Knowledge of overvoltage in power system

There are several types of overvoltage in the power system, including atmospheric overvoltage, power frequency overvoltage, operational overvoltage, and resonant overvoltage.   

Atmospheric overvoltage: Sudden addition of direct lightning strikes or lightning induction to the power system, causing electrical equipment to withstand voltages far exceeding their rated values. Atmospheric overvoltage can be divided into direct lightning overvoltage and induced lightning overvoltage. After the power system is subjected to atmospheric overvoltage, it can cause insulation breakdown or flashover of transmission and distribution lines and electrical equipment, resulting in power outages and endangering human life. The characteristics are short duration, strong impact, directly related to the intensity of lightning strike activity, and independent of equipment voltage level. Therefore, the insulation level of systems below 220KV is often determined by atmospheric overvoltage. For gas overvoltage, measures such as installing lightning rods, lightning conductors, and lightning arresters, reasonably improving the insulation level of the line, and using automatic reclosing devices are usually taken.

Power frequency overvoltage: an overvoltage that occurs in a system during operation or grounding faults, with a frequency equal to or close to the power frequency (50 Hz) and higher than the system operating voltage. The characteristics are long duration, low overvoltage multiple, and generally low risk to equipment insulation. However, it plays an important role in determining the insulation level during high voltage and long-distance transmission. Within a few hundred milliseconds after system operation and grounding tripping, due to the inertia of the generator's automatic voltage regulator, the electromotive force of the generator remains constant. The power frequency overvoltage during this period is called temporary power frequency overvoltage. As time goes on, the automatic voltage regulator of the generator comes into effect, causing a decrease in the electromotive force of the generator. At this time, the power frequency overvoltage is called steady-state power frequency overvoltage. The main reasons for generating power frequency overvoltage are the capacitance effect of long no-load lines, the positive, negative, and zero sequence voltage components caused by asymmetric grounding, and the sudden load shedding of the system causing the generator to accelerate rotation. Special measures should be taken to limit power frequency overvoltage according to specific situations. Common methods include using parallel reactors to compensate for the capacitance effect of no-load long lines, selecting a reasonable system neutral point operation mode, and quickly adjusting the voltage control of the generator.

Operation overvoltage: Overvoltage caused by sudden changes in system parameters due to operation (such as closing and opening of circuit breakers), faults, or other reasons, resulting in the system transitioning from one state to another, and the electromagnetic energy oscillation of the system itself during this transition process. The characteristic is randomness, but under unfavorable conditions, the overvoltage multiplier is relatively high. Reasons for overvoltage operation and avoidance measures

1. The operating overvoltage of the power grid is generally caused by the following reasons

A. Line closing and reclosing; B. Open the no-load transformer and shunt reactor; C. Asymmetric fault tripping and oscillation disconnection of the line; D. Open the circuit without load. The overvoltage caused by line closing and reclosing has a significant impact on the insulation coordination of power grid equipment, and circuit breakers with closing resistors should be used to limit this overvoltage. The lightning arrester can serve as a backup protection device for overvoltage operation of electrical equipment in substations, and it is also a protection device for lightning overvoltage in substations. When designing for Class A and C overvoltage, it should be predicted based on the power grid conditions.

2. Overvoltage during line closing and reclosing operations

When the no-load line is closed, overvoltage will occur due to the oscillation of the line inductance capacitance. When the lines overlap, the high power supply potential and the presence of residual charges on the lines intensify this electromagnetic oscillation process, further increasing the overvoltage. Therefore, circuit breakers should be equipped with closing resistors to effectively reduce closing and reclosing overvoltage. According to the predicted conditions of the power grid, the overvoltage distribution of no-load line closing, single-phase closing, and successful and unsuccessful three-phase closing (such as during operation) should be calculated, and the relative ground and phase to phase statistical operation overvoltage including the line receiving end should be calculated. The conditions for predicting overvoltage in such operations are as follows: A. The no-load line is closed, and before the line circuit breaker is closed, the power bus voltage is higher than the grid voltage; B. Before the successful three-phase reclosing, a single-phase grounding fault occurred at the receiving end of the line; When the three-phase reclosing is unsuccessful, there is a single-phase grounding fault at the receiving end of the line. The relative ground operation overvoltage generated at the receiving end of the no-load line closing, single-phase closing, and successful three-phase closing (if used during operation) should not exceed 2 2UXG.

3. Operation overvoltage of 3-terminal no-load transformer and shunt reactor

The operating overvoltage caused by forced arc extinguishing when the circuit breaker breaks the inductive current of these devices should be determined based on factors such as the circuit breaker structure, circuit parameters, transformer (shunt reactor) wiring and characteristics. The overvoltage of this operation can generally be limited by a lightning arrester installed between the circuit breaker and the transformer (parallel reactor). For transformers, lightning arresters can be installed on the low voltage side or high voltage side, but if the neutral grounding method of the high and low voltage power grids is different, magnetic blow valve type lightning arresters should be used on the low voltage side. When the lightning arrester may frequently operate, it is advisable to use a circuit breaker with a high-value opening resistor.

