Research on three-stage current protection for phase-to-phase short circuit in single-side power supply network

introduction

Current protection is generally divided into three stages: the first stage is current quick-break protection; the second stage is time-limited current quick-break protection; the third stage is time-limited overcurrent protection, also known as overcurrent protection. These three stages of current protection can ensure the safe operation of the power system.

1. Current quick-break protection

The current quick-break protection is a current protection that can be operated without time limit when the short-circuit current increases rapidly when a line fault occurs. As shown in Figure 1, an example is given to illustrate how the current quick-break protection is set.

Assuming that a fault occurs in section A-B, it is hoped that protection 1 can act quickly, and will not cause protection 1 to act when a fault occurs in section B-C at point k2, but protection 2 of section B-C to act. This requires that the setting value of protection 1 is greater than the maximum short-circuit current value when k2 is short-circuited, that is, greater than the current Ik.B.max when the three-phase short circuit occurs on the busbar B of the substation under the maximum operating mode. Therefore, the setting principle of the current quick-break protection is to set it according to the maximum short-circuit current value at the end exit of a certain section of the line, that is:

Where, Eφ is the phase electromotive force of the system equivalent power supply; Zs.min is the minimum impedance between the protection installation and the system equivalent power supply; ZA-B is the impedance between the short-circuit point and the protection installation; IEQ * jc3 * hps10 oal(sup 3(et.1 is the setting value.

The operating current is:

Here KEQ * jc3 * hps10 oal(sup 3(l is a reliability coefficient, and its value is about 1.2~1.3. The reason for introducing this reliability coefficient is to consider the influence of non-periodic components. The actual short-circuit current may be greater than the calculated value and a certain margin is left. Therefore, when setting the setting value, it is impossible to protect the full length of the line. In actual production, the minimum protection range of this protection must be greater than 15%~20% of the full length of the line. The minimum protection range refers to the short circuit type occurring at the end of the line when the system is in the minimum operating mode and the short circuit type is two-phase short circuit, that is, the minimum protection range calculation formula is:

Where Lmin is the minimum protection range length of the current quick-break protection; :1 is the positive sequence impedance per unit length of the line.

Advantages and disadvantages of current quick-break protection: Current quick-break protection only needs to set a setting value greater than the maximum short-circuit current value at the end of the line, so its structure is relatively simple. In addition, since the device has almost no time limit when a fault occurs, it can quickly cut off the fault. But at the same time, the biggest disadvantage of this protection is that it cannot protect the entire length of the line, and the protection range needs to be sacrificed. Therefore, if a line is too short, this protection device cannot be used.

2. Time-limited current quick-break protection

Since the first section of protection, i.e., the current quick-break protection, cannot protect the entire length of the line, it is necessary to install another set of protection devices for the remaining unprotected lines, which can protect the entire length of the line. However, as can be seen from the above current quick-break protection, it is contradictory to want to protect the entire length of the line and at the same time not to operate the line protection device when the next-level line is short-circuited. Therefore, this problem can be solved by adding a time-limited protection device, thus producing a time-limited current quick-break protection.

Still taking Figure 1 as an example, when a fault occurs in the A-B section, the current quick-break protection action of protection 1 first meets the rapidity requirement, and for the remaining unprotected area of ​​the A-B section, the time-limited current quick-break protection device is selected. If you want to protect the entire length of the A-B line, the protection range must be extended to the B-C line. If k2 of the B-C section fails, the current quick-break protection of the B-C section should be activated, and the time-limited current quick-break protection of A-B cannot be activated. This requires that the range of the time-limited current protection of the A-B section cannot exceed the quick-break protection range of the B-C section, and the action time of the A-B section must be higher than that of the B-C section by one time period, namely A1, so that the two protections will not collide. Therefore, the setting principle of the time-limited current quick-break protection is to coordinate with the next-level line coordination section for setting, that is:

Current setting: IEQ * jc3 * hps10 oal(sup 3(.1 = KEQ * jc3 * hps10 oal(sup 3(lIsEQ * jc3 * hps10 oal(sup 3(t.2

