer’s requirements to determine the next power outage maintenance time and maintenance category. When arranging the maintenance plan, the maintenance cycle of related equipment should be coordinated and arranged as unified as possible to avoid repeated power outages.

For equipment defects, they should be handled according to the nature of the defects and the relevant provisions of defect management. If there are multiple defects in the same equipment, they should also be arranged to be handled in one maintenance as much as possible. If necessary, the maintenance category can be adjusted.

The normal cycle of Class C maintenance should be consistent with the test cycle. The non-stop maintenance items and test items of various types of equipment should be arranged according to the specific actual situation.

When the defect handling of current transformers and lightning arresters involves internal defect handling or disassembly, it is generally not arranged to be handled on site. Usually, spare parts are directly used for replacement, and then the faulty equipment is returned to the factory or handled in the maintenance workshop of the repair unit.

3.2 Maintenance strategy

The maintenance strategy of the filter and shunt capacitor device based on the status score is shown in Table 2.

Table 2 Maintenance strategy table of filter and shunt capacitor device

3.2.1 “Normal state” maintenance strategy

AC and DC filters and shunt capacitor device evaluated as “normal state” shall perform Class C maintenance. According to the actual condition of the equipment, Class C maintenance can be performed according to the normal cycle or extended for one year. Before Class C maintenance, Class D maintenance can be appropriately arranged according to actual needs.

3.2.2 “Attention state” maintenance strategy

AC and DC filters and shunt capacitor devices evaluated as “attention state” shall be subject to Class C maintenance. If the evaluation result is “attention state” due to the deduction of points for a single state quantity, Class C maintenance should be arranged in advance according to the actual situation. If the evaluation result is “attention state” due to the total deduction of points for multiple state quantities, it can be performed according to the normal cycle, and necessary maintenance and test content should be added according to the actual condition of the equipment.

AC and DC filters and shunt capacitor device evaluated as “attention state” should appropriately strengthen Class D maintenance.

3.2.3 “Abnormal state” maintenance strategy

AC and DC filters and shunt capacitor device evaluated as “abnormal state” shall determine the maintenance category and content according to the evaluation results, and arrange maintenance in a timely manner. Class D maintenance should be strengthened before implementing power outage maintenance.

3.2.4 “Severe Status” Maintenance Strategy

For AC and DC filters and shunt capacitor devices that are evaluated as “severe status”, the maintenance category and content shall be determined according to the evaluation results, and maintenance shall be arranged as soon as possible. Class D maintenance shall be strengthened before power outage maintenance.

[/fusion_text][/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]

The post Maintenance of AC/DC filter and shunt capacitor devices appeared first on Xuansn Capacitor.

The basic score, test score, family defect score, and defect score of each device in the AC and DC filters and shunt capacitor device are calculated comprehensively to obtain the comprehensive score of the AC and DC filters and shunt capacitors, as shown in formula (1).

                 (1)

Where B–basic score;

T–test score;

F–family defect score;

Q–defect score.

Except for the basic score which is a percentage score, the test score, family defect score, and defect score are all coefficient scores between 0 and 1, which should be noted when scoring.

2. Basic score (B) for shunt capacitor device

(1) The basic score of the equipment refers to the evaluation of the basic information of the equipment. The basic score can be evaluated by referring to Table 1 and formula (2). The full score of the basic score is 100 points. In general, the basic score of the equipment is greater than 80 points. The overall basic score of the filter and shunt capacitor device is the lowest basic score of each device. The detailed indicators of the basic evaluation score of the equipment can be referred to Table 2.

Table 1 Equipment basic score reference

                                                                (2)

Table 2 Equipment basic score detailed indicator table

(2) Basic score calculation example. Example description: ① The manufacturers of the reactors, resistors, lightning arresters, and current transformers of the filter and shunt capacitor device in a converter station are all well-known domestic manufacturers with a good reputation. In the past five years, there have been no major quality problems (generally referring to the poor product quality caused by unreasonable manufacturer design, serious problems in key production processes, etc., which have been officially reported by relevant companies); ② The supplier of the capacitor equipment of the converter station has significantly higher fault damage than other manufacturers in the past one or two years. The capacitor equipment has passed the factory test, but some test state values are close to the attention value, and the handover test value has a large deviation from the factory test; ③ There is no rework during the equipment installation process.

The following analysis can be obtained from the above description: The basic scores of reactors, resistors, lightning arresters, and current transformers can all reach 100 points. Basic score of capacitors: 13 points for manufacturing and factory testing, 6 points for transportation, installation, and handover testing, 10 points for equipment without family defects, and the total basic score of capacitors is 60+29=89 (points).

Therefore, the basic score of the filter and shunt capacitor device composed of the above equipment installed on site takes the lowest basic score of all equipment, and the basic score of the filter and shunt capacitor device is 89 points.

3. Family defect score (F) for shunt capacitor device

When the equipment has a family defect, the status score of those equipment that have not yet occurred or detected the family defect should be evaluated by formula (3) before the hidden danger is eliminated to evaluate the impact of the family defect. When calculating the family defect score, / is determined based on the location and nature of the defect (refer to Table 3, the value principle of ƒ).

Table 3 ƒ Value selection principle

                                                            (3)

Where N–the total number of family equipment;

n–the number of equipment with the family defect (N>n≥1).

If the hidden dangers involving family defects have been eliminated, that is, their impact is no longer considered,

The following example explains the process and precautions for evaluating family defects in shunt capacitor device.

Taking capacitors as an example, the situation description: During the maintenance process, a converter station found that the capacitance of quite a few faulty capacitors was larger than the rated value.

Situation analysis: As can be seen from the previous chapters, the capacitance value is larger than the rated value, indicating that the fuse in the capacitor element is not reliably blown when a single capacitor element fails, resulting in a large parallel series element being short-circuited. If it is an individual case, the capacitance value of the faulty capacitor should not be larger than the rated value in large quantities, which obviously violates the original design intention of the internal fuse of the capacitor element. After being recognized by relevant units or institutions, the unreliable blowing of the internal wire can be determined as a family defect. Generally, family defects are closely related to the production process and material selection of the product batch. Before the process and materials are improved, such failures caused by a certain specific reason may exist in large numbers.