4. Asymmetric fault tripping and oscillation disconnection operation overvoltage in 4 lines

The weak connection between the transmission and reception ends of the power grid, such as the opening of the circuit due to asymmetric faults or disconnection during grid oscillation, will result in the opening of the circuit due to asymmetric faults or overvoltage caused by oscillation disconnection. To predict the overvoltage caused by asymmetric fault tripping of a power line, the condition of single-phase grounding fault at the receiving end of the line can be selected. The potential power angle difference between the sending and receiving ends of the line during tripping should be selected according to the actual situation. Circuit breakers with opening resistors can reduce the overvoltage caused by asymmetric fault opening and oscillation disconnection in power lines. When this condition is not met, lightning arresters installed on the line should be used to limit it.

5. Open circuit overvoltage for unloaded lines

Attention should be paid to using circuit breakers that do not reignite when opening under the condition of a power to ground voltage of 1.3UXG.

6. Lightning arresters should be installed in substations

Damage to electrical equipment caused by overvoltage during operation. The installation position is as follows: A. On each line entrance side of the outgoing circuit breaker, the lightning arrester installed at that position is called the line lightning arrester; B. The lightning arrester installed at the substation side of the outgoing circuit breaker is called the substation lightning arrester. The specific installation location and quantity of all lightning arresters should be determined in conjunction with 4.4.2. Note: When there is no parallel reactor at the entrance of the line, if predicted (for circuit breaker closing, the condition of one phase failure of the closing resistance needs to be considered)

When the overvoltage at this location is below the level of overvoltage protection for lightning arrester operation, lightning arrester should not be installed here.

7. rated voltage with series gap lightning arrester

It should not be lower than the power grid frequency overvoltage level at the installation point.

8. When using metal oxide surge arresters to limit operating overvoltage

The manufacturer's product manual should be followed to ensure that the long-term operating voltage, power frequency overvoltage, and resonant overvoltage allowable duration meet the requirements of the power grid.

9. Operating overvoltage current capacity of 9 lightning arresters

Allowing energy absorption should meet the requirements of the power grid (considering the condition of one phase failure of the closing resistance when closing the circuit breaker). In addition, it is necessary to verify whether the voltage on the lightning arrester exceeds its specified protection level. When outdated, its impact on insulation coordination should be considered.

10. Monitor the power frequency overvoltage of the operating power grid

It is advisable to install automatic recording devices for overvoltage waveform or amplitude in the substation for resonance overvoltage and operation overvoltage, and collect the measured results properly.

Measures to limit overvoltage during operation include:; Choose high-voltage switches with strong arc extinguishing ability; Improve the synchronicity of switch actions; Install parallel resistors on the switch interface; Using high-performance lightning arresters, such as zinc oxide lightning arresters; Make the neutral point of the power grid directly grounded for operation.

Resonant overvoltage: Some inductance and capacitance components in the power system can form various oscillation circuits when the system is operated or malfunctions. Under certain energy sources, series resonance phenomenon will occur, causing serious overvoltage in some components of the system. There are several types of resonant overvoltage:

(1) The linear resonant overvoltage resonant circuit is composed of inductive elements without iron cores (such as the inductance of transmission lines and leakage inductance of transformers) or inductive elements with iron cores that have excitation characteristics close to linearity (such as arc suppression coils) and capacitive elements in the system.

(2) The ferromagnetic resonance overvoltage resonance circuit is composed of inductive components with iron cores (such as no-load transformers and voltage transformers) and capacitive components of the system. Due to the saturation phenomenon of iron core inductance components, the inductance parameters of the circuit are nonlinear. This type of circuit containing nonlinear inductance components will generate ferromagnetic resonance when certain resonance conditions are met.

(3) Parameter resonance overvoltage is composed of a loop consisting of inductance components with periodic changes in inductance parameters (such as the synchronous reactance of a salient pole generator that varies periodically between Xd and Xq) and system capacitance components (such as no-load lines). When the parameters are matched, energy is continuously transmitted to the resonance system through the periodic changes in inductance, causing parameter resonance overvoltage.

The main measures to limit resonance overvoltage are:

(1) Improving the synchronicity of switch actions is important as many resonant overvoltages are caused under non full phase operating conditions. Therefore, increasing the synchronicity of switch actions and paying attention to non full phase operation can effectively prevent the occurrence of resonant overvoltages.

(2) Installing a small reactance at the neutral point of a parallel high-voltage reactor can block the transmission of power frequency voltage and series resonance during non full phase operation. (3) Disrupting the conditions for the generator to generate self excitation, parameter resonance overvoltage