The 35 kV system in the power system is ungrounded or grounded through the arc suppression coil. The abnormal voltage of 35 kV system is very common, and there are many reasons. How to accurately judge and deal with it is very important for the corresponding dispatching and operation department.

1. Causes and manifestations of abnormal voltage in 35 kV system

The 35 kV system voltage anomalies can be summarized into the following 7 types:

(1) The high-voltage fuse is blown. When the one phase, two phase or three-phase high-voltage fuse is blown, the secondary voltage of the fused phase will be significantly reduced and a "bus grounding" signal will be sent. When the fuse is not completely blown, the "bus grounding" signal may not be issued.

(2) Single phase grounding. When single-phase grounding occurs, the voltage of the grounding phase is close to 0, and the voltage of the other two phases rises to line voltage, and a "bus grounding" signal is sent (the voltage is taken from the open triangle voltage 3 U0).

(3) Resonance. If the three-phase voltage rises abnormally, the meter may reach the full scale, and the three-phase voltage is basically balanced. Generally, the "bus grounding" signal will not be sent. Busbar voltage transformer will make buzzing sound. The theoretical calculation shows that the overvoltage generally does not exceed 1.5~2 times of the phase voltage, and in some cases, it is as high as 3.5 times. It lasts for a few tenths of a second until it exists.

(4) The low-voltage fuse is blown. The secondary voltage will be significantly reduced and will not give a "bus ground" signal.

(5) The secondary voltage circuit is abnormal. In particular, it refers to the abnormality of bus voltage transformer and the following circuits. When this happens, the voltage cannot be predicted. The causes are usually secondary small wire burning, wire collision, wrong circuit connection, meter abnormality, etc.

(6) The gear of arc suppression coil is inappropriate. Some 110 kV substations are equipped with 35 kV neutral point arc suppression coil. When the gear position is not suitable (usually abnormal after gear adjustment), the three-phase voltage is unbalanced, but the difference is small, and the grounding signal may be sent. At this time, the voltage of relevant substations may be inconsistent.

(7) Line phase failure. It can be divided into one phase fusing and two phase fusing. It is difficult to distinguish the abnormal bus voltage of the substation on the load side. The probability of occurrence in actual operation is small.

The above seven conditions are the characteristics of voltage anomaly caused by a single cause, which can be used as the basis for judgment and processing. The sixth type only exists in the substation grounded by arc suppression coil, which is easy to judge and handle. The treatment of the seventh case is the same as that of single-phase grounding. Therefore, the following analysis focuses on the first five reasons.

As each operator, especially the dispatcher, should grasp these characteristics to accurately judge and quickly handle various abnormalities that may occur during operation. It is relatively easy to judge a single feature, and it is more complex to judge and handle the voltage anomaly caused by two or more complex faults. For example, single phase grounding or resonance is often accompanied by the fusing of high-voltage fuse and low-voltage fuse. When the high-voltage fuse is not completely fused, whether the grounding signal is sent depends on the setting value of the secondary voltage of the grounding signal and the fusing degree of the fuse. From the actual operation, when the voltage is abnormal, the secondary circuit is often abnormal. At this time, whether the voltage and the grounding signal are sent out is of little reference value. It is particularly important to find the troubleshooting rules for handling voltage anomalies.

2 Judgment and analysis method

Any abnormality in the circuit above the secondary voltmeter may lead to abnormal measured voltage. There are two types of voltage anomalies.

(1) Abnormal measuring circuit: the voltage below the knife switch of 35 kV bus voltage transformer is abnormal, and the primary voltage is actually normal. Usually, the most likely events are: high-voltage fuse, low-voltage fuse fusing, and abnormal secondary circuit.

(2) Abnormal bus voltage: the voltage above the knife switch of 35 kV bus voltage transformer is abnormal. It can be divided into resonance, single-phase grounding, phase failure and improper gear of arc suppression coil. It is also not ruled out that there are abnormal measuring circuits at the same time.

When the operator on duty reports that the 35 kV bus voltage is abnormal, there may actually be other signals, such as AC circuit disconnection, protection device abnormality, etc. First, check whether the equipment in the substation is abnormal, and then further understand:

(1) A, B, C three-phase phase voltage.

(2) Whether the bus grounding signal is sent.

(3) Understand the voltage of relevant substations (this article refers to substations with common 35 kV power supply, such as the substation on the opposite side of 35 kV outgoing line, the same below), and whether the voltage of another section of the 35 kV bus is abnormal when it operates in parallel.

The above three points can be called "three elements". Among them, the third one is used to judge whether the bus voltage is abnormal or the measuring circuit is abnormal. If the voltage of the relevant substation is normal, the measuring circuit is abnormal; Otherwise, it means that the bus voltage is abnormal.