After determining the family defect, the family defect score should be included in the evaluation of the batch of products (even if some capacitors have not yet exposed the family defect and are normal equipment). This is also the difference between the family defect and the general defect score (general defects are included in the score if they exist, and not included in the score if they do not exist or have been eliminated).

The value of the family defect formula is determined, N is the total number of family defects (generally refers to the number of products of the same batch within a certain range or a certain area, which is relative). The wider the sampling of N and the larger the range, the more reasonable the calculation. In most cases, the unit where the unit is located will count the number of products of the batch used by the unit. For example, a converter station uses 7892 capacitors of this batch from this manufacturer (of course, this is a relative concept, and capacitors of this batch may also be found in other places). There are 87 capacitors with large capacitance values ​​and the fuses in the capacitors fail to operate reliably. The formula (4) calculates:

                                            (4)

Formula (4) needs to further determine the value of ƒ, which is determined based on the location and nature of the defect. According to the value selection principle of Table 3 ƒ and the degree of influence of the family defect on the equipment, the family defect such as the unreliable action of the fuse in the capacitor is a major factor that seriously affects the capacitor equipment and poses a real threat to the safe and stable operation of the filter and parallel capacitor equipment. The conservative value can be set as the upper limit of 0.15, thus:

Notes on family defect evaluation:

(1) After the fuse fault in the capacitor is determined to be a family defect, even if there are only a few or even no products of the same process and batch in other stations that have this fault, the status score should also consider the family defect score.

(2) The first condition for a family defect is that the relevant unit determines that the fault or equipment hidden danger is a family defect. The determination of a family defect is generally determined by at least the provincial grid company or above, which is completely different from the determination of a defect.

(3) The person who conducts the family defect evaluation of the equipment must be an experienced professional and have a good understanding of the structural principles of the equipment (otherwise, it is easy to cause large deviations when some values ​​need to be obtained by human intervention according to the regulations) in order to make a relatively reasonable evaluation.

(4) The determination of the N(n) value in the family defect evaluation is generally obtained through reasonable statistics of the number of equipment applications within a certain range.

4. Test scoring method (T)

The test score is the weighted geometric mean of the scores of individual test items. The score of a single item is between 0% and 100%, and 100% corresponds to the state quantity of each item in the item being far below the attention value or warning value, and there is no obvious deterioration trend. Suppose a device has undergone m single tests, the score of the i-th test is G_i, and the weight is W_i; (take 1 when not given), then the test score T can be expressed by formula (5):

                                                       (5)

4.1 Single test item scoring method-conventional method

This method is applicable to the analysis of positive degradation and negative degradation state quantities with attention value or warning value requirements

Suppose the attention value is x_z, the warning value is x_j, and the values of the last three tests are x, x₁, and x₂, where x is the current test value, x₁ is the test value t₁ years ago (relative to x), and x₂ is the test value t₂ years ago (relative to x), and t₂>t₁. In the following formulas, if the state quantity gives a warning value, then xˊ=x_j; if the state quantity gives a attention value, then xˊ=1.3x_z (positive degradation) or xˊ=x_z/1.3 (negative degradation). The single test scoring method is shown in formula (6) to formula (12), where xƒ is the average value of the state quantity in similar new equipment. If there is no such value, it is replaced by the factory or acceptance test value of the equipment.

(1) When there is only one test record (i.e. x₁ and x₂ do not exist):

                                                     (6)

In formula (6), when G<0, let G=0; when G>100, let G=100.

The following is a calculation example of the current test value of the arrester under 0.75 DC reference voltage in the AC and DC shunt capacitor device filter device.

The leakage current of a lightning arrester under 0.75 times DC in a converter station filter is 15μA when it is installed on site. According to Q/GDW506-2010 “Guidelines for the Status Evaluation of AC Filter and shunt Capacitor Device in High Voltage DC Transmission”, the attention value of this test item is 50μA, and the attention value of 15μA is far from exceeding the attention value. When the equipment is put into operation after completing the infrastructure handover test, it is necessary to evaluate the equipment results. The evaluation method of this test project is as follows:

Since there is only one test project, the calculation is based on formula (6). As the average value of x_f in similar products is not easy to obtain as an operation and maintenance unit, the factory test value is selected here (which can be found in the factory test report). The leakage current value of the arrester at 0.75 times the DC reference voltage is 14.5μA.

There is only one test record and the single test score is 0.99.

(2) There are 2 test records (i.e. x₂ does not exist):

Positive degradation is shown in formula (7):

                                             (7)

Negative degradation is shown in formula (8):

                               (8)

When there are 2 test records, the algorithm is as follows: Taking the leakage current of the arrester at 0.75 times the DC reference voltage as an example (the state quantity of the arrester leakage current test is a positive degradation state quantity), when the first The leakage current value of the one-year test is 16μA, then x₁=15μA (the acceptance test value one year ago), x=16μA (the current test value), substituting into formula (7) to obtain:

Then there are 2 lightning arresters with a leakage current score of 0.95 at 0.75 times the reference voltage.

(3) If there are 3 or more tests, select the values of the most recent 3 tests:

Positive degradation is shown in formula (9):

                                        (9)

Negative degradation is shown in formula (10):

                                                  (10)

When there are 3 test records, the algorithm is as follows: still taking the leakage current of the arrester at 0.75 times the DC reference voltage as an example, when the test result of the project in the second year is 15.6μA, the values in formula (7-9) are: x=15.6μA (current value of leakage current), x₁=16μA (leakage current value one year ago), x₂=15μA (leakage current value two years ago), and the above values are Substituting into formula (9), we get:

Then there are 3 leakage current records at 0.75 times the reference voltage, and the score is 0.978.

The above uses a specific example to introduce the state quantity calculation method of the positive degradation test.

4.2 Single test item scoring method

This method is suitable for state quantity analysis with positive and negative deviation requirements.

