The DC system is a critical equipment system in substations, serving as the power supply for protection devices, control circuits, signal circuits and emergency lighting. Improper operation and maintenance of the DC system can lead to faults and accidents with severe consequences. Combining 20 years of operation and maintenance experience in the power system, Wuhan Haomai Electric Power has summarized several major types of common fault judgment and handling methods for DC systems for reference.

I. UPS Power Supply Fault Handling

The UPS power supply device is installed on the UPS power panel as an integrated unit, and internal faults are rare. Faults mainly occur in peripheral equipment in the following aspects:

1. AC input power supply: AC power failure, phase loss and other faults in the AC line from the input terminal of the UPS power supply device or the output of the remote AC panel (to the UPS power panel).Handling method: Quickly identify and eliminate the fault cause through fault indications such as panel displays and monitoring messages of the UPS power supply device.
2. DC input power supply: DC power failure and other faults in the DC line from the input terminal of the UPS power supply device or the output of the remote DC distribution panel (to the UPS power panel).Handling method: Quickly identify and eliminate the fault cause through fault indications such as panel displays and monitoring messages of the UPS power supply device.
3. DC switching fault: Switching faults during the transition from AC input power failure to DC battery power supply. The causes may include out-of-limit DC voltage of the battery pack, poor contact of the line from DC to the inverter input terminal, blown fuse, and poor contact of the connector for the fuse blow signal contact.Handling method: Quickly identify and eliminate the fault cause through fault indications such as panel displays and monitoring messages of the UPS power supply device.

Notes on the above fault handling:① Operators must be proficient in judging fault phenomena, troubleshooting and handling faults in correct and reasonable steps, and conduct standardized and safe operations.② No unsafe acts or phenomena are allowed during operation to avoid electric shock to personnel or damage to equipment.

II. Verification of Comprehensive Alarm Information of High-Frequency Switching Power Supply (Abnormality of Charging Device)

Abnormal alarms of the high-frequency switching power supply charging device are verified by temporarily modifying the alarm parameters and changing the operating status of the charging panel monitoring device:

1. Abnormal AC input voltage

1. Set the overvoltage and undervoltage parameters of AC voltage in the high-frequency switching power supply monitoring device to verify whether the monitoring device can send corresponding AC overvoltage/undervoltage alarm signals and messages.
2. Disconnect the AC input power supplies of Channel I and Channel II of the high-frequency switching power supply charging device respectively to verify whether the monitoring device can send corresponding power failure alarm signals and messages for AC Channel I and II.
3. Set phase loss for the AC input power supplies of Channel I and Channel II of the high-frequency switching power supply charging device respectively to verify whether t he monitoring device can send corresponding AC abnormality alarm signals and messages.

 

2. Charging module fault

1. Module communication fault: Unplug and reinsert the communication cable plug at the back of a certain module or change the module communication address code to verify whether the monitoring device can send corresponding module communication abnormality alarm signals and messages.
2. Module fault: Internal faults of the module are rare. The fault type shall be judged according to on-site conditions considering the actual equipment differences of each manufacturer. Some manufacturers equip each module with an independent power air switch, and the module will send a "module fault" signal when the corresponding air switch is disconnected. However, modules produced by some manufacturers will not alarm when the power air switch is disconnected. Sometimes, if the internal fault of the module occurs in the communication unit, a module communication fault will also be reported.

3. Lightning protection alarm

Pull out the lightning protection sheet of a certain phase of the lightning protector in the lightning protection unit at the AC input end of the charging device; the monitoring device shall send lightning protector alarm signals and messages.

4. The signals of overhigh/overlow floating charge voltage and overhigh/overlow equalizing charge voltage are default-set in the equipment of some manufacturers at the factory, but not in others, and shall be judged according to on-site conditions.
5. Blown fuse of battery pack

The main fuse of the battery pack is generally installed at the charging panel. The blown fuse signal may be caused by poor contact of the micro switch contact next to the battery fuse. Verify whether the monitoring device can send corresponding abnormality alarm signals and messages.

