The series resonance under UHV power can help many power workers conduct various power tests more conveniently.

In the power system, some inductive and capacitive components can form various oscillation circuits during system operation or failure. Under certain energy sources, series resonance phenomena can occur, leading to severe overvoltage in certain components of the system.

classification

(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.

Restrictive measures

(1) Improving the synchronicity of switch actions is important as many resonant overvoltages are caused under non full phase operating conditions. Therefore, improving the synchronicity of switch actions and preventing 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) Destroy the conditions for the generator to generate self excitation and prevent parameter resonance overvoltage.

When the system resonates, under the action of resonance voltage and power frequency voltage, the magnetic density of the PT iron core quickly saturates, and the excitation current increases rapidly, which can cause serious overheating and damage to the PT winding (all PTs in the same system are threatened), and even cause bus faults and large-scale power outages. Therefore, how to quickly eliminate resonance when it occurs is the key to ensuring the safe operation of the equipment.

Characteristics of conditions

In a neutral ungrounded power system, due to the nonlinear excitation characteristics of electromagnetic voltage transformers (TVs), resonant overvoltage occurs when voltage fluctuations cause the reactance in the network to approach the capacitive reactance. Especially when encountering TVs and systems with poor excitation characteristics (prone to saturation) that experience single-phase to ground flashover or grounding, it is more likely to cause resonance overvoltage. Mild cases may cause the fuse of the TV to melt, short circuit between turns, or explode; Serious accidents such as lightning arrester explosions, bus short circuits, and power outages that pose a serious threat to the safe operation of power systems and electrical equipment can occur in severe cases.

Resonance processing

For our current 6kV ungrounded system, the main methods are to activate the arc suppression coil and change the operating parameters. Generally, activating the arc suppression coil can eliminate resonance. For fundamental and high-frequency resonance, as long as the harmonic eliminator operates reliably, resonance can also be eliminated. However, for frequency division resonance, which has zero sequence properties, the harmonic eliminator cannot eliminate resonance. Switching three-phase symmetrical loads does not work. For systems without arc suppression coils, the following methods can be used according to the actual situation:

1. Processing of fundamental or high-frequency resonance:

1) When there is a running capacitor, cut off the running capacitor; When no capacitors are running, put in a set of capacitors;

2) When the above measures cannot eliminate harmonics, cut off all capacitors of the busbar and apply to the dispatch to cut off some feeders, preferably by cutting the long line first.

2. Handling of Frequency Division Resonance:

1) Remove all capacitors from the busbar;

2) When resonance cannot be eliminated, apply to the dispatch to cut off the line on the busbar until resonance is eliminated;

3) If resonance cannot be eliminated even after all lines are cut off, apply to the dispatcher to cut off the transformer switch and cut off the power to the busbar;

4) Restore power supply to the busbar and lines.