Assume that the current test value of a state quantity of the converter station is x, and the zero deviation value (usually the initial value or rated value) is x₀, then the deviation (E) of x is as shown in formula (11)

                                                              (11)

Assume that the allowed positive deviation is k_+, and the allowed negative deviation is k_–, and the scoring method is as shown in formula (12):

                                                   (12)

When G≤0, let G=0.

The following uses the capacitor bank bridge arm capacitance test value (this state quantity belongs to the deviation degradation state quantity, and only the current test value is used when the deviation degradation is scored) as an example to illustrate the calculation method of using this type of test item as a single state quantity score. It is known from Q/GDW506-2010 that the bridge arm capacitance value deviation should not exceed ±2%. The following uses a domestic converter station as an example to illustrate the calculation method of deviation degradation. When the measured value of the current arm capacitance is x=1.082μF and the rated value of the bridge arm is x₀=1.084μF, the x deviation (E) can be obtained according to formula (11) as follows:

Then the deviation degradation score is 91.2%.

Based on the state quantities of the test items selected in the above examples (assuming that the evaluation of other test state quantities is 1), the total test score of the filter and parallel capacitor is

The weight values of the above test state quantities are obtained according to Q/GDW506-2010: the weight of the state quantity of 0.75 times the leakage current of the lightning arrester is 3, and the weight of the state quantity of the bridge arm capacitance of the filter and parallel capacitor capacitor group is 4.

5. Equipment defect score (Q)

Each equipment defect of AC and DC filters and shunt capacitors is scored, and the equipment defect level is divided according to the total defect deduction and single defect deduction. Defect scores are divided into four levels according to the severity: severe state, abnormal state, attention state, and normal state. Each defect score level corresponds to a defect score coefficient. The normal state Q is 1, the attention state Q is 0.7~0.9, the abnormal state Q is 0.4~0.6, and the severe state Q is 0~0.3.

When the total deduction of all defect state quantities reaches the provisions of Table 4, it is regarded as a normal state; when the single deduction of any defect state quantity or the total deduction of all defect state quantities reaches the provisions of Table 4, it is regarded as a attention state; when the single deduction of any defect state quantity reaches the provisions of Table 4, it is regarded as an abnormal state or a severe state.

Table 4 Overall evaluation standard table for equipment defects

The defect score should be the existing defects that have not been eliminated, and the defect state quantity score of the participating equipment. Before scoring the defects of the filter and shunt capacitor, collect all the equipment defects of the group of filters or shunt capacitor device. For example, the uneliminated defects collected by the 3615 HP3 filter of a domestic converter station are shown in Table 5.

Table 5 Uneliminated defects of 3615 HP3 filters in a domestic converter station

The maximum deduction for a single item of this group of filters is 12 points, and the total deduction is 16 points. According to the regulations Q/GDW506-2010, the more serious deduction reasons are taken, and the defect evaluation of this group of filters is in the attention state, which is the lower limit of the deduction value in the attention state. After a comprehensive comparison, the defect evaluation coefficient is determined to be 0.85.

In summary, through the above-mentioned filter and parallel capacitor basic scoring examples, family defect scoring examples, test scoring examples, and defect scoring examples, assuming that these examples are the actual conditions of a group of AC filter evaluations, the overall comprehensive score of the filter can be obtained. According to formula (6-1), the following is obtained:

Equipment comprehensive score G=B×F×T×Q=89×0.2332×0.94×0.85=16.58 (points).

If the filter does not have family defects, the equipment comprehensive score G=B×F×T×Q=89×0.2332×0.94×0.85=71 (points).

From this, it can be seen that once it is characterized as a family defect, it will have a very large impact on the equipment score, which may cause the equipment to be in a normal state. As a result, due to the existence of family defects, the equipment score is suddenly classified as a serious state, which will also have a huge impact on the maintenance strategy.

The post Evaluation methods for filters and shunt capacitor device appeared first on Xuansn Capacitor.

1.1 Parallel capacitor status evaluation background

Before 2008, the AC and DC filters and shunt capacitors of DC converter stations had been inspected once a year in the traditional way (of course, the same is true for other equipment in DC converter stations). We usually call this centralized maintenance method of DC converter stations the annual comprehensive overhaul of converter stations. The annual comprehensive overhaul of AC and DC filters and shunt capacitors has always been carried out in accordance with DL/T 596-1996 “Procedure for Preventive Tests of Power Equipment”. Because there are no special test items for filters and shunt capacitors in DC converter stations in this procedure, the test personnel working on site are actually just referring to it. With the increasing number of DC systems and the international requirements for indicators such as the availability of DC systems, this annual comprehensive overhaul based on equipment preventive tests has become increasingly unable to meet the urgent needs of society and enterprises for improving efficiency. The annual power outage maintenance test items are numerous and the cycle is long. Because of the total power outage, the power outage time is long, and the equipment test rate is too high, the DC equipment availability and other indicators cannot be compared with the international advanced level, which leads to high equipment operation and maintenance costs. Sometimes there will be some negative effects during the maintenance process (such as: some equipment that was originally in good operation, due to various reasons during the maintenance test, the equipment is not as good as before the maintenance after the maintenance). By introducing the method of parallel capacitor status evaluation and combining the concept of condition maintenance, the maintenance strategy can be optimized more effectively to avoid the shortcomings of traditional overhaul methods.

Therefore, in view of the above situation, the concept of equipment condition maintenance is proposed. The condition maintenance of equipment is based on safety, reliability, environment, and cost. Through equipment status evaluation, risk assessment, and maintenance decision-making, it is a strategy to achieve safe and reliable operation and reasonable maintenance cost. Before the equipment status maintenance, the equipment needs to be evaluated. Specifically, the parallel capacitor status evaluation of the filter and shunt capacitor equipment is the basis and basis for the implementation of status maintenance. This chapter focuses on the relevant standards and requirements for the evaluation of AC and DC filters.

