Electric encyclopedia: Power system over-voltage knowledge

Over-voltage power system is divided into the following types: atmospheric over-voltage, power over-voltage, over-voltage operation, resonant over-voltage.

Atmospheric overvoltage: Sudden lightning or lightning induction suddenly added to the power system, so that the voltage withstand electrical equipment far exceeds its rating. Atmospheric over-voltage can be divided into direct lightning over-voltage and induced lightning over-voltage. When the power system suffers from atmospheric over-voltage, the breakdown or flashover of the insulation of the transmission and distribution lines and the electrical equipment may occur, resulting in the power failure so as to endanger the life and safety of human beings. Features are short duration, high impact, and intensity of lightning activity has a direct relationship with the device voltage level has nothing to do. Therefore, the insulation level of the system below 220KV is often decided by preventing atmospheric overvoltage. To prevent atmospheric over-voltage, usually taken to install lightning rods, lightning lines, surge arresters, a reasonable increase in line insulation levels, the use of automatic reclosing devices and other measures.

Power frequency overvoltage: The frequency at which the system will operate or cause a ground fault to be equal to the mains frequency (50 Hz) or to an overvoltage above the system maximum operating voltage close to the mains frequency. Characteristics are long duration, over voltage multiple is not high, the general risk of insulation of the equipment is not large, but in the ultra-high voltage, long distance transmission to determine the level of insulation play an important role when the system operation, grounding hundreds of milliseconds after the trip, As the generator in the chain of mutations can not be sudden, the generator automatic voltage regulator inertia, so that the generator electromotive force remains unchanged during this period of power over-voltage known as temporary power over-voltage. With the increase of time, the generator automatic voltage regulator produces the effect, causes the generator electromotive force to drop and tends to be stable, at this moment the power frequency over-voltage is called the steady-state power frequency over-voltage. The main causes of over-voltage caused by industrial frequency are: no-load long-line capacitance effect, asymmetrical grounding caused by the positive sequence, negative sequence and zero sequence voltage components, the system suddenly load to accelerate the rotation of the generator. Limit frequency over-voltage should take specific measures for the specific circumstances, commonly used methods are: the use of shunt reactor compensation for no-load long-term capacitive effect, select a reasonable neutral point of operation, the generator for rapid voltage adjustment control Wait.

Operation over voltage: due to the operation (such as circuit breaker closing and opening), failure or other reasons, the sudden changes in system parameters, the system from one state to another state transition in the transition of the system itself, electromagnetic Overvoltage that can oscillate. Is characterized by randomness, but the most adverse case of over-voltage multiple higher. Operation over voltage causes and avoidance measures

1 grid over-voltage operation is generally caused by the following reasons

A. Line closing and reclosing; B. No-load transformers and shunt reactors open; C. Line asymmetrical fault opening and oscillating out; D. No-load line sub-gate.

Circuit closing and reclosing overvoltages have an important influence on the insulation coordination of grid equipment. Circuit breakers with closing resistors should be used to limit this overvoltage. Lightning arrester can be used as substation electrical equipment over-voltage protection of the backup device, which is also the surge arrester lightning over-voltage protection device. A, C design over-voltage, should be combined with the grid conditions to be predicted.

2 line closing and reclosing operation over voltage

No-load line closing, due to the line inductance - capacitance oscillation will produce over-voltage switch. Line coincidence, due to higher power supply potential and the presence of residual charge on the line, exacerbating this electromagnetic oscillation process, the over-voltage to further improve. Therefore, the circuit breaker should be installed closing resistance, in order to effectively reduce the closing and reclosing over-voltage. According to the grid forecasting conditions, find the overvoltage distribution of the no-load line closing, single-phase reclosing and the successful and unsuccessful three-phase reclosing (when in operation) Statistics over-voltage operation. The conditions for predicting such operation overvoltage are as follows:

A. no-load line closing, line circuit breaker closing before the power bus voltage is the highest voltage grid;

B. Before a successful three-phase reclosing, a single-phase ground fault has occurred at the receiving end of the line; when the unsuccessful three-phase reclosing occurs, the line has a single-phase ground fault at the receiving end.

No-load line closing, single-phase reclosing and successful three-phase reclosing (as used in operation), the relative statistics of the operation of the line over-voltage, should not exceed 2 2UXG.

3 Breaking the over-voltage of the no-load transformer and shunt reactor

The operation overvoltage caused by forced arc extinguishing when the circuit breaker breaks the inductive current of these devices should be determined according to the structure of the circuit breaker, the circuit parameters, the wiring and characteristics of the transformer (shunt reactor) and other factors. This overvoltage can be limited by arresters installed between circuit breakers and transformers (shunt reactors). On the transformer, arrester can be installed in the low-pressure side or high-pressure side, but if the high and low voltage power grid neutral grounding method is different, low-pressure side should use magnetic blow valve arrester. When the arrester may be frequent movements, it is appropriate to use circuit breaker with high opening resistance.