Keywords

1 Introduction

With the construction of high-voltage transmission lines and the rapid development of communication technology, the transmission line generally adopts a double differential configuration mode. When the main protection configuration is strengthened, the timing of backup protection can be simplified and not fully coordinated [1, 2]. However, under extreme operating conditions such as protection blocking caused by protection against DC power loss in the power grid, loss of multiplex channel communication equipment, and communication channel failure, the extreme main protection of the substation single, multiple, or even the entire station line may be lost. These extremes short-circuit faults on the line under working conditions often need to be removed through backup protection. If the backup protection is simplified or mismatched, the backup protection will refuse to act or skip actions [3,4,5,6], which will cause the stability of the power grid and the safety of the equipment.

Domestic and international researches have been carried out on the optimization of related backup protection under extreme operating conditions. The literature [7] has studied the strategy of simply accelerating the backup protection section II when the line transmission channel is abnormal while ensuring that the delay of the adjacent line backup protection meets the coordination relationship, but there is also the risk of accelerating overstepping misoperation of the faulty adjacent line backup protection in the line area when the multi-interval or the total station line channel is abnormal. Literature [8] studied the identification of DC power loss in total station protection and put forward the concept of regional protection master station-substation protection system. This regional protection requires the use of additional communication channels to achieve data transmission to achieve rapid removal of regional distance protection and achieve cost high communication requirements.

In view of the extreme power grid conditions where the line differential main protection is lost, the switch information in the substation is used to identify the extreme conditions that lead to the loss of differential protection, and then the accelerated coordination strategy for backup protection is completed, and distance protection is achieved according to the main protection lost conditions adaptive range adjustment strategy. The optimized solution eliminates the problems of far-delay action of backup protection, the risk of disordered misoperation and insufficient protection sensitivity, and improves the reliability of backup protection to quickly remove faults.

2 Adaptability Analysis of Backup Protection

2.1 Backup Protection Setting Principle

In the ultra-high voltage line setting specification, for the line of 220 kV–750 kV voltage level, the setting is generally based on the near reserve and near-backup principle. When setting the distance protection section II, first consider the sensitivity, and cooperate with the adjacent line section I or longitudinal protection. If the time is considered to be in conjunction with the general time setting value of 0.5 s–1.7 s, the simplified section will set the operation time consistency of section II [9, 10]. The distance protection section III is set according to sufficient sensitivity, and cooperates with the adjacent line distance section II or section III. Only when the conditions permit, the combination of remote and near is adopted, and the sensitivity coefficient of remote backup is not required, and the action time avoids the maximum oscillation period, generally 1.7 s–4.0 s [9,10,11].

The sensitive section of the zero sequence protection is set according to the sensitivity of the metallic fault at the end of the line. At the same time, it cooperates with the sensitive section of the zero sequence protection of the adjacent line, and the time is not less than 1.0 s. The end of zero sequence protection is used to supplement the backup distance protection. The setting value is usually 300 A. The action time is set according to the end of the zero sequence overcurrent of the adjacent line. The starting time avoids the longest non-full phase running time [9,10,11].

2.2 Analysis of the Problem of Backup Protection When the Main Protection is Lost

According to the analysis of the existing setting principles, the backup protection setting has a far action delay when it is matched, and the misselection of the mismatch setting timing cannot meet the protection scope and selectivity requirements, which in turn leads to mismatch or overstepping action to the lower line bus in complex grid extreme conditions malfunction. In Fig. 1, lines L1–L3 adopt dual configuration line differential protection, and at the same time the backup protection of each line is configured according to the principle of simplification. Assuming extreme operating conditions such as device lockout, communication equipment abnormality, and channel failure caused by protection power failure at substation B, a backup protection adaptability analysis is performed.

Fig. 1.
figure 1

Schematic diagram of power transmission system protection configuration.

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  1. (1)

    Substation B protection device blocking.

    When protection P3 occurs DC power loss in substation B, the differential main protection is lost, as shown by ① in Fig. 1. When a fault occurs in line L2, it is hoped that P1 and P4 will quickly remove the fault. As shown in Fig. 1, if the fault point K1 is within the backup protection range of the protection P1, if there is no special treatment, normal coordination delay action will be performed, and the fault removal will be slower, which may cause serious burnout of the device. If the fault point is at K2, P4 can remove the fault, but P1 cannot be removed according to the near backup setting, which may cause the protection to refuse to operate. When the whole station protection is blocked, there is also the problem that the backup protection of the adjacent station refuses to move or the action is slow.

  2. (2)

    Substation B communication equipment is abnormal.

