Problems that are easily neglected in 10kV relay protection and their countermeasures

1. Excitation surge current problem in the line

(1) Impact of excitation surge current in the line on relay protection devices

The excitation inrush current is unique to transformers. It is caused by the fact that when the transformer is dropped, the magnetic flux in the transformer core cannot change suddenly, and a non-periodic component magnetic flux appears, which saturates the transformer core and causes a sharp increase in the excitation current. The maximum value of the transformer excitation inrush current can reach 6 to 8 times the rated current of the transformer, and it is related to the capacity of the transformer. The smaller the transformer capacity, the greater the excitation inrush current multiple. The excitation inrush current has a large non-periodic component and decays with a certain time coefficient. The decay time constant is also related to the transformer capacity. The larger the capacity, the larger the time constant, and the longer the inrush current exists.

10kV lines are equipped with a large number of distribution transformers. When the line is put into operation, these distribution transformers are hung on the line. At the moment of closing, the excitation surge currents generated by each transformer are superimposed on each other and reflected back and forth on the line, resulting in a complex electromagnetic transient process. When the system impedance is small, a large surge current will appear, and the time constant is also large. The current quick-break protection in the two-stage current protection often has a small action current value due to the sensitivity, especially in long lines or when the system impedance is large. The excitation surge current value may be greater than the device setting value, causing the protection to malfunction. This situation is not prominent when the number of line transformers is small, the capacity is small, and the system impedance is large, so it is easy to be ignored, but when the number and capacity of line transformers increase, it may occur. Our bureau once encountered the problem that the 10kV line could not be put into normal operation due to the surge current after the substation capacity was increased.

(2) Methods to prevent false operation caused by inrush current

An obvious characteristic of the excitation inrush current is that it contains a large amount of secondary harmonics. This characteristic is used in the main transformer main protection to prevent the excitation inrush current from causing the protection to malfunction. However, if it is used in 10kV line protection, the protection device must be modified, which will greatly increase the complexity of the device, so the practicality is very poor. Another characteristic of the excitation inrush current is that its size decays over time. At the beginning, the inrush current is very large, and after a period of time, the inrush current decays to zero. The current flowing through the protection device is the line load current. By using this characteristic of the inrush current, a short time delay is added to the current quick-break protection to prevent the false operation caused by the excitation inrush current. The biggest advantage of this method is that there is no need to modify the protection device (or only a simple modification). Although it will increase the fault time, it is still applicable to places such as 10kV that have little impact on the stable operation of the system. In order to ensure reliable avoidance of the excitation inrush current, the acceleration circuit in the protection device should also add a delay. After several years of exploration, our bureau has added a time limit of 0.15 to 0.2s to the 10kV line current quick-break protection and acceleration circuit. Judging from the operation in recent years, it has been safe and can effectively avoid false operation of protection devices due to excitation surge current in the line.

2. TA saturation problem

(1) Impact of TA saturation on protection

The short-circuit current at the exit of a 10kV line is generally small, especially for substations in rural power grids, which are often far away from the power supply and have a large system impedance. For the same line, the magnitude of the short-circuit current at the exit will vary with the scale of the system and the mode of operation. As the scale of the system continues to expand, the short-circuit current of the 10kV system will increase, and can reach hundreds of times the primary rated current of the TA. Some TAs with small transformation ratios that can operate normally in the system may be saturated; on the other hand, short-circuit faults are a transient process, and the short-circuit current contains a large number of non-periodic components, which further accelerates the saturation of the TA. When a 10kV line is short-circuited, due to the saturation of the TA, the current induced on the secondary side will be very small or close to zero, causing the protection device to refuse to operate. The fault must be removed by the busbar circuit breaker or the main transformer backup protection, which not only prolongs the fault time, expands the fault range, affects the power supply reliability, but also seriously threatens the safety of the operating equipment.

(2) Methods to avoid TA saturation

TA saturation actually means the saturation of magnetic flux in the TA core, and the magnetic flux density is proportional to the induced potential. Therefore, if the secondary load impedance of the TA is large, the induced potential of the secondary circuit will be large under the same current conditions, or under the same load impedance, the larger the secondary current, the larger the induced potential. Both of these situations will increase the magnetic flux density in the core. When the magnetic flux density reaches a certain value, the TA will be saturated. When the TA is severely saturated, the primary current will all become the excitation current, the secondary side induced current will be zero, the current flowing through the current relay will be zero, and the protection device will refuse to operate.

There are two main ways to avoid TA saturation. First, when selecting TA, the transformation ratio cannot be too small. The TA saturation problem when the line is short-circuited should be considered. Generally, the TA transformation ratio of 10kV line protection is better than 300/5. On the other hand, the secondary load impedance of TA should be reduced as much as possible, and the protection and measurement of TA should be avoided as much as possible. The length of TA secondary cable should be shortened and the cross-section of secondary cable should be increased. For comprehensive automation substations, 10kV lines should use products that integrate protection, measurement and control as much as possible, and install them on the control panel. This can effectively reduce the secondary circuit impedance and prevent TA saturation.

3. Transformer protection used

(1) Problems with the transformer protection used

The transformer is a special device with small capacity but very high reliability requirements. The installation location is also very special. It is usually connected to the 10kV busbar. The short-circuit current on the high-voltage side is equal to the system short-circuit current, which can reach more than ten kA. The short-circuit current at the low-voltage side outlet is also large. We have always paid insufficient attention to the reliability of transformer protection, which will pose a great threat to the safe operation of the transformer and even the entire 10kV system.

Traditional transformer protection uses fuse protection, which is relatively safe and reliable. However, with the increase of system short-circuit capacity and the requirements of integrated automation, this method has gradually failed to meet the requirements. Now, most of the newly built or renovated substations, especially the integrated automation substations, are equipped with transformer switch cabinets, and the protection configuration is similar to that of 10kV lines, but people often ignore the saturation problem of the TA used for protection. Due to the small capacity of the transformer, the primary rated current is very small, and the protection metering often shares the TA. In order to ensure the accuracy of the metering, the TA ratio will be selected very small during the design, and some places even choose 10/5. In this way, when the transformer fails, the TA will be severely saturated, and the induction secondary circuit current is almost zero, causing the transformer protection device to refuse to operate. If it is a high-voltage side fault, the short-circuit current is enough to cause the busbar protection or the main transformer backup protection to operate and disconnect the fault. If it is a low-voltage side fault, the short-circuit current may not reach the starting value of the busbar protection or the main transformer backup protection, so that the fault cannot be removed in time, and eventually the transformer is burned, seriously affecting the safe operation of the substation.

(2) Solution

To solve the problem of transformer protection refusal to operate, we should start with the reasonable configuration of protection. The selection of TA should take into account the saturation problem of the transformer failure. At the same time, the TA used for measurement must be separated from the TA used for protection. The TA used for protection is installed on the high-voltage side to ensure the protection of the transformer, and the TA used for measurement is installed on the low-voltage side of the transformer to improve the measurement accuracy. In terms of setting the fixed value, the current quick-break protection can be set according to the short circuit of the low-voltage outlet of the transformer, and the overload protection can be set according to the capacity of the transformer.