1.2 Terms related to status evaluation and the composition of status quantities

As the relevant standards for equipment status evaluation and maintenance have only been gradually issued and put into production in China in recent years, the following introduces the commonly used terms for status quantity evaluation:

(1) Status quantity: various types of information that directly or indirectly represent the status of the equipment, such as data, sound, image, phenomenon, etc., which are divided into general status quantities and important status quantities.

(2) General status quantity: status quantity that has a relatively small impact on the performance and safe operation of the equipment

(3) Important status quantity: status quantity that has a greater impact on the performance and safe operation of the equipment.

(4) Normal state: all status quantities are within a stable and good range, and the equipment can operate normally.

(5) Attention state: the trend of a single (or multiple) status quantity changes in the direction of approaching the standard limit, but does not exceed the standard limit, or some general status quantities exceed the standard value. The equipment can continue to operate, but the monitoring during operation should be strengthened.

(6) Abnormal state: a single important status quantity changes greatly and has approached or slightly exceeded the standard limit

The operation should be monitored and power outages and maintenance should be arranged in a timely manner.

(7) Severe status: A single important status quantity seriously exceeds the standard limit and a power outage for maintenance needs to be arranged as soon as possible.

(8) Equipment status score (referred to as status score): A method of expressing the status of equipment in percentage. 100 points represent the best equipment status, and 0 points represent equipment that needs to be repaired as soon as possible. The status score for other situations is between 0 and 100 points.

(9) Positive degradation: The degradation of the status quantity is manifested as an increase in the value of the status quantity, such as dielectric loss factor, etc.

(10) Negative degradation: The degradation of the status quantity is manifested as a decrease in the value of the status quantity, such as insulation resistance, etc.

(11) Deviation degradation: The degradation of the status quantity is manifested as the inconsistency between the status quantity and the initial value, such as the DC resistance of the transformer winding, etc.

(12) Basic score: For new equipment that has passed the handover test and is ready for commissioning, or equipment that has passed the acceptance test after maintenance and can be put into operation again, its status is scored once as the basis for subsequent scoring. This score is called the basic score.

2.Composition, weight and degree of degradation of state quantity

The parallel capacitor status evaluation of filters and reflects the internal conditions, surface phenomena, common problems, equipment parameters and other elements (or factors) of the equipment through state quantities. These elements together form the evaluation parameters of the entire equipment. The quality of the parameters determines the quality of the equipment evaluation.

2.1 Specific composition of state quantity

(1) Original data. The original data of filter and shunt capacitor equipment mainly include: nameplate, type test report, order technical agreement, equipment supervision report, factory test report, transportation and installation record, handover acceptance report, installation and operation manual, etc.

(2) Operation data. The operation data of filter and shunt capacitor equipment mainly include: daily inspection records of equipment, records of equipment defects and abnormalities, infrared temperature measurement records, etc.

(3) Test data. The test data of filter and shunt capacitor equipment mainly include case reports, online monitoring information, special test reports, and relevant countermeasures.

(4) Other data. Other information of filter and shunt capacitor equipment mainly includes: the operation, defects and failure of the same type (class) equipment; changes in the equipment operating environment, changes in the system operation mode; other factors that affect the safe and stable operation of the equipment, etc.

2.2 Weight of equipment state quantity

Depending on the degree of influence of the state quantity on the safe operation of AC and DC filters and shunt capacitors, it is divided into four levels from light to heavy, and the corresponding weights are weight 1, weight 2, weight 3, weight 4, and the coefficients are 1, 2, 3, and 4. Weight 1 and weight 2 correspond to general state quantities, and weight 3 and weight 4 correspond to important state quantities

2.3 Deterioration degree of state quantity

Depending on the degree of degradation of the state quantity, it is divided into four levels from light to heavy, namely, industrial,, and level. The corresponding basic deduction values ​​are 2, 4, 8, and 10 points.

2.4 Deduction value of the state quantity of AC and DC filters and shunt capacitors

The deduction value of the parallel capacitor status evaluation is determined by the degree of degradation of the state quantity and the weight. The deduction value of the state quantity is equal to the basic deduction value of the state quantity multiplied by the weight coefficient. No points will be deducted when the status is normal. The relationship between the status weight, degradation degree and deduction value is shown in Table 1.

Table 1 The relationship between the status weight, degradation degree and deduction value

The post Parallel capacitor status evaluation and maintenance appeared first on Xuansn Capacitor.

(1) Aging of liquid dielectric insulation. The main manifestations of aging: increased loss, usually accompanied by an increase in trace moisture, the appearance of macromolecules and small molecules, gas precipitation (causing capacitor bulging), etc. Causes of insulating oil aging: mainly due to the chemical action of free radicals and the catalytic action of impurity ions. These factors are one of the causes of capacitor damage and directly affect its service life and performance.

The production process of oil is a reversible chemical reaction process, and it is difficult to avoid the existence of free radicals in insulating oil. Free radicals are atomic groups with lone electrons and have great chemical activity. Modern science has confirmed that many chemical reactions are completed with the help of the activity of free radicals. Under certain conditions, free radicals will react chemically with the unsaturated bonds on the benzene ring, resulting in an increase in molecular weight and the appearance of macromolecules, which may be accompanied by an increase in the viscosity of the liquid medium. Condensation reactions may also occur between free radicals, which also results in the production of macromolecules, but the micro-water content of the oil sample will increase. Free radicals may also cause oil molecules to crack and produce small molecules. The chemical reaction of free radicals causes the performance of the oil to deteriorate, which results in the aging of the insulating oil and is one of the causes of capacitor damage

(2) Aging of solid dielectric (polypropylene film). The main manifestation of aging is a decrease in breakdown voltage, not an increase in loss. Because the small molecules produced by film aging will dissolve in the oil, they will not affect the loss of the film, and the deorientation of the film will not significantly increase the loss. The main cause of aging is similar to that of oil, mainly caused by the chemical action of free radicals and the catalytic action of impurity ions.

