summary
Researchers Mao Yan and Feng Cancheng from the Zhongshan Power Supply Bureau of Guangdong Power Grid Co., Ltd. pointed out in an article published in the 11th issue of "Electrical Technology" magazine in 2018 that the DC system is an important component of the safe operation of substations. The insulation fault location test of the substation DC system is a crucial part of the DC system's routine maintenance work. It can effectively verify whether the insulation monitoring devices installed in the DC system can accurately issue alarms for branches with reduced insulation when the insulation of the DC load branch decreases.
This paper proposes a simple and reasonable control circuit that uses a changeover switch to select the grounding busbar. By adjusting the grounding resistance and designing a leakage current sensor clamp, the efficiency of insulation line selection tests for substation DC systems is improved, the risk of accidental contact with the DC busbar during the test is reduced, and the ultimate goal of improving human-machine interface efficiency is achieved.
The substation DC system [1-2] consists of charging devices, maintenance-free lead-acid batteries, feeder circuits, voltage regulating devices, and related control, measurement, signal, protection, and regulation units, providing reliable power for control, signal, relay protection, automatic devices, and emergency lighting. The insulation monitoring device is used to identify the grounded busbar and grounded DC branch when the insulation of the DC branch is low, and to quickly issue an alarm signal.
During the acceptance and routine maintenance of DC systems, personnel need to effectively verify the integrity of the insulation line selection function of the equipment. Currently, most routine maintenance work on substation DC systems uses self-made simple grounding test conductors. This requires multiple contacts with the DC bus and passing the test conductor through narrow branch leakage current transformers (CTs), resulting in high risks and low efficiency. Therefore, designing a safe and efficient insulation line selection test device for substation DC systems is crucial for substation DC system maintenance.
1 Design Scheme
1.1 Working principle of insulation monitoring device for line selection
Each branch in the DC feeder cabinet of the substation is equipped with a leakage current sensor to detect the insulation condition of the branch. The leakage current sensors are installed around the positive and negative output lines of the DC circuit, and the system monitors the signals output by the sensors of each branch in real time during operation.
When the insulation of a branch is normal, the current flowing through the sensor is equal in magnitude but opposite in direction, and its output signal is zero. When the branch is grounded, the leakage current sensor has a differential current flowing through it. The sensor transmits the differential current signal to the insulation monitoring device. The device uses parameter calculation to promptly select the grounded busbar and grounded branch, issues an alarm signal, and sends the signal to the plant and main station monitoring system.
Figure 1 shows a schematic diagram of the insulation line selection working principle of the DC system in the substation.
Figure 1 Schematic diagram of the insulation line selection working principle of the DC system of the substation
1.2 Working principle of insulation line selection test device
The structure of the test device [3-6] mainly consists of three parts: grounding bus switching circuit, clamp continuity detection circuit and grounding current control circuit, as shown in Figure 2.
Figure 2 Schematic diagram of the insulation line selection test device
The specific principle is as follows: The retractable conductor draws power from the DC bus (control bus + and control bus), uses a changeover switch to select the grounding bus, and passes the grounding current through the leakage current sensor of each DC branch through the clamp-type wire clamp. The adjustable resistor is used to adjust the grounding current, and finally meets the requirements of the DC system insulation line selection test.
As shown in Figure 2, when the changeover switch YK is switched to the "0" position, the YK contact of the wire clamp continuity detection circuit in the wire frame 1 is closed. When the wire clamp is inserted into the J2 hole and closed, the L2 indicator light is lit [7], indicating that the J2 wire clamp is closed properly. The functions of the L3-L7 indicator lights are the same as those of the L2 indicator light. When all the L2-L7 indicator lights are lit, the J2-J7 wire clamps can be used normally.
If a lamp does not light up, check if the clamp is properly closed, or replace it with a spare clamp. If the socket is not used, simply toggle the corresponding short-circuit switch to short-circuit the socket; the indicator light for that socket will then illuminate. The DC power supply voltage in the continuity detection circuit is 3V, the LED's forward voltage drop is approximately 1.8V, and the forward current is approximately 15mA. Considering the YK contact resistance and the diode's brightness requirements, and according to Ohm's law, the series resistors R2-R7 should be 100Ω.
