Discussion on the verification method of power dir

2022-08-16
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Discussion on the verification method of transformer power directional protection

Abstract: a verification method of transformer interphase and grounding power directional protection is proposed. Through the analysis of transformer phase to phase and grounding faults in power system, combined with the wiring polarity of Pt and CT, this method simulates the system fault and carries out the whole group test, which can simply and reliably verify the transformer power direction protection

key words: polarity analysis of power directional protection phase to phase fault ground fault

1 introduction

transformer power directional protection (including phase to phase power directional protection and zero sequence power directional protection) is one of the important backup protection of transformers. As a backup protection for the internal faults of adjacent components and transformers, it plays an important role in preventing the expansion of the fault range and ensuring the safe operation of the system. The correctness of its directivity is related to the primary and secondary wiring of current transformer, the secondary wiring of voltage transformer and the secondary wiring of protection device. In actual operation, it is easy to cause protection malfunction or refusal due to the wrong polarity of wiring. This paper attempts to summarize a simple and reliable verification method through the discussion of power direction protection. The results show that by simulating the actual faults of power system and combining the analysis of CT and Pt wiring polarity, the correctness of power direction protection direction can be tested simply and reliably, and the workload is greatly simplified in equipment acceptance and daily regular inspection

2 problem

the correctness of the power direction protection direction can be checked by checking the polarity of the voltage and current wiring of the protection. However, for the actual device on site, there are many secondary lines, the connection method is complex, it is difficult to clarify the direction of each line, and it is easy to make mistakes. Moreover, for microcomputer transformer protection, which is increasingly widely used, the direction of power direction protection is generally set by software control word, and the determination of directivity is controlled and determined by protection calculation software under the condition that the protection software module defaults to the polarity of voltage and current wiring of the system. For example, for wbz2500 microcomputer transformer protection, its configuration has the function of direction, and the direction must be determined under the following polarity wiring mode: CT polarity is that when the primary current flows into the transformer, the induced current of the device is the positive current flowing into the device; The polarity of Pt is positive, and 1500 people are arranged for employment. In this way, it is impossible to check the directionality of the power directional protection by checking the polarity of the voltage and current connection method of the relay as the discrete component protection. The relatively simple and reliable method is to simulate the forward and reverse faults of the system according to the protection setting and the wiring polarity of CT and Pt, add the simulated fault voltage and current to the protection, and verify the angle and sensitivity of its action

3 verification of phase to phase power direction

to simulate system faults and carry out the whole group of tests, we must first analyze the situation of a system fault

the positive direction setting of 220kV Transformer interphase power direction protection in our bureau is all directed to the bus. First, consider the case of positive direction fault. As shown in Figure 1, when phase to phase fault occurs in the line outside the bus, the CT of the transformer is protected, and the positive direction of the current is from the bus to the transformer. If the line impedance angle is 70 °, the vector diagram of primary voltage and current can be drawn, as shown in Figure 2. It can be seen that the fault current IK lags behind the interphase voltage uk160 °

Figure 1 system positive phase to phase fault

the vector relationship of secondary voltage and current depends on the polarity of Pt and CT connection. Generally, the PT adopts the polarity reduction connection method, and the polarity end of the secondary winding is connected to the protection (see Figure 9 below for the PT wiring diagram). For CT, the polarity reduction connection method is also adopted (see Figure 10 below for CT wiring diagram). When the primary winding L1 points to the bus and the secondary side current flows out from K1, it can be considered that the secondary current and the primary current are in the same phase. At this time, the secondary voltage and current vector diagram can be made, as shown in Figure 3; On the contrary, when the secondary side current flows out of K2, the phase of the secondary current and the primary current is opposite, and the secondary voltage current vector relationship is shown in Figure 4

Figure 2 primary vector diagram of positive phase to phase fault Figure 3 secondary vector diagram and action area of positive phase to phase fault Figure 4 secondary vector diagram and action area of positive phase to phase fault

if the protection can operate correctly in case of positive direction fault, and the protection should be reliable and inoperative in case of reverse direction fault, it indicates that the protection wiring is correct and the performance is intact

for example, CT primary winding L1 points to the bus, and the secondary side current flows out from K1. When the secondary voltage uk2 and secondary current ik2 as shown in Figure 3 are added to the protection, it is equivalent to the external fault of the system bus at this time. When the direction pointing to the bus is positive, the fault belongs to the positive direction fault, and the protection should act correctly. Thus, the action area and sensitive angle of the protection can be verified, as shown in Figure 3

take the lg211 interphase power direction relay as an example. When its sensitive angle is set to 230 ° and 90 ° wiring is adopted, in the case of the polarity and direction of the above Pt and CT wiring, if the protection wants to act under the positive direction fault, it is required that the relay current coil and the reverse polarity of the voltage coil are connected with the secondary voltage current. If the polarity of the voltage coil is terminated with the polarity end of the PT secondary, the polarity end of the current coil is connected with the non polarity end of the CT secondary, In this way, the action area can be consistent with the fault, and the directivity can be guaranteed. At this time, the range of the action area of the relay is IK leading uk120 ° to 300 °

when the primary winding L1 of CT points to the bus and the secondary side flows out from K2, the relationship between secondary voltage and current is exactly 180 ° in the above positive direction fault, as shown in Figure 4. When the secondary voltage uk2 and secondary current ik2 of this relationship are added to the protection, it is also the case of fault outside the system bus, and the protection should act correctly. At this time, the range of action area is IK lagging uk60 ° to leading uk120 °. If lg211 interphase power direction relay is used, it can be inferred that the voltage and current coils of the relay are positive and connected to the secondary voltage and current

it can be analogized:

when the primary winding L1 of CT points to the transformer and the secondary side flows out from K1, the correctness of power direction protection can be determined when the action area of protection is the same as that shown in Figure 4

when the primary winding L1 of CT points to the transformer and the secondary side flows out from K2, the action area should be the same as Figure 3

it can be seen that when verifying the power direction protection, according to the polarity of Pt and CT wiring and the setting of protection direction, simulate the primary fault of the system, and add secondary voltage and secondary current to the protection to conduct a group test, which can not only verify the integrity of protection function, but also verify the correctness of protection power direction wiring. The method is simple and reliable