    When the substation B communication DC power loss line L2 adopts the multiplexing channel, the line loses the main protection, as shown in ② in Fig. 1. If the K1 fault point is within the range of the near-backup section II, the action time is not less than 0.5 s, and when the multi-level difference is involved, the action delay is longer, which is not conducive to the rapid removal of the fault. However, due to the loss of the main protection, the line L2 fault may enter the near-backup protection range of the adjacent lines P1 and P6 at the same time. When the time setting is consistent or mismatched, the adjacent lines will misstep and expand the fault removal range. If all station line protections are multiplexed channels, the communication equipment is abnormal and the main station line main protection is lost. The near-backup protection time is set according to the step difference and the action delay is long. If the action time acceleration is shortened to the same, it may cause the loss of the main protection phase. Adjacent line near-backup protection overstepping action expands the scope of fault removal and affects the safety of power equipment. For dedicated fiber differentials, the main protection is not affected.

  3. (3)

    Channel failure

The substation B line L2 channel fault and the fault in the area are within the range of backup section II, as shown in ③ in Fig. 1. When the multi-level difference is involved, the protection action delay is long, which is not conducive to the rapid removal of the fault. When all substation B channels are faulty, the main protection of lines L1 and L2 is lost. The fault of line L1 or L2 may enter the near-backup protection range of the adjacent line. When the time setting is consistent or mismatched, the adjacent line will be misstepped and expand the fault. The resection range, and the resection time may be longer.

Blocking of protection device, abnormal communication equipment, and channel failure will cause the main protection to be lost and output an abnormally signal for the channel. When the line protection uses a dedicated optical fiber or multiplexed channel, the channel quality judged by the protection is determined. In the differential protection, the channel error occurs when the data check fails in the received frame. The line differential protection requires a channel error rate of 10–3–10–5. In order to ensure high sensitivity, the current line protection channel error rate is set at a fixed value 10–5. In addition to data verification, When the received data frame header is completely wrong, it is considered that there is no valid frame. When the channel has no valid frames or the channel error exceeds 10–5, the main protection and output channel abnormal signal are blocked after a certain time delay. This delay is 200 ms according to the guaranteed discrimination reliability. There is no delay output when the protection device is locked. After the channel quality is restored, the main protection and channel anomaly signals are restored after 1 s widening. This widening time is greater than the blocking time of the protection device when the setting value is modified. The main protection blocking and channel anomaly discrimination logic are shown in Fig. 2.

Fig. 2.
figure 2

Differential main protection blocking and reset logic.

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3 Optimized Scheme for Backup Protection

Regardless of the line differential protection caused by protection blocking, communication equipment abnormality or channel failure, according to the existing backup protection setting scheme, there is a risk of long action delay, rejection or misleading.

The design specifications of the substation require the protection power supply and the communication power supply to be independent, because there may be different DC power loss under extreme working conditions, so the full substation switching quantity acquisition equipment adopts the protection power supply and channel power supply dual power supply mode for the discrimination of extreme working conditions.

3.1 Judging Extreme Conditions

  1. (1)

    Judgment of protection blocking in the substation

When the protection device is blocked, the dual line protection at each interval outputs the “device blocking” and “channel abnormal” signals. If the full substation protection fails, the DC screen in the substation outputs the “protection DC loss” signal.based on these signals, it is determined that the line protection device in the substation has failed. Its discriminating logic is shown in Fig. 3.

Fig. 3.
figure 3

Discrimination of blocking of line protection interval in substation.

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In Fig. 3, the switching information of the substation domain meets the delay time T1 and outputs the “protection blocking signal in the substation”, which is output to each interval line protection device. T1 avoids the restart time of the protection device to modify the fixed value, which is generally less than 500 ms.

  1. (2)

    Abnormal identification of communication equipment in substation

When the multiplex channel MUX communication is blocked, the dual MUX equipment in each interval outputs the “communication device blocking” signal, and the corresponding channel protection device outputs the “channel abnormal” signal. If the communication power of the entire substation loses power, the communication DC screen in the substation outputs “communication power loss” signal to identify the blocking of communication equipment in the substation. The discrimination logic is shown in Fig. 4.

Fig. 4.
figure 4

Abnormal discrimination of line communication equipment in the substation.

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In Fig. 4, T2 is the confirmation delay of the communication abnormal signal in the station, avoiding the restart time of the MUX device. The abnormality of the communication equipment makes the main protection of the line lost but the logic of the backup protection function of the protection device is normal.

  1. (3)

    Channel fault identification

When the external channel is abnormal and then short-circuited, it is assumed that the double-channel or multi-circuit operation between the two stations is considered to be a single-interval channel failure. If the substation is connected to multiple substations, the multi-interval channel abnormality is considered to be the multi-interval channel failure. The discriminant logic is shown in Fig. 5.

Fig. 5.
figure 5

Fault identification of multi-interval line channels in the substation.