The increase in the temperature of the capacitor during operation increases the activity of free radicals and the catalytic action of impurity ions, which accelerates the aging of the polypropylene film. Studies have shown that when the temperature exceeds a certain value, the polypropylene film will deorientate. After biaxial stretching, the polypropylene molecules of electrical polypropylene film have a certain degree of regularity and the film has sufficient dielectric strength. This is the fundamental difference between it and general packaging film. Deorientation means that the electrical film returns to the characteristics of packaging film, and its dielectric strength becomes very low. For filters and parallel capacitor devices in high-voltage DC transmission systems, the harmonic content is large during operation, and the local thermal effect is greater than that of capacitors in other occasions, and the aging problem of the film is relatively prominent. If other impurities are mixed into the polypropylene film during the production or rolling process, it will lead to more frequent breakdown of the capacitor film during operation. When this effect accumulates to a certain extent, the filter or capacitor unbalance protection will trip. As shown in Figure 1, a polypropylene film of a capacitor element inside the capacitor is broken down during the operation of the equipment or at the moment of circuit breaker closing.

Figure 1 A broken-down capacitor element (located on the large surface of the element)

(3) Methods to solve the problem of insulation aging. In solving the problem of equipment insulation aging, equipment manufacturers should play a more active role. During the factory supervision of the equipment manufacturing stage, the operation and maintenance unit should pay attention to tracking whether the manufacturing links of the equipment manufacturer have achieved the following points.

Is the cleanliness control of the production process strict: Controlling the cleanliness is an effective way to reduce the entry of impurity particles into the capacitor, which can not only reduce the loss of the capacitor, but also reduce the impact of heat in the capacitor. It can also slow down the aging of insulation, especially the aging process of the film, thereby avoiding one of the causes of capacitor damage.

Whether to use oil with high aromaticity: Choosing oil with high aromaticity can not only reduce the electric field strength on the oil layer and delay the aging process of the oil, but also absorb free radicals in the oil and inhibit the growth of the number of free radicals.

Whether to use additive technology: The additive should be a polar mesh medium, which is polarized and carries a strong polarization charge under the action of the electric field. With the mesh structure, it can easily absorb free radicals and inhibit the activity of free radicals.

Whether to use a reasonable structure: A reasonable structure, coupled with an optimized structural parameter design, can reduce the internal temperature rise of the capacitor. The reduction in capacitor temperature during operation means a reduction in free radical activity, which weakens the catalytic effect of impurity ions, thereby delaying the insulation aging inside the capacitor.

2.Internal fuse performance issues causes of capacitor damage

Compared with capacitors used in other occasions, capacitors used in high-voltage DC converter stations have higher performance requirements in all aspects, which is also the reason why capacitors used in converter stations are more easily damaged during operation.

(1) The capacitor has high requirements for surge current impact resistance. The tuning frequency of the capacitor bank for HVDC transmission is high, and the series reactance rate is small, so the surge current multiple is large when switching. In addition, the short-circuit discharge voltage of capacitors for HVDC transmission is relatively high, basically around 2.8 times, 12% higher than the 2.5 times specified in the standard, and the discharge energy is 25% greater.

(2) The steady-state overcurrent capacity of the capacitor is required to be relatively large. The harmonic content of the HVDC system is large, and the current of the capacitor is even greater. For example, the shunt capacitor of the Xingren Converter Station of the Southern Power Grid has a single-phase rated capacity of 94Mvar and a rated voltage of 430kV. The rated current calculated based on the rated capacity and rated voltage is 218A, and the actual rated current is 254A, a difference of 16.5%. The heating of the internal fuse increases by 35.4% during normal operation.

(3) The rated voltage selection of capacitors is different from that of ordinary shunt capacitors. When the harmonic content is large enough, the rated voltage of the capacitor is determined by the rated capacity.

When the harmonic content is small, the rated voltage of the AC capacitor for HVDC is determined by the arithmetic sum of the fundamental and harmonic voltages. The energy storage of the capacitor element during operation is smaller than that during the standard test. Therefore, if the internal fuse is selected according to the current standard, even if the test is passed, the internal fuse may not operate reliably at the lower limit voltage or operate too much at the upper limit voltage during operation. This makes the selection of the internal fuse performance range too narrow, which can easily lead to inappropriate selection of the internal fuse, thus becoming one of the causes of capacitor damage.

(4) The consequences of incorrect internal fuse operation. Each capacitor element in a single capacitor is connected in series with an internal fuse, as shown in Figure 2. The figure shows the internal structure of a capacitor currently used in a domestic converter station. The capacitor adopts a 15-parallel-4-series component connection method. The purpose of setting an internal fuse for each small element is that when the internal capacitor element breaks down, the internal fuse should operate reliably and melt, isolating the faulty capacitor element, while the other elements operate normally. Since only one element inside the capacitor is damaged, the overall capacitance value is not greatly affected, and the overall capacitance value is slightly smaller.

Figure 2 Capacitor internal cross-section

                 (1)

(where n is 1, 2, 3, 4)

When the internal components of the capacitor fail due to internal or external reasons, the internal fuse will not work properly (referring to unreliable melting) and will not work properly, which will lead to two opposite results in the external capacitance of the capacitor, as shown in formula (1). Assume that the dielectric of a capacitor element in C₄ is broken down, and the internal fuse used to protect the capacitor element is not disconnected. The element is equivalent to a wire that short-circuits the other 14 elements in the entire C₄ parallel string. The capacitance value of the entire capacitor C_total is obtained by formula (2):

                          (2)

When the other parallel string elements are intact, C_total will be 4/3 times the original capacitance value, which is larger than the normal rated value. If it is greater than 33% of the rated value, it is an unqualified capacitor. Q/GDW 496-2010 “High Voltage DC Transmission AC/DC Filter and Shunt Capacitor Device Maintenance Specification” stipulates that the difference between the initial value of the filter (assuming the initial value is the rated value) is required to be -5%~+10% to be qualified.

