In the power system, due to the complexity of the system architecture and the flexibility of the operation mode, it is easy to cause the randomness of the operation parameters and promote the change of the system parameters, resulting in the power grid with inductive and capacitive elements, the oscillation transformation or transmission of electromagnetic energy and the overvoltage of the power grid. The phenomenon of ferroresonance overvoltage has obvious external characteristics, which directly threatens the safe operation of power system. In serious cases, it will cause the explosion of voltage transformer (PT) and cause accidents. Effective preventive measures must be taken to ensure system and personal safety.

1、 Inducement of ferromagnetic resonance and overvoltage

The complex structure of power system, flexible operation mode and random operation parameters promote the change of system parameters. Under the stimulation of system operation or failure, the inductive and capacitive elements in the system form various oscillation circuits, especially the electrical equipment with iron core and winding such as transformer and transformer. Under certain excitation, it is easy to produce series parallel resonance phenomenon due to electromagnetic coupling, resulting in serious ferromagnetic resonance overvoltage. For example, the 35kV Shibali substation of our Bureau failed. According to the operation duty record of the day, the voltage at 10kV side was 16.8kv, resulting in the burning of 35kvpt. According to statistics, ferroresonance overvoltage in 35kV system accounts for 55% of the total faults; Ferroresonance overvoltage in 10kV system accounts for 50% of the total faults. Most of the 35kV and below power grids operate in the neutral ungrounded mode. Due to the relatively weak structure of the 35kV and below power grids, the probability of failure is high due to the influence of geography, environment, climate, insulation aging and other factors. In addition, the system operation mode is changeable and the operation is frequent, which leads to the phenomenon of ferromagnetic resonance overvoltage of the system from time to time.

1. The cause analysis of ferromagnetic resonance overvoltage of induced generation network is as follows:

(1) After the fault occurs, the strong saturated current flows through the electrical equipment with iron core and winding. Due to the transient electromagnetic coupling, ferromagnetic resonance is generated and ferromagnetic resonance overvoltage is formed.

(2) The system has various operation modes. Under different operation modes, the impedance, inductive reactance and capacitive reactance of the system also change. Under specific conditions, it will cause system resonance, and then transition to ferromagnetic resonance to form ferromagnetic resonance overvoltage.

(3) In the neutral ungrounded system, stimulated by the operation or system fault, it will cause the electromagnetic coupling of the voltage transformer, make the iron core quickly saturated and produce ferromagnetic resonance, force the neutral point of the system to shift, and lead to the phenomenon of system overvoltage is particularly prominent.

2. Analysis of external characteristics of equipment damage caused by ferromagnetic resonance.

Ferromagnetic resonance leads to equipment damage. For the equipment, there are three obvious external characteristics:

(1) Before the equipment is damaged, various test data are normal and there is no abnormal performance in operation, which is not easy to arouse people's vigilance.

(2) After the equipment is damaged, the electrical equipment with iron core and winding will burn down the winding due to strong transient coupling excitation current, resulting in equipment damage.

(3) Ferromagnetic resonance causes overvoltage and damages electrical equipment, which is obviously different from the system fault or accident caused by equipment short circuit. In case of overvoltage caused by ferromagnetic resonance, the system will fluctuate up and down with different amplitudes, and the meter shall respond; When the equipment is short circuited, the voltage at the short-circuit point approaches zero and the short-circuit current approaches infinity. Of course, ferromagnetic resonance in the system will also transition to short-circuit accident.

2、 Harm of ferromagnetic resonance

(1) Ferromagnetic resonance overvoltage will rapidly saturate the iron core in the electrical equipment with iron core, resulting in the rapid increase of the excitation current of the winding. In case of serious, it can reach more than 100 times of the rated excitation current. As a result, the fuse of the voltage transformer is blown, the oil is injected, the winding is burned or even exploded.

(2) In some specific cases, ferromagnetic resonance overvoltage may be very high (about 3 times of phase voltage), which will break down the insulation of electrical equipment and damage these equipment.

(3) Ferromagnetic resonance overvoltage will cause the porcelain skirt surface of dirty electrical equipment (such as voltage transformer, current transformer, lightning arrester, insulator, etc.) to flash over and explode, and even form a short circuit.

(4) When ferromagnetic resonance overvoltage occurs, there may be no grounding point in the power grid, but there will be a false grounding phenomenon in which the voltage decreases in one phase and increases in two phases, resulting in illusion and misjudgment by the operators on duty.