Notes on the above fault handling:① Operators must be proficient in simulating fault phenomena, verify the correspondence and accuracy of fault signals sent by the monitoring device in correct and reasonable steps, and conduct standardized and safe operations.② No unsafe acts or phenomena are allowed during operation to avoid electric shock to personnel or damage to equipment.

III. Precision Test of High-Frequency Switching Power Supply

Connect the equipment correctly with the FDT-220/110 DC System Comprehensive Tester to test the voltage regulation precision, current regulation precision and ripple coefficient of the high-frequency switching power supply module. Refer to the user manual of this product of Wuhan Haomai Electric Power for the equipment connection method.

 

1.Test of voltage regulation precision

In accordance with GB19826-2014 General Technical Specifications and Safety Requirements for the Characteristics of DC Power Supply Equipment in Electric Power Engineering, when the charging and floating charge device is in floating charge or equalizing charge (voltage stabilization) state, the AC output voltage changes within 85%～120% of its rated value, the output current changes within 0～100% of the rated value, and the output voltage remains stable at any value within its floating charge voltage adjustment range, its voltage regulation precision shall be ≤±0.5%.The voltage regulation precision is calculated by the following formula:δU=Uz​Um​−Uz​​×100%Where:δU – Voltage regulation precision;Um​ – Limit value of output voltage fluctuation;Uz​ – Tested value of output voltage.

 

2.Test of current regulation precision

In accordance with GB19826-2014 General Technical Specifications and Safety Requirements for the Characteristics of DC Power Supply Equipment in Electric Power Engineering, when the charging and floating charge device is in constant current charging (current stabilization) state, the AC output voltage changes within 85%～120% of its rated value, the output voltage changes within the specified adjustment range, and the output current remains stable at any value within 20～100% of its rated value adjustment range, its current regulation precision shall be ≤±1%.The current regulation precision is calculated by the following formula:δI=Iz​Im​−Iz​​×100%Where:δI – Current regulation precision;Im​ – Limit value of output current fluctuation;Iz​ – Tested value of output current.

 

3.Test of ripple coefficient
In accordance with GB19826-2014 General Technical Specifications and Safety Requirements for the Characteristics of DC Power Supply Equipment in Electric Power Engineering, when the charging and floating charge device is in floating charge or equalizing charge (voltage stabilization) state, the AC output voltage changes within 85%～120% of its rated value, the output current changes within 0～100% of the rated value, and the output voltage remains stable at any value within its floating charge voltage adjustment range, its ripple coefficient shall be ≤±0.5%.The ripple coefficient is calculated by the following formula:δ=2Udc​Upp​​×100%Where:δ – Ripple coefficient;Upp​ – Peak-to-peak value of pulses in DC voltage;Udc​ – Average value of DC voltage.

 

Notes on the above fault handling:① Operators shall proficiently, standardly and correctly connect and operate the FDT-220/110 DC System Comprehensive Tester.② Record the test results item by item, and be able to correctly analyze and draw conclusions from the test results.③ No unsafe acts or phenomena are allowed during operation to avoid electric shock to personnel or damage to equipment and instruments.

IV. Handling of Multiple Faults of Storage Batteries

Battery packs have many types of faults, which are often caused by poor contact. The fault points shall be found according to specific conditions, and the following aspects can be considered:

1. Loose bolt of connecting plate terminal of individual batteries: Loose internal bolts of the nut cover of some batteries may cause low voltage signals or even open circuit of some batteries.Handling method: Quickly identify and eliminate the fault cause through fault indications such as monitoring messages.
2. Broken or poorly contacted inspection wiring of individual batteries: Broken or poorly contacted wiring of some batteries may cause low voltage signals or even open circuit of some batteries.Handling method: Quickly identify and eliminate the fault cause through fault indications such as monitoring messages.
3. Overhigh terminal voltage of individual batteries: May be caused by replacing some batteries with those with overhigh terminal voltage.Handling method: Quickly identify and eliminate the fault cause through fault indications such as monitoring messages.
4. Overlow terminal voltage of individual batteries: May be caused by replacing some batteries with those with overlow terminal voltage.Handling method: Quickly identify and eliminate the fault cause through fault indications such as monitoring messages.
5. Abnormal internal resistance of individual batteries: May be caused by replacing some batteries with those with excessively high internal resistance.Handling method: Quickly identify and eliminate the fault cause through fault indications such as monitoring messages.

