Fault analysis of an abnormal current sampling alarm of a rectifier transformer protection device

2022-07-15 0 By

The module conversion system and current loop of the protection device are the key to realize the protection of transmission lines or components, so the module conversion system inspection and sampling accuracy verification of the protection device is one of the essential work in the protection device test.The author describes in detail the inspection and analysis of pcS-977 current circuit sampling accuracy of the commutator protection device in a dc ice-melting device test, when the commutator protection device reported abnormal differential current alarm and abnormal alarm light on the device panel without adding any analog value.And focus on the cause of the protection alarm, the harm of similar problems and preventive measures are analyzed.A overview, field anomalies in the protection test of two rectifier as the direct power supply of complete sets of dc melting ice system, dc respectively melting ice power device and D Y bridge bridge, rectifying change USES is 35 kV three-phase two winding transformer, high voltage side take to pick up to 35 kV bus bar, low voltage side after unknown variable output power 11.3 kV voltage of rectifier valve group,Rcs-977, a complete set of thawing transformer protection device, is used for commutation and transformer protection. The high-voltage side current of the current circuit of the protection device is taken from the CT winding of the 35kV high-voltage side switch, and the low-voltage side current is taken from the CT winding of the bushing of the commutation and transformer and low-voltage side.During the sampling test, after sampling and verifying the three-phase current on the high voltage side of the #1 rectifier transformer protection with the protection tester, when changing the test line to prepare sampling and verifying the three-phase current on the low voltage side, when only changing the A-phase current test line, the alarm of the protection device will be on and the differential current abnormal alarm will be reported.After an alarm occurs, the staff immediately draws attention to it and stops working. The wiring diagram of field test is shown in Figure 1 below.Figure 1 site validation test wiring schematic tester in stopped at this time no added amount, no analog output, tester is A test phase lines connect to low voltage side of protection device input terminal, A phase B, C, N phase test line is still on the high side B, C and N inputs, high protection, low voltage side foreign current terminal connector not poke,The current loop of the device is not isolated from the CT current loop connected to the site, and the sampling of the protection device shows that the Ia on the low-voltage side has 0.37IE current (the device differential current alarm is set to 0.2IE after inspection).In view of the above situation, the possible cause of the abnormal device is analyzed and verified.Analysis 1: there is short-circuit grounding or no-load state inside the tester output current to the protection device, resulting in differential current protection, abnormal sampling device alarm.On-site staff with a multimeter and megger test instrument source earthing terminal, shell and test instrument of output current, voltage N insulation measurement, good insulation, rule out the possibility that the tester is generated by internal grounding current, at the same time, the test equipment validation under no-load condition, keep the tester is in the idle state,It was found that the output voltage N of the tester was 0.96V, and the test line was short-connected with the ground. No current would be generated, which ruled out the possibility that the tester could provide current to the protection current loop in the no-load state.Analysis of two:Access protection device of high and low pressure CT secondary winding current circuit ground, does not meet the anti-accident measures of CT of CT in the differential protection circuit must be at the request of the one-point earthing protection device place, respectively in CT terminal box on the ground, two ground through the test of CT loop line form a loop, cause the current circuit has two grounding,Due to the induction voltage in the substation, the long cable of the current loop leads to the voltage difference between the two grounding points of the current loop, thus generating the current causing device alarm.Site wiring of A simulation, and analyze the practical situation of wiring, because of the tester is A phase test line in A phase input terminals on the low voltage side of protection device, B, C and N test line (pick up on the high side B, C and N inputs, and because of the rectifier transformer protection device current input circuit terminal connector is not broken, and the scene to current loop is not isolated,As a result, the ground point of the field terminal box of the secondary loop of the two CT windings on the high and low voltage sides taken by the protection device forms a loop through the test line. Finally, the current is generated due to the existence of the induced voltage, which leads to the alarm of abnormal sampling of the device.The inspection found that the secondary loop of the CT winding of the high-voltage side switch was grounded on site, and the secondary loop of the CT winding of the low-voltage side casing was grounded at the terminal box of the rectifier body on site, and there was a certain distance (about 25 meters) between the two grounding points.Further inspection found that the current terminal connecting piece of the current N wire connected to the low-voltage side of the protection device was removed, which effectively isolated the low-voltage side N wire connection (the grounding point was isolated). Under the condition that other wiring was not changed, the current sampling of the device became zero and the abnormal alarm disappeared.Similarly, remove the current terminal connecting strip with current N wire on the high voltage side of the protection device. When other conditions remain unchanged, the current sampling of the device also