When the internal fuse operates correctly, the result should be smaller. Assume that a capacitor element in C₄ is broken down, the internal fuse operates correctly, the branch of the capacitor element is disconnected, and the other elements in C₄ work normally. The capacitance C_total of the entire capacitor is obtained by formula (3):

                   (3)

When other parallel series elements are intact, C_total will be 56/57 (98.25%) of the original capacitance value, which is slightly smaller than the normal rated value. If the deviation of capacitor design and manufacturing is considered, when one capacitor element is broken down and the internal fuse operates correctly, it can be considered as a fully qualified capacitor. When two capacitor elements are broken down and the internal fuse operates correctly, it is concluded that C_total will be 52/54 (96.3%) of the original capacitance value, and the initial value difference is -3.7%, which is still a qualified product. When three capacitor elements are broken down and the circuit is disconnected, the capacitor will be 94.1% of the rated value, and the initial value difference is -5.9%, which is an unqualified product. Therefore, for a capacitor with a structure of 15 parallel and 4 series, no more than 3 internal fuses should be blown, and the internal fuse should not be blown incorrectly due to the breakdown of the capacitor element. As shown in Figure 3, the left side shows a large surface of the capacitor element being broken down and short-circuited, and the internal fuse not operating correctly, and the right side shows a normal internal fuse blowing.

Figure 3 Diagram of the breakdown of internal components of the capacitor and the blowing of the internal fuse

For metal-cased capacitors, the capacitance of the capacitor is affected by temperature and even by the influence of insulating oil (the capacitor with oil leakage is basically unchanged compared with the capacitor without oil leakage). The state of the internal series and parallel components affects the capacitance of capacitors and capacitor banks. For ordinary metal-cased capacitors with this series and parallel connection mode, the breakdown and short circuit of the components are the fundamental reasons for the change in the capacitance of the external capacitor, and are also one of the causes of capacitor damage.

From the above analysis, it can be seen that as a converter station operation and maintenance personnel, the internal structures of various capacitors in the converter station should be clearly understood from the manufacturer during the equipment tracking and installation stage, so as to facilitate the analysis of the internal fault conditions of the capacitor in future work.

3.Joint heating

The problem of temperature rise caused by joint heating is mainly caused by the unreasonable connection structure of capacitor joints. In the early days, some domestic capacitor manufacturers had some defects in joint design. However, as a large number of high-voltage DC transmission capacitors in my country were put into operation, equipment manufacturers are also actively improving joint design technology. In recent years, the problem of heating of capacitor joints caused by unreasonable design has been basically eliminated. However, this is also one of the causes of capacitor damage.

The common practice of equipment manufacturers to solve the problem of joint heating is to use threaded wire clamps and spring washers, as shown in Figures 4 and 5. The focus of the wire clamp plus thread is to solve the problem of current flow channel, so that the current conduction surface of the wire clamp and the wire is the largest and the contact resistance is the smallest. The spring washer can perform temperature compensation to ensure that the contact pressure of each conductive contact surface does not change with the ambient temperature. It is an effective way to control the heating of the joint. Figure 6 shows a schematic comparison of different joint forms of capacitors, and Figure 7 shows an infrared spectrum of the joint heating measured by infrared temperature measurement during operation.

Figure 4 Comparison of old and new capacitor clamps (a) Clamp without internal thread; (b) Clamp with internal thread

Figure 5 Comparison of capacitor joints with spring washers and without spring washers (a) No spring washers at the joint connection; (b) With spring washers at the joint connection

Figure 6 Comparison of schematic diagrams of different capacitor joint forms (a) Wiring method of non-elastic washers and non-threaded tile-shaped clamps; (b) Wiring method of elastic washers and threaded tile-shaped clamps

Figure 7 Infrared spectrum of heating conditions of capacitor joints

4.Oil leakage

Oil leakage at the capacitor joint or at the root of the capacitor casing is another typical fault defect of capacitors during operation. Taking the first half of 2009 as an example, the converter stations under the jurisdiction of State Grid Corporation of China (Echeng Converter Station, Lingbao Converter Station, Yidu Converter Station, Huaxin Converter Station, Gaoling Converter Station) had a total of 244 capacitor defects of various types, of which 59 were oil leakage defects, accounting for 24.2% of the total fault defects. The oil leakage defect of capacitors is a long-standing problem for equipment operation and maintenance units and even the entire capacitor industry. There are many reasons for the oil leakage of capacitors during operation, mainly the inadequacy of product manufacturing process and the incorrect methods during installation, construction and transportation. The former mainly relies on the equipment manufacturer to improve the process, and the latter requires the construction unit or operation and maintenance personnel to master the correct construction or maintenance installation process.

Defects in the equipment manufacturing process can lead to oil leakage, which is also one of the causes of capacitor damage. As shown in Figure 8, the traditional outlet bushing. The traditional capacitor porcelain bushing outlet bushing is prone to oil leakage at the joint, as shown in Figure 10; the improved heat-resistant and leak-proof outlet bushing is used, as shown in Figure 9. The bushing and the capacitor shell are soldered with a flange. In addition, the early domestic capacitor products did not install positioning washers, and only relied on nuts to tighten. The change in the temperature of the contact will easily cause the nut to loosen, resulting in heating of the contact. Overheating positions can easily cause the end solder to melt and leak oil. As shown in Figures 9 and 11, the improved blind hole terminal and the rolled sealed sleeve are shown. The one-piece crimped sleeve combines the cover and the sleeve into one without any welds, which improves the sealing reliability. At the same time, during processing, the shell and the bottom, the bottom and the cover are welded by CNC argon arc machine, with small errors and precise positioning, ensuring the high consistency of the welds, further reducing the possibility of product leakage.

Figure 8 Traditional outlet sleeve

Figure 9 Heat-resistant and leak-proof outlet sleeve

Figure 10 Solder melt causes oil leakage in the joint

Figure 11 Rolled sealed sleeve

Improper installation or handling methods lead to oil leakage, which is also one of the causes of capacitor damage. Directly lifting the porcelain sleeve during transportation causes the flange welding to lose silver and cracks to appear; excessive force on the nut during wiring causes the porcelain sleeve to lose silver; the paint layer at the shell welding falls off and the shell is corroded. In order to avoid invisible oil leakage defects caused by installation or transportation, on-site operation and maintenance personnel should master the correct methods: when transporting and transferring capacitors, it is strictly forbidden to move the porcelain sleeve, do not tighten the nuts with force, and tighten them according to the torque requirements; when the paint layer falls off, remove rust and repaint in time. In view of this, during the infrastructure installation or maintenance stage, the operation and maintenance personnel should focus on strengthening the supervision and inspection of the construction process on site, and supervise the on-site personnel to use the correct construction methods and methods to complete the maintenance and installation of the capacitors.