3 Treatment method

3.1 Handling of abnormal measurement circuit

It is relatively simple to handle the abnormality of the measuring circuit. Just replace the low-voltage fuse and observe whether the voltage is normal. If it is still abnormal, the bus voltage transformer can be changed to maintenance and the high-voltage fuse can be replaced. If the voltage is still abnormal after the high-voltage fuse is replaced, it is determined that the secondary voltage circuit is abnormal.

3.2 Handling of abnormal bus voltage

The resonance and single-phase grounding shall be eliminated first (they will not occur at the same time), and then other abnormalities shall be handled. According to the "three elements", determine whether it is single-phase grounding or resonance. If you are not sure, you can follow the steps below:

3.2.1 Eliminate abnormal bus voltage

It can be realized by splitting or paralleling the power grid, usually by pulling (or closing) the 35 kV bus switch, which is a very practical method. The cause of abnormal voltage can be quickly "surfaced". If there is resonance, the resonance will disappear. According to the change of voltage, single-phase grounding or high-voltage fuse fusing can also be distinguished. This narrows the search scope. The following two cases are described:

(1) When the 35 kV bus is normally operated in separate trains (i.e. hot standby of the 35 kV bus tap changer), the 35 kV bus tap changer can be closed. The following analysis is made according to the voltage of this section of bus:

1) If the voltage drops to normal, the resonance disappears;

2) When the voltage drops below the normal voltage, it indicates that the resonance disappears and the fuse may be blown at the same time;

3) The abnormal voltage "harms" another bus (rises), indicating that there is single-phase grounding;

4) The voltage is basically unchanged, indicating that the high-voltage fuse or low-voltage fuse is blown.

(2) When the 35 kV bus normally operates in parallel (that is, the 35 kV bus branch switch operates), the 35 kV bus branch switch can be opened and the bus can be treated in sections. At this time, the resonance can be eliminated, and the low-voltage fuse can be checked to see if it is in good condition. Then, according to the voltage of the relevant substation, it is easy to distinguish whether there is single-phase grounding and which bus section is grounded, and eliminate it according to the single-phase grounding treatment method.

3.2.2 Eliminate the abnormality of measuring circuit

If the above methods can not restore to normal, replace the high-voltage fuse. If the voltage is still abnormal, it is determined that the secondary voltage circuit is abnormal.

The above methods are applicable to substations with two main transformers. If there is only one main transformer, the same purpose can be achieved by closing 35 kV tie lines. 10 kV system can also be solved by reference. The above steps can be shown in Figure 2.

From the above analysis, it can be seen that the processing sequence can be taken as follows: resonance, low-voltage fuse fusing, single-phase grounding, high-voltage fuse fusing, and secondary circuit abnormality.

The abnormal voltage may also be caused by special circumstances, such as poor contact of the bus voltage transformer. There may also be several reasons mixed together, but they can also be found according to the above ideas. If the cause of the abnormality is still unclear, the abnormal part shall be removed from operation and handed over to the maintenance personnel for handling. As a dispatcher and operator, judge that the cause of the abnormality is the bus voltage transformer and the following circuits, and restore the system voltage to normal.

4 Judgment example of 35 kV voltage display abnormality

Example 1 A 110 kV substation (normal parallel operation).

Phenomenon: voltage of section I Ua=0 kV, Ub=37 kV, Uc=34 kV; Section II voltage Ua=28kV Ub=37kVUc=22kV "Section I, section II bus grounding".

Treatment: after opening the 35 kV bus switch, the voltage is displayed as:

Section I voltage Ua=22kV, Ub=22kV, Uc=21kV; Section II voltage Ua=28kV Ub=27kV Uc=21kV "section II bus grounding".

It indicates that section I is normal and the grounding is in section II. The inspection found that the low-voltage fuse of phase A and phase C of 35kV section II bus voltage transformer was blown. After replacing the low-voltage fuse, the voltage of section II Ua=3kV, Ub=36kV, Uc=33kV, indicating that phase A was grounded. It is found on the test pull section II that the grounding is on the Lanzhou Huangshan 3526 line.

Phenomenon: section II voltage Ua=25 kV, Ub=27 kV, Uc=13 kV bus is grounded, and the voltage of outgoing substation is normal.

Treatment: judging from the normal voltage of the relevant substation, it should be that the high-voltage fuse of section II voltage transformer is blown, but the voltage rise is abnormal.

In case of closing the 35 kV bus tap changer, the voltage value remains unchanged, and single-phase grounding and resonance can be completely eliminated. Check whether the low-voltage fuse is in good condition. After the high-voltage fuse of section II voltage transformer is replaced, the voltage will not change. It is only possible that the secondary circuit is abnormal, and it is found through inspection that the secondary small wire has been fused.

The final judgment result is that the B phase high-voltage fuse of section II voltage transformer is blown (the watchman replaced the fuse that was still blown at that time), and the secondary circuit is abnormal.