becomes zero, and the abnormal alarm disappears.Further confirm there is no output device on the tester alarm for abnormal samples under the condition of high and low voltage side circuit two ground for testing line to form a loop, is commonly referred to as the current loop of two ground [2], and because of the existence of the induced voltage lead to two different ground potential and current input to protection, which leads to device of differential current faults.Iii. Further Simulation verification of cause Analysis After the cause was determined, in order to further confirm and verify the abnormal analysis, wave recording verification was carried out in combination with the wiring situation of the wave recording device in the melting chamber.The melting chamber wave record device of current sampling circuit connection with rectifier transformer protection situation, respectively connected to the two rectifier for analog monitor current high and low voltage side, and each rectifier transformer protection PCS – 977 access to the high and low voltage side current is derived from the same different winding of CT, CT loop grounding of consistent, both in CT earthing terminal box.For further validation of rectifier transformer protection, sampling current loop two ground and caused by the existence of the induced voltage, the wave record device for the staff of the two rectifier becomes low voltage side current loop (ground respectively in each rectifier ontology terminal box, 20 meters apart) to analyze wave record verification, validation wiring figure 2 as follows:Figure 2 fault wave record verification wiring diagram tester B, C and N in wave record up the device output test line # 2 CT on the low voltage side of rectifier changes of B, C, N the secondary current circuit connected to the terminal, when A phase test line access # wave record device 1 rectifier transformer low voltage side A phase when the secondary current circuit connected to the terminal, wave record starting wave record device, wave record waveform analysis message shown below in figure 3.From the waveform, it can be seen that phase A current (effective value 0.203a) on the low-voltage side of the #1 rectifier and phase B and C current (effective value 0.066A and 0.063A, respectively) on the low-voltage side of the #2 rectifier are generated at the same time with A phase difference of 180°.Three times and into A relationship (# for 2 rectifier transformer low voltage side CT circuit with the three test thread B, C, N, and CT secondary winding is high resistance [3], so the # 1 CT on the low voltage side of rectifier becomes ground electricity flows through A phase, the tester is B, C, N line respectively to # for 2 rectifier transformer low voltage side CT location, form three lines in parallel, as shown in chart 2).As can be seen from the wiring of the current loop of the wave recording device, since the wave recording device is not connected to the external zero sequence current, the current divided by the N-phase test line does not flow into the device and directly flows back to the ground, so the n-phase current 3i0-n of the low-voltage side of the #2 rectifier in the wave recording device is zero (the actual current divided by the N-phase test line is about 0.074A).Therefore, it is further confirmed that the sampling abnormal alarm is that the ground point of the two CT circuits forms a loop with the test line, and the current circuit is usually called two-point grounding fault.By searching and analyzing the reasons for the abnormal sampling of the rectifier variable protection, the abnormal reasons are finally found out. In view of the abnormal reasons, the harm of two-point grounding in CT loop and the preventive measures are summarized.Hazards: in the current secondary loop, if there are grounding points on both sides of the sampling coil of the protection device, on the one hand, the parallel circuit composed of two grounding points and ground will short-circuit the current coil, so that the current through the sampling device is greatly reduced.In addition, in the event of a ground fault, the ground potential difference between the two ground points due to the zero-sequence current flowing through the ground grid will generate a significant additional current in the current coil.These two reasons will make the current sampled by the device greatly different from the secondary fault current of the current transformer, which will lead to incorrect action of protection or inaccurate measurement.Preventive measures: 1. Strictly implement the requirements of CT loop related countermeasures to ensure that the secondary loop of CT winding has only one grounding point.For the secondary circuit of current transformer which is independent and has no electrical connection with other secondary circuit of current transformer, it is advisable to realize a little grounding in the switching field.The current loop composed of several sets of current transformers should be grounded at the point where the first level combined current, especially the CT winding secondary loop connected to the differential protection where two sets of current transformers merge must be grounded at the point where the protection device merges.The secondary winding of the standby current transformer should be short-circuited in the switching field and grounded at one point.2, in the process of debugging, debugging personnel arrangement in isolation measures must check grounding of electric current loop and the site location, especially in CT a power outage, only one set of CT loop device test, be sure to test current loop and outer loop and reliable security isolation, to ensure that the current secondary loop must one and only one place,Avoid the situation of one-point grounding of the current secondary circuit due to the failure of N phase isolation after the arrangement of safety measures, which leads to the misoperation of the protection device.This paper is compiled from “Electrical Technology”. The title of the paper is “Fault Analysis of abnormal current sampling Alarm of a rectifier transformer protection device”. The author is Rong Jun.