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In Fig. 5, T3 is the confirmation delay of the multi-interval channel fault signal in the station, generally 1 s. The channel fault is eliminated due to the device blocking and the abnormal communication device. The channel’s main protection is lost when the channel fails, but the protection device’s own near-backup protection function logic is normal.

Through the collection and logic judgment of the switching value in the substation, it can distinguish between protection blocking, abnormal communication equipment and channel failure, laying a foundation for the configuration and optimization of backup protection of line protection.

3.2 Accelerated Self-adjustment Strategy for Backup Protection

In order to ensure versatility, the “backup self-adjusting control word” is added to the line protection to switch the function on the standard line protection device. Adding the input of “protection blocking signal in substation”, “abnormal signal of communication equipment in substation” and “multi-interval line channel fault signal in substation” to realize different backup self-adjustment strategies. At the same time, the “remote backup distance protection setting” is added for adaptive adjustment of the distance protection range under extreme power grid conditions.

  1. (1)

    Backup protection strategy for line protection blocking in substation

Taking the protection blocking of substation B in Fig. 1 as an example, with the substation B full station protection blocking or partial protection blocking, adaptive acceleration of substation A protection P1 and station C protection P4 backup protection is realized.

The line protection that the main protection in the substation does not lose when the partition protection of the substation B is blocked will transmit the received “blocking protection signal in the substation” to the opposite side through the optical fiber to trigger the optimization of the backup protection of the opposite side. The abnormal and extreme conditions in the substation are judged by passive adaptive acceleration, so the passive adaptive acceleration trigger logic of the backup protection of the neighboring station is shown in Fig. 6.

Combined with the protection passive acceleration self-adjustment trigger logic in Fig. 5, the conditions are that the interval line protection device is not blocked, there is no protection blocking in the substation, the communication equipment is abnormal, and the multi-interval channel fault, and the interval channel is abnormal or received in the opposite substation Protection blocking signal. When substation B has interval line protection blocking, protection of P1 and P4 requires an adaptive acceleration adjustment backup strategy. The distance protection section II accelerates to tz2 to ensure the sensitivity of the end fault of the protection line itself. The adjacent line bus fault has sufficient sensitivity, and the distance III setting value is adaptively changed to the set “remote backup distance protection setting value“. Accelerate zero sequence II section to tL2, accelerate zero sequence III section to tL4, in order to ensure the fast removal of short circuit faults in the area and the operation time of the main protection and failure protection of multiple normal adjacent lines, tz2 = tL2 = 0.5 s , tz4 hide the longest oscillation time over the line is 1.8 s, and tL4 and tz4 retain a time difference, its value takes 2 s.

Fig. 6.
figure 6

Protection passive acceleration self-adjustment trigger logic.

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  1. (2)

    Backup protection strategy for abnormal blocking of communication equipment in substations

Taking the MUX failure of substation B in Fig. 1 as an example, regardless of the loss of the main protection of the substation B's total station line or part of the interval line, the substation B communication equipment abnormal interval line near-backup protection and the loss of main protection adjacent line near-backup protection action time need to cooperate and accelerate. The line protection device in substation B will receive the “abnormal signal of the line communication equipment in the substation”, combined with the channel abnormal signal of the line protection device, to actively trigger acceleration automatically. The protection device performs passive adaptive acceleration triggering, and the logic is in Fig. 6.

Taking the active acceleration self-adjustment trigger condition of Fig. 7 as the interval device is not blocked but the device channel is abnormal, and the communication equipment in the substation is abnormal or there are other interval channels in the substation. Combined with the protection passive and active acceleration self-adjustment trigger logic in Figs. 6 and 7, in Fig. 1 it is assumed that when the lines in substation B are multiplexed channels and the communication power supply is powered off, protection P1 cooperates with protection P3, protection P4 cooperates with protection P2, protection P1, P2, P3, P4 all need adaptive acceleration. The zero sequence protection stage II accelerates to tL1, and the zero sequence protection stage III accelerates to tL3. The adaptive acceleration strategy for protecting P1 and P4 is the same as in Fig. 6. In order to ensure the rapid elimination of the fault in the area and the near-backup protection coordination characteristics of the adjacent lines, the action time coordination relationship is tz1 < tz2 < tz3 < tz4, tL1 < tL2 < tL3 < tL4, tz3 < tL3, tz4 < tL4. In order to retain the level difference tz1 = tL1 = 0.3 s, tz3 = 1.6 s, tL3 and tz3 retain a time difference, its value takes 1.8 s.

Fig. 7.
figure 7

Protection active acceleration self-adjustment trigger logic.