4 verification of zero sequence power direction

verify the zero sequence power direction by simulating fault. First, analyze the relationship between zero sequence voltage and current in case of positive direction grounding fault. As shown in Figure 5, a zero sequence network diagram is made when a ground fault occurs at point K of the system. It can be seen from the figure that the zero sequence current IK flows through the M side of the zero sequence network, and the zero sequence voltage um0 on the bus side is:

Figure 5 system positive direction ground fault zero sequence equivalent network um0 = IK × ZM, ZM side zero sequence impedance; ZM mainly depends on the zero sequence impedance of the neutral grounding transformer in the substation, and the impedance angle is about 85 °

according to the formula, the phasor relationship between primary zero sequence voltage uk1 and primary zero sequence current IK1 is shown in Figure 6. The zero sequence voltage is about 85 ° ahead of the zero sequence current. The phasor relationship between secondary zero sequence voltage and secondary zero sequence current is related to the wiring of

pt and CT

the setting of the positive direction of the zero sequence power directional protection of 220kV Transformer in our bureau is all directed to the bus. The zero sequence voltage is obtained through the PT open triangle, and its wiring adopts 23 U0 to connect the etc line, that is, the primary zero sequence voltage and the secondary zero sequence voltage are in reverse phase. As shown in Figure 9

when the zero sequence current flows through the secondary neutral line of bushing CT, when the primary side L1 of CT points to the bus, and the secondary side is led out from K1 to the protection, it can be considered that the primary zero sequence current is in phase with the secondary zero sequence current, and the vector relationship between the secondary zero sequence voltage uk2 and the secondary zero sequence current ik2 is shown in Figure 7; On the contrary, when L1 on the primary side of CT points to the bus, the secondary zero sequence current is in reverse phase, and the secondary zero sequence voltage uk2 and the secondary zero sequence voltage are led out from K2 to the protection, it can be considered that the vector relationship between the primary zero sequence current and the secondary current ik2 is shown in Figure 8

Figure 6 positive direction ground fault primary zero sequence voltage and current figure 7 positive direction ground fault secondary vector diagram and action area figure 8 positive direction ground fault secondary vector diagram and action area

Figure 9 PT secondary voltage 23 U0 wiring diagram

similarly, when verifying the zero sequence power direction protection, first find out the wiring mode of Pt and CT, and then simulate the fault situation, Add the secondary zero sequence voltage uk2 and secondary zero sequence current ik2 of the above relationship to the protection terminal, check the protection action, and determine the action area and sensitive angle of the protection. If the protection can act correctly in case of positive direction fault, and the protection should be reliable in case of negative direction fault, it indicates that the protection wiring is correct and the function is intact

for example, C takes the average value as the hardness value of the test piece T, the primary winding L1 points to the bus, and the secondary side current flows out from K1. For the zero sequence power directional protection with the direction pointing to the bus as the positive direction, add the secondary zero sequence voltage uk2 and secondary zero sequence current ik2 as shown in Figure 7 to the protection, then it is equivalent to the external fault of the bus at this time, and the protection should act correctly, but should not act in the case of the reverse direction fault. The action area and sensitive angle of the protection are determined according to the above action conditions, as shown in Figure 7. The operating range of the protection is 5 ° ~ 185 ° current lead voltage, and the maximum sensitive angle is 95 °

Figure 10 CT wiring diagram for lg212 zero sequence power direction relay, when the sensitivity angle is 70 °, in the above case, the voltage and current coil of the relay should be reverse polarity connected to the secondary zero sequence voltage and secondary zero sequence current (if the polarity of the voltage coil is connected to the polarity end of the PT secondary, the polarity end of the TV coil should be connected to the non-polar end of the CT secondary), so as to ensure that in the case of positive direction fault, The relay can operate correctly when the secondary zero sequence current is ahead of the secondary zero sequence voltage

when the CT primary winding L1 points to the bus and the secondary side current flows out from K2, the secondary zero sequence current is exactly 180 ° reverse, as shown in Figure 8. For the zero sequence power directional protection with the direction pointing to the bus as the positive direction, add the secondary zero sequence voltage uk2 and secondary zero sequence current ik2 shown in Figure 8 to the protection, then it is just the case of positive direction fault, and the protection should act correctly, but should not act in the case of reverse direction fault. The action area and sensitive angle of the protection are determined according to the above action conditions, as shown in Figure 8. For lg212 relay, it can be deduced that the voltage and current coils of the relay should be positive connected to the secondary zero sequence voltage and secondary zero sequence current

it can also be deduced:

when the primary winding L1 of CT points to the transformer and the secondary side flows out from K1, the correctness of zero sequence power directional protection can be determined when the action area of protection is the same as that shown in Figure 8

when the primary winding L1 of CT points to the transformer and Shandong innovation group starts to make the secondary side of conventional cable flow out of K2, the action area should be the same as Figure 7

it can be seen that when verifying the zero sequence power directional protection, it is the same as verifying the phase to phase power directional protection. According to the polarity of Pt and CT wiring and the direction of protection, it simulates the primary fault of the system, and adds secondary voltage and secondary current to the protection to conduct a group test. Noteworthy

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