(5) When ferromagnetic resonance overvoltage occurs, if the grounding resistance of the grounding grid such as work and protection is unqualified, this overvoltage can also jump into the grounding grid through the grounding lead of the equipment to increase the grounding voltage, thus endangering the personal safety of on-site personnel.

3、 Method for eliminating ferromagnetic resonance

Due to different power grid parameters and external excitation conditions, ferromagnetic resonance overvoltage may be caused. Ferromagnetic resonance overvoltage will damage the equipment, lead to the occurrence and expansion of accidents, even cause the collapse of the system and endanger personal safety. Its impact and harm are great. Measures to limit ferromagnetic resonance overvoltage must be taken.

There are several common harmonic elimination methods.

1. Change electrical parameters

(1) Installation of relay protection equipment

When a single-phase grounding fault occurs in the power grid, in order to change the resonance parameters of the voltage transformer, it can be realized by installing a set of relay protection equipment. The device uses the large resonance current generated by single-phase grounding to start the current relay and short-circuit the winding at the triangular opening on the secondary side of the voltage transformer. After the fault is eliminated, the protection device returns to the original state and the voltage transformer returns to normal operation.

(2) Select three-phase or five column voltage transformer that is not easy to be saturated

For voltage transformers used in 10kV system, voltage transformers with excitation reactance greater than 1.5 m Ω shall be selected.

(3) Reduce the number of voltage transformers

In the same power grid, the number of voltage transformers should be reduced as much as possible, especially the number of neutral grounding voltage transformers should be limited. For example, the substation is only used as a voltage transformer for measuring instrument and protection, and the neutral point is not allowed to be grounded.

(4) Series connected single-phase transformer

The series connection of single-phase transformer at the primary neutral point of three-phase voltage transformer can significantly increase the equivalent reactance of three-phase voltage transformer and avoid resonance caused by deep saturation.

(5) Each phase is equipped with a capacitor

This method can make the network equivalent capacitance smaller, and the network equivalent reactance can not match it, so as to eliminate resonance.

(6) Install arc suppression coil at neutral point

When resonance occurs in 10kV system and the single-phase grounding current is large or close to 30 A, the neutral point can be grounded through arc suppression coil.

2. Consumption of resonant energy

(1) Parallel damping resistor on the triangular side of Pt opening.

This measure plays a role in changing the parameters of voltage transformer. It can not only prevent magnetic saturation of voltage transformer, but also effectively consume resonance energy and prevent resonance overvoltage. However, this method is often used in substations with low requirements. The reason is that the harmonic elimination resistance mostly adopts electric bulbs or resistance wires. When it is damaged, it will not have harmonic elimination effect; Moreover, when the system is single-phase grounded, a voltage of 100 V will be generated at the open triangle side. Because the cold resistance of the electric bulb or resistance wire is small, it is easy to flow a large current at the open triangle side of Pt, causing Pt damage.

(2) The harmonic elimination resistance is connected in series between the neutral point at the primary side of the voltage transformer and the ground

This resistor can be used to weaken or eliminate the high-order harmonics that cause the resonance of the system. The simulation test shows that the frequency division ferromagnetic resonance will not be excited even if the system has a single-phase grounding fault. However, if the resistance value is too large, it will affect the sensitivity of system grounding protection.

(3) Installation of harmonic elimination device

A harmonic elimination device can be directly installed at the open triangular winding of the voltage transformer. When resonance occurs and the voltage reaches the action value under the design cycle, the frequency discrimination system of the device will automatically input the "harmonic elimination resistance" to absorb the resonance energy and eliminate the ferromagnetic resonance. The harmonic elimination device operates reliably and can also record parameters such as voltage and oscillation frequency in case of fault, which is conducive to accident analysis. Now this method is widely used.

4、Summary

Ferromagnetic resonance in 10 ~ 35 kV power grid is one of the reasons that lead to the burning loss of voltage transformer, power failure and endanger the safe power supply; Changing power system parameters, destroying resonance conditions, and absorbing and consuming resonance energy to suppress resonance can effectively eliminate ferromagnetic resonance, eliminate the unsafe impact of ferromagnetic resonance on the power grid, and ensure the reliable and economic operation of the power grid. After studying the causes and elimination methods of ferromagnetic resonance, 15 35kV substations in our bureau have adopted the method of directly installing harmonic elimination devices at the PT opening triangle. So far, no accident caused by ferromagnetic resonance has occurred, which ensures the safe and reliable operation of the substation.