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  1. (3)

    Multi-interval line channel fault backup protection strategy in substation

Taking the channel failure of substation B in Fig. 1 as an example, there is no loss of the near-backup function of the line when the main protection of the multi-interval line in the substation B is lost, and the near-backup protection of the channel fault interval line needs to cooperate with the near-backup protection action time of the adjacent line that lost the main protection speed ​​up protection action time. The line protection device in substation B will receive the “multi-interval line channel fault signal in station”, combined with the channel abnormal signal of the line protection device, to perform active adaptive acceleration trigger. The adaptive acceleration trigger logic is the same as that in Fig. 7. Passive adaptive acceleration triggering on the opposite line protection device with abnormal channel is the same as that in Fig. 6. Combined with the protection passive and active acceleration self-adjustment trigger logic in Figs. 6 and 7, the multi-interval line channel fault backup protection strategy in the substation is the same as the communication device in the substation abnormally blocking backup protection strategy, but passive adaptation is performed only when the single-interval channel fails. Aiming at the extreme working conditions such as line protection device blocking, communication equipment abnormality and channel failure in the power grid, the active and passive acceleration self-adjustment trigger logic of near-backup protection is proposed. From the backup retention strategy of Fig. 6 and Fig. 7, when the line that loses the main protection fails, the near-backup protection can expedite the removal of the fault. At the same time, it ensures that the action time and protection range between the protection device in the substation and the protection in the opposite substation after the main protection of the multi-interval line is lost. The risk of expanding the fault range and the risk of refusal due to mismatch of backup protection are avoided.

4 Simulation Test

The above-mentioned backup protection acceleration self-adjusting trigger logic is implemented on standard ultra-high voltage line protection. At the same time, the distance, zero-sequence backup action time and action range of the line protection are self-adjusted according to the trigger logic. Identify the device under extreme conditions, and carry out systematic simulation tests on the protection logic and backup protection strategy.

According to Fig. 1, a simulation test environment is established. Each substation is equipped with a dual-power station domain switch quantity acquisition device. The unit positive sequence impedance of lines L1–L3 is \(Z_{1} = {0}{\text{.02 + j0}}{.28}\,{\rm \Omega}/{\rm km}\), and the unit zero sequence impedance is \(Z_{0} = {0}{\text{.19 + j0}}{.84}\,{\rm \Omega}/{\rm km}\).The lengths of lines L1–L3 are 100 km, 40 km and 60 km respectively. The protection P1–P6 are all put into the differential protection function, the backup protection distance I is set according to 70% reliability, the distance II is set according to the simplified consideration of sensitivity, and the other settings are set according to cooperation. The protection once is set in Table 1.

Table 1. Primary value of simulation test protection setting.
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Short circuit faults at different locations on line L2 under different extreme conditions of analog substation B. The location of the selected fault point is 20%, 80% and 100% of the total length of the line at the protection P2 installation, and the backup protection of different fault points P1–P6 before and after the optimization of the action, through the “backup self-adjusting control word” to switch back and forth to achieve the comparison before and after optimization, the action results are shown in Table 2, Table 3 and Table 4. In addition, it simulates the action behavior of substation B under different extreme conditions when the L3 line is faulty.

Table 2. Protection action when L2 line has a short circuit fault at 20%.
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Table 3. Protection action when L2 line has a short circuit fault at 80%.
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Table 4. Protection action when L2 line has a short circuit fault at 100%.
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Under different extreme working conditions of substation B, protection P5 and P6 are reliably cut off by differential protection when line L3 has a short-circuit fault, and protection P1–P4 are not activated. The protection action results from Table 2, Table 3 and Table 4 show that before optimization, according to the existing backup protection principle and setting principle, the line fault backup protection under different extreme conditions has the problems of long fault removal, overstepping or refusal to operate. Identify the grid protection blocking, communication equipment abnormalities and communication channel faults under extreme operating conditions based on the switching value of the station domain. The optimized backup protection action speeds up and reduces the risk of backup protection overstepping action when line faults occur after extreme operating conditions.

5 Conclusions

Aiming at the loss of the main protection of the line in the substation due to the extreme operating conditions of the power grid, the paper proposes an extreme working condition identification method based on station domain information, near-backup and remote-backup protection accelerated self-adjustment strategy. Introducing dual power supply domain switching devices in substations to realize the discrimination of the main protection loss of single-interval and multi-interval lines caused by protection device blocking, communication equipment abnormality and channel failure, and then provide external decision-making information for emergency protection strategies. According to the discrimination of the extreme working conditions in the station, an active and passive adaptive backup protection acceleration trigger mechanism for line protection is proposed to realize the line protection of the main protection and the line protection of the neighboring station due to the loss of extreme protection in the station. The time is reasonably coordinated, and the simulation test shows that the emergency strategy of near-backup protection and remote-backup protection shortens the fault removal time of the fault line and the remote backup adjacent line, and avoids the long action time of the adjacent line or the occurrence of overstepping misoperation.