[/fusion_text][/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]

The post Analysis of the causes of capacitor damage to HVDC appeared first on Xuansn Capacitor.

1 The absolute minimum number of filter groups does not meet the requirements during operation

1.1 Phenomenon description

The monitoring system background reports that the absolute minimum filter is not met and an emergency fault alarm is issued.

1.2 Operation personnel processing steps

(1) First, report the situation to the on-duty dispatcher of the National Electric Power Dispatching and Communication Center (hereinafter referred to as the National Dispatching Center) and the head of the station, and closely monitor the operation status of the DC system.

(2) The duty officer checks whether the reactive power control method is normal.

(3) Check whether the filter switch control mode and energy storage are normal.

(4) If all of the above are normal, apply to the national dispatching department to put in a spare AC filter or parallel capacitor if necessary.

(5) If there is no spare filter (parallel capacitor) available temporarily, the on-site filter maintenance progress should be accelerated to restore the filter standby as soon as possible.

(6) During the emergency treatment of electrolytic capacitor, it is necessary to strengthen the status monitoring of the capacitor to avoid further damage that affects the operation of the filter group.

2 Oil leakage of filter capacitors and emergency treatment of electrolytic capacitor

2.1 Description of the phenomenon

The filter capacitor was found to be leaking oil during the inspection.

2.2 Processing steps

(1) Observe the oil leakage rate of the capacitor. If the oil leakage is very weak, the operator should strengthen the oil leakage monitoring and regularly monitor the unbalanced current of the capacitor. If it is at a low level, it can continue to operate (generally, capacitors with slight oil leakage can continue to operate), but maintenance should be arranged as soon as possible.

(2) If the leakage rate is very fast and a large oil stain has formed on the ground, the national dispatching department should be immediately requested to shut down the faulty filter for maintenance.

(3) Take safety measures for the filter that has been taken out of operation and notify the maintenance personnel to handle it immediately. Especially for the emergency treatment of electrolytic capacitor, the leaking parts should be replaced or repaired in time to avoid causing greater impact on the filter system.

3 Overheating of AC filter/parallel capacitor equipment joints

3.1 Phenomenon description

Infrared temperature measurement shows that the temperature of the equipment joint is too high.

3.2 Processing steps

(1) Report to the head of the converter station, closely monitor the operation status of the entire equipment, shorten the infrared temperature measurement cycle of the joint, and monitor whether the heating has a trend of further deterioration.

(2) According to the requirements of DL/T 664-2008 “Application Specification of Infrared Diagnosis Technology for Live Equipment”, determine the level of heating defects. If it is an emergency defect, apply to the national dispatching agency to withdraw the group of filters (consider the operation needs of the DC system and follow the principle of first investment and then withdrawal), and carry out emergency repairs. Especially in this case, the emergency treatment of electrolytic capacitors needs to be particularly cautious to ensure that the heating parts will not affect the performance of the capacitor or further damage it.

4 AC filter group protection action in the emergency treatment of electrolytic capacitor

4.1 Phenomenon description

(1) The AC filter protection action alarm in the background of the monitoring system.

(2) The large filter switch trips and is locked.

4.2 Processing steps

(1) Whenever the large filter of the converter station trips, the National Dispatching Office and the superior supervisor should be reported immediately.

(2) At the same time, check the input status of the backup AC filter (parallel capacitor) to ensure the normal operation of the backup equipment, and pay special attention to the relevant matters of emergency disposal of electrolytic capacitors.

(3) Check whether the DC system has power reduction.

(4) Check the operation of the equipment in the fault area. If the small filter switch does not trip, immediately pull it open manually.

(5) Check the DC system power and AC bus voltage, and apply to the National Dispatching Office to adjust the DC system operating power if necessary.

(6) On-site inspection of the equipment within the protection range of the protection action. If obvious faults such as flashover and fracture are found, apply to the National Dispatching Office to transfer the corresponding equipment to maintenance according to the fault location, take safety measures, and notify the maintenance process.

(7) If no obvious primary equipment fault is found during the on-site inspection, notify the maintenance personnel to check and analyze the secondary circuit and the cause of the protection action.

(8) If the failure is caused by a small group filter, the small group filter has not tripped, causing the failure to start the large group filter to trip. The small group filter should be withdrawn and turned over for maintenance, and the other small group filters in the large group should be put back into operation. If the large group filter protection is indeed tripped, the equipment should be repaired immediately. During the emergency process, when emergency treatment involves electrolytic capacitors, the status of the relevant capacitors should be checked quickly to avoid further damage.

5 Alarm of AC filter C1 capacitor unbalance protection and emergency treatment of electrolytic capacitor

5.1 Description of the phenomenon

(1) The monitoring system background AC filter C1 capacitor unbalance protection stage I alarm.

(2) The monitoring system background AC filter C1 capacitor unbalance protection stage II alarm, the filter circuit breaker trips after a delay of 2h.

(3) The monitoring system background AC filter C1 capacitor unbalance protection stage III is activated, and the circuit breaker trips.

5.2 Processing steps

(1) Immediately report to the on-duty dispatcher of the national dispatching and notify the supervisor of this station.

(2) When a section alarm occurs, if the system conditions permit, the faulty AC filter can be withdrawn from operation for processing. If it has not been shut down, monitoring should be strengthened. If the alarm signal disappears by itself, it should be reported to the national dispatcher in time.

(3) When the alarm of stage II occurs, if the spare AC filter is in operating conditions, the faulty AC filter should be shut down within 2 hours for processing; if there is no spare AC filter in operation, the faulty AC filter should be shut down by reducing the DC transmission power.