Notes on the above fault handling:① Operators must be proficient in judging fault phenomena in accordance with the requirements of commissioned equipment, troubleshoot and handle faults in correct and reasonable steps, and conduct standardized and safe operations.② No unsafe acts or phenomena are allowed during operation to avoid electric shock to personnel or damage to equipment.

V. Handling of DC Grounding Faults

DC grounding faults in substations may be caused by various reasons and are relatively complex to locate. Generally, DC grounding may occur in feeder circuits or battery packs, requiring operators to have certain fault troubleshooting capabilities.

1. DC grounding fault of feeders: The grounding point in the DC distribution panel cabinet or a certain point of the DC feeder output line is judged and located by the DC monitoring panel and DC fault locator. Grounding points are usually hidden and require testers to search patiently and carefully.
2. DC grounding fault of battery packs: The grounding point at a certain position of the battery pack is judged and located by the DC monitoring panel and DC fault locator.
3. DC system grounding fault: Among DC grounding faults, two-point grounding is more harmful and may cause serious consequences. A two-point grounding fault in the DC system may form a ground short circuit, resulting in maloperation or refusal to operate of relay protection and automatic devices, and may cause blowing of DC fuses or tripping of air switches, leading to power failure of the entire DC system.

1. Single-point grounding of DC bus: Will not cause serious consequences, but requires operation and maintenance personnel to handle it in a timely manner to avoid two or more point grounding faults. The grounding point is judged and located by the DC monitoring panel and DC fault locator.
2. Positive pole grounding of DC system: May cause maloperation of protection and automatic devices. Normally, tripping coils and relay coils are connected to the negative power supply (act when connected to the positive pole). If another point grounding occurs in these circuits, maloperation may be caused. The grounding point is judged and located by the DC monitoring panel and DC fault locator.
3. Negative pole grounding of DC system: May cause refusal to operate of protection and automatic devices. Normally, tripping and closing coils and relay coils are all connected to the negative pole. If negative pole grounding occurs in these circuits again, the coils will be short-circuited by the grounding point and fail to act. The grounding point is judged and located by the DC monitoring panel and DC fault locator.
4. Grounding of both positive and negative poles of DC system: Will cause blowing of power fuses and power loss of protection and operation circuits, and this grounding method may cause equipment damage. Generally, the probability of such faults is low.

4. AC inrush DC grounding fault: The device will alarm when the AC inrush DC reaches a certain amount. If the DC system is equipped with an AC-DC inrush line selection device, the faulty DC branch can be located quickly. The grounding point is judged and located by the AC-DC inrush device.

Notes on the above fault handling:① Operators shall proficiently, standardly and correctly connect and use the DC grounding fault locator.② Be able to correctly analyze and judge the test results of the instrument.③ No unsafe acts or phenomena are allowed during operation to avoid electric shock to personnel or damage to equipment and instruments.

VI. Handling of Current Sharing Fault of Charging Modules

The probability of current sharing function faults of charging modules is low. The module current sharing communication cable can be inspected, and such faults usually trigger other alarms. The mounting base of charging modules of some manufacturers is integrally formed, making it difficult to find faults from the outside. If the settings are correct, the fault may be with the charging module itself.

VII. Handling of Abnormal AC Switching Fault of DC Charging Device

Switch on and off the two AC input lines of the DC charging device respectively to check whether the other power supply can be automatically and reliably put into operation.