In the DC transmission system, AC filters and shunt capacitors provide reactive power for the DC system and filter out harmonics. DC filters are used to filter out harmonics on the DC side to prevent interference with communications. Usually, there is sufficient spare capacity for filters and shunt capacitors in the DC converter station. Therefore, when a group fails, the system will automatically put other spare reactive power reserves into use. However, since capacitors are relatively easy to damage during operation, operation and maintenance personnel need to take correct treatment methods in some emergency treatments to avoid power loss or even lockout of the DC system due to filter problems. The following introduces some common abnormal conditions during filter operation and their corresponding emergency treatment of electrolytic capacitor steps.
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.