(4) When stage III trips, check whether the spare filter is in normal operation, report to the dispatcher and strengthen monitoring.

(5) Check on-site for obvious faults in the filter capacitor.

(6) Take safety measures for the shut down filter and notify the maintenance personnel to handle it.

6 AC filter differential protection action and emergency treatment of electrolytic capacitor fault

6.1 Phenomenon description

Monitoring system alarm, event record, fault recording action; filter circuit breaker tripping; filter circuit breaker locking.

6.2 Processing steps

(1) The operator immediately reports to the on-duty dispatcher of the national dispatch and the supervisor of the station.

(2) Check whether the spare filter is in normal operation, report to the dispatcher and strengthen monitoring, pay special attention to the emergency treatment of electrolytic capacitor, and ensure stable system operation.

(3) Check on-site for any obvious faults in the equipment within the filter protection range.

(4) Take safety measures for the filter to be removed and notify the maintenance personnel to immediately handle the fault.

7 Alarm signals for AC filter resistor overload protection and reactance overload protection

7.1 Phenomenon description

(1) Monitoring system overload stage I alarm.

(2) Monitoring system overload stage II alarm, filter circuit breaker trips with a delay of 40 minutes;

(3) Monitoring system overload stage III emergency fault, filter circuit breaker trips.

7.2 Processing steps

(1) The operator should immediately report the on-site situation to the on-duty dispatcher and the superior supervisor of the National Dispatching Office.

(2) When an overload stage I alarm occurs, if the system conditions permit, the faulty AC filter can be removed from operation for processing, especially the electrolytic capacitor emergency disposal, to ensure its normal operation. If the conditions for removal from operation are not met for the time being, monitoring should be strengthened. If the alarm signal disappears on its own, it should be reported to the National Dispatching Office in a timely manner.

(3) When an overload alarm occurs in stage II, if the standby AC filter is in operating condition, the faulty AC filter should be shut down within 40 minutes for inspection and processing; if there is no standby AC filter available for operation, the faulty AC filter should be shut down within 40 minutes by reducing the DC transmission power.

(4) When an emergency overload fault occurs in stage III, the standby filter should be checked on site to see if it is operating normally, and the dispatcher should be informed and monitored more closely.

(5) Check on site whether the overloaded AC filter has any obvious fault.

(6) Take safety measures for the shut-down AC filter and notify the maintenance personnel to handle the fault.

8 Reactive load shedding action and emergency treatment of electrolytic capacitor

8.1 Phenomenon description

(1) The monitoring system background reports an emergency fault alarm of reactive power failure and power reduction.

(2) DC power reduction.

8.2 Processing steps

(1) Immediately report the situation to the on-duty dispatcher of the National Dispatching Office and the supervisor of this station.

(2) If a reactive load shedding alarm occurs during the power increase process, the reactive power control mode and the on-site filter control mode should be checked immediately to see if they are normal. After manually or automatically switching on the standby AC filter, apply to restore the operating power, paying special attention to the emergency treatment of electrolytic capacitor.

(3) If the reactive load shedding action occurs due to filter tripping, the reactive power control mode and the on-site standby filter control mode should be checked immediately to see if they are normal. After manually or automatically switching on the standby AC filter, apply to the national dispatching department to restore the original power operation.

(4) If there is no standby AC filter or the standby filter is available on site, apply to the national dispatching department to adjust the system power.

(5) Notify the maintenance personnel to conduct inspection and treatment.

9 DC filter overload protection action and emergency treatment of electrolytic capacitor

9.1 Phenomenon description

(1) Monitoring system background alarm, event recording, and fault recording action.

(2) Remove the DC filter.

(3) When the number of DC filter groups does not meet the system requirements or a large fault current (greater than 180A, this value varies slightly depending on the project) occurs, lock the converter.

9.2 Processing steps

(1) The operator shall immediately report the situation to the on-duty dispatcher of the National Dispatch Center and strengthen monitoring.

(2) Take safety measures for the withdrawn filter and notify the maintenance personnel to handle it.

10 DC filter C1 capacitor unbalance protection alarm

10.1 Phenomenon description

(1) The monitoring system background DC filter C1 capacitor unbalance protection stage I alarm.

(2) The monitoring system background DC filter C1 capacitor unbalance protection stage II alarm and delay 2h to cut off the DC filter.

(3) The monitoring system background DC filter C1 capacitor unbalance protection stage III alarm and cut off the DC filter. When the number of DC filter groups does not meet the system requirements, the converter is locked.

10.2 Processing steps

(1) The operating duty personnel shall report the situation to the on-duty dispatcher of the National Dispatch Center and the supervisor of this station to ensure that the emergency treatment of electrolytic capacitor.

(2) When the background stage I alarm occurs, if the DC system conditions permit, the faulty DC filter can be withdrawn from operation for processing.

(3) When the backstage II alarm occurs, the faulty DC filter should be shut down for processing within 2 hours.

(4) When the backstage III fault trips, the DC system operation should be monitored more closely and the operation status of the DC filter in the station should be reported to the dispatcher.

(5) Check whether the capacitor bank, reactor, resistor or other primary equipment of the faulty DC filter has obvious abnormalities.

(6) Take safety measures for the shut-down filter and notify the maintenance personnel to handle it.

11 DC filter differential protection action

11.1 Phenomenon description

(1) The monitoring system backstage alarm, event recording and fault recording start.

(2) Cut off the DC filter.

(3) When the DC filter group does not meet the fault current required by the system, the converter is locked.

11.2 Processing steps

(1) Report to the on-duty dispatcher of the national dispatching and the supervisor of the station, and strengthen the operation monitoring;

(2) Take safety measures for the shut-down DC filter and notify the maintenance personnel to check and handle it. When handling it, it is necessary to focus on checking whether the emergency disposal of the electrolytic capacitor needs to be started.

The post DC filters and emergency treatment of electrolytic capacitor appeared first on Xuansn Capacitor.