Action time limit selection: 1EQ * jc3 * hps10 oal(sup 3(I=1EQ * jc3 * hps10 oal(sup 3(+A1

In the formula, I is the II-stage protection of protection 1 on the A-B section, that is, the setting value of the time-limited current quick-break protection; IEQ * jc3 * hps10 oal(sup 3(t.2 is the I-stage protection of protection 2 on the B-C section, that is, the setting value of the current quick-break protection; KEQ * jc3 * hps10 oal(sup 3(1 is the reliability coordination coefficient, which is generally taken as 1.1~1.2; 1EQ * jc3 * hps10 oal(sup 3(I is the time limit of the upper limit time quick-break of the A-B section; 1EQ * jc3 * hps10 oal(sup 3(is the action time limit of the current quick-break on the B-C section; A1 is the time step value, which is generally between 0.3~0.5, and is generally taken as 0.5s in engineering.

Sensitivity check: Since the protection is to protect the entire length of the line, the system units are only turned on to meet the power demand during the check, that is, the system is running in the minimum mode. In addition, the short circuit type at the end of the line is checked by two-phase short circuit, and the sensitivity coefficient Ksen is used for measurement, that is:

Ksen is generally required to be ≥1.3~1.5.

Advantages and disadvantages of time-limited current quick-break protection: Its advantage is that it can protect the entire length of the line and has high sensitivity; its disadvantage is that it has a delay of about 0.5s and its speed is relatively poor.

3. Time-limited overcurrent protection

When a circuit fails, the protection device needs to have backup protection in addition to the main protection on the faulty line. The first two protection devices are the main protection of the faulty line, and the backup protection of the line is undertaken by the time-limited overcurrent protection, that is, as the near backup protection of the faulty line, and as the far backup protection of the lower-level line adjacent to the faulty line, and the protection action time is constant and will not change with the size of the short-circuit current. Therefore, the actual role of the time-limited overcurrent protection is a backup protection. It will only act when the main protection on each line fails, otherwise it will not act. This requires that the starting current must be greater than the maximum load current IL.max of the line. In addition, when the fault on the outside line is resolved, the voltage must be restored. This requires that under the self-starting current condition, the protection device can return to work again, and its return current should be greater than the load self-starting current, that is:

Where, Ie is the return current of the protection device; KEQ * jc3 * hps10 oal(sup 3(EQ * jc3 * hps10 oal(sup 3(is the reliability coefficient, generally 1.15~1.25; Kss is the self-starting coefficient, the value is greater than 1.

The operation of the protection device and the re-operation after the fault is removed are realized by the current relay. Therefore, the relationship between the start and return of the relay can be used to obtain the relationship of the protection device, and thus the setting value of the current, that is:

Where Kre is the return coefficient of the current relay, which is generally 0.85~0.95.

The setting principle of the time-limited overcurrent protection is to avoid the maximum load current in the line.

The setting of the action time limit is based on the principle that one level is higher than the next, just like a ladder. The action time of the protection close to the power supply should be longer than the action time of the adjacent protection far from the power supply by a time difference At, that is,

Its sensitivity calibration:

Although the time-limited overcurrent protection is mainly used as a backup protection, it can sometimes be used as the main protection of the line. At this time, the current verification when the two phases are short-circuited at the end of the line under the minimum operation mode is used, and Ksen ≥ 1.3 ~ 1.5 is generally required: when used as a backup protection for adjacent lines, the current verification when the two phases are short-circuited at the end of the adjacent line under the minimum operation mode is used, and Ksen ≥ 1.2 is required at this time. In addition, in practice, the sensitivity of the protection is required to cooperate with each other. Simply put, the closer to the fault point, the higher the sensitivity of the protection is required, which is also considered for rapid fault removal.

Advantages and disadvantages of time-limited overcurrent protection: Its advantages are small operating current, higher sensitivity, and the protection range can reach the full length of the line and adjacent lines: Its disadvantage is that the operating time limit is too long. Therefore, an inverse time-limited overcurrent protection is introduced, which will not be introduced here.