Realization of Resonance Elimination in Power System

Overvoltage is common in power systems. The main causes of overvoltage in power grids include resonant overvoltage, operating overvoltage, lightning overvoltage, sudden changes in system operation mode, and system overvoltage caused by severe load fluctuations. Among them, resonant overvoltage occurs frequently and is very harmful.

Once overvoltage occurs, it often causes damage to the system's electrical equipment and large-scale power outages. According to the records of power production and operation over the years and accident analysis, most overvoltage accidents in medium and low voltage power grids are caused by resonance. In daily work, it is found that in special weather such as windy, rainy, etc., the frequency of intermittent grounding of substation 35kV and below systems is high. When grounding makes the system parameters meet the resonance conditions, resonance will occur.

At the same time, resonance overvoltage is generated. Resonance can cause destructive consequences to the power system: resonance causes a large amount of additional harmonic losses in the components of the power grid, reduces the efficiency of power generation, transmission and power consumption equipment, affects the normal operation of various electrical equipment; causes relay protection and automatic devices to malfunction, and makes electrical measuring instruments inaccurate; it can interfere with adjacent communication systems, generate noise, reduce communication quality, and even make the communication system unable to work normally.

Resonance and Ferroresonance

Resonance is a steady-state phenomenon. Therefore, the resonant overvoltage in the power system will not only be generated during the transition process during operation or accidents, but may also remain stable for a long time after the transition process ends, until a new operating resonance condition is destroyed. Therefore, the duration of the resonant overvoltage is much longer than the operating overvoltage. Once this overvoltage occurs, it often causes serious consequences. Operation experience shows that resonant overvoltage can be generated in networks of various voltage levels, especially in power grids of 35kV and below. There are many accidents caused by resonance, which has become a common concern in the system.

Therefore, it is necessary to make necessary calculations and arrangements in advance during design, or take certain additional measures (such as installing damping resistors, etc.) to avoid the formation of unfavorable resonant circuits, and to prevent the generation of resonances through reasonable operations in daily work, reduce the amplitude of resonant overvoltages, and eliminate resonances in a timely manner. In the case of 6~35kV system operation or failure, the system oscillation circuit often excites a continuous high-amplitude ferromagnetic resonance overvoltage due to the magnetic circuit saturation of the iron core inductance of transformers, voltage transformers, arc suppression coils, etc.

Ferromagnetic resonance can be fundamental resonance, high-order harmonic resonance, and sub-harmonic resonance. Their common characteristics are that the system voltage increases, causing insulation flashover or lightning arrester explosion; or generating high-value zero-sequence voltage components, resulting in phantom grounding and incorrect grounding indication; or overcurrent in the PT, causing the fuse to blow or the transformer to burn out; the open delta winding of the bus PT has a higher voltage, causing the bus insulation monitoring signal to operate. The different characteristics of each sub-harmonic resonance are mainly: the three-phase voltage of the sub-harmonic resonance increases in turn, exceeding the line voltage, generally not exceeding 2 times the phase voltage, and the pointer of the three-phase voltmeter swings at a low frequency in the same range.

When the fundamental wave resonates, the two-phase voltage increases and exceeds the line voltage, but generally does not exceed 3 times the phase voltage, and one-phase voltage decreases but is not equal to zero.

When high-order harmonics resonate, the three-phase voltages increase at the same time or one of the phases increases significantly, exceeding the line voltage but not exceeding 3 to 3.5 times the phase voltage.

Resonance accident solution

When the PT is working normally, the core magnetic flux density is not high and is not saturated; but if the switch is suddenly closed or opened when the voltage passes through zero, or the single-phase grounding disappears, the core magnetic flux will reach several times the steady state and be in a saturated state. At this time, the excitation current of one or two phases increases significantly. When the inductive reactance and capacitive reactance parameters match properly (meet the resonance condition), resonance will occur, that is, ferromagnetic resonance. When resonance occurs, an overvoltage of 2 to 3.5 times the rated voltage and an overcurrent of several dozen times the rated current will be generated at both ends of the inductor and capacitor. The current passing through the PT is much larger than the excitation current, which will burn out the PT and other equipment in severe cases.

General measures to prevent resonant overvoltage

Improve the synchronization of circuit breaker action. Since many resonant overvoltages are caused by non-full-phase operation, improving the synchronization of circuit breaker action and preventing non-full-phase operation can effectively prevent the occurrence of resonant overvoltage.

A small reactor is installed at the neutral point of the parallel high-voltage reactor. This measure can block the power frequency voltage transmission and series resonance during non-full-phase operation.

Destroy the conditions for the generator to produce self-excitation and prevent parameter resonance overvoltage.

Specific measures