After the continuity test of the wire clamp is completed, clamp the leakage current sensors of each branch to be tested with the wire clamp. Then adjust the adjustable resistor R1 to a suitable resistance value (7kΩ for 110V DC system and 15kΩ for 220V DC system), and use a multimeter to measure the actual resistance value of R1 in the resistance test hole. The circuit shown in frame 2 in Figure 2 is the grounding current control circuit.
After adjusting R1 to a suitable resistance value, insert the grounding clamp into the J8 socket and connect it to the grounding copper busbar of the DC feeder cabinet. Then insert the positive and negative DC bus clamps into the J0 and J1 sockets respectively. Then use the grounding bus switching circuit (as shown in frame 3 in Figure 2) to switch the YK position. When switched to "1", the positive DC bus is simulated to be grounded, and the red indicator light is lit. When switched to "2", the negative DC bus is simulated to be grounded, and the green indicator light is lit. A fuse is connected in series [8] to ensure circuit safety.
At this point, the insulation monitoring device will display the grounding busbar and select the DC branches clamped by the clamps. Repeating this test will complete the insulation line selection test for the entire substation DC system. Given that there are approximately 60 DC branches in the current substation DC feeder cabinets, and each row is designed with 10 branches, totaling 6 rows, 6 clamps are designed for convenient field use.
Taking the testing of a single DC branch in a 110V DC system as an example, first, adjust YK to the "0" position and insert the clamp into the J2 socket. At this time, L2 will light up. Then, close the short-circuit switches 2-6, and L3-L7 will light up. Next, insert the grounding clamp into J8, insert the bus clamp into J0 and J1, clamp the leakage current transformer (CT) of the DC branch to be tested with the J2 clamp, and adjust R1 to 7kΩ. After confirming that the measurement is correct, switch YK to the "1" position. The red indicator light will light up. Check that the insulation testing device should generate a positive DC bus grounding signal and select the branch to be tested. Switch YK to the "2" position. The green indicator light will light up. Check that the insulation testing device should generate a negative DC bus grounding signal and select the branch to be tested.
1.3 Appearance Design of Insulation Line Selection Test Device
To facilitate the safe and efficient use of the testing device by substation maintenance personnel, researchers innovatively designed the device's appearance.
As shown in Figure 3, the device panel wiring adopts a plug-in structure, utilizing the headphone jack for easy wiring and storage of test leads. The wire clamps feature a wire-type design, are small and lightweight, and can be effectively applied to leakage current sensors in confined spaces. The indicator lights on the panel are all low-power soft-light types, and different circuit operating states are distinguished by color differences.
The resistance adjustment and measurement port facilitates resistance adjustment and measurement during testing. The grounding busbar switching switch is a toggle switch, with corresponding auxiliary contacts configured for each state to meet the functional requirements of the device.
Figure 3 Front view of the insulation line selection test device
Figure 4 is a rear view of the test setup. The back panel design mainly includes two parts: heat dissipation and device fixation. Heat dissipation utilizes strip-shaped ventilation holes, providing excellent heat dissipation. The device is fixed using permanent magnets, which are mounted on both sides of the back panel. During the test, they can firmly adhere to the DC feeder cabinet of the substation, freeing the test personnel's hands.
Figure 4 Rear view of the insulation line selection test device
Comparison of test data from 2 devices
Taking the DC system of a 110kV substation as an example, according to the configuration of one DC feeder cabinet [9-10], there are a total of 50 DC feeders. Using the traditional simple test line, one person is required to monitor and another person is required to operate, which takes 1 hour. With this test device, one person is required to monitor and another person is required to operate, which takes 0.5 hours.
By comparing experimental data, this insulation fault location testing device can reduce the testing time of traditional insulation fault location tests by half. The main source of the increased efficiency is that it avoids the need to repeatedly connect test wires and busbar clamps and grounding clamps in the leakage current transformer (LCT) as in traditional tests. In addition, this testing device reduces the risk of personnel accidentally touching the DC busbar, ensuring the safety of personnel and equipment.
Figure 5 Comparison of experimental data from the device
in conclusion
The insulation fault location test device can efficiently complete insulation fault location tests of substation DC systems without frequent contact with the DC bus. The device features a simple panel design with clear and concise indicators, facilitating on-site operation by test personnel and making it suitable for widespread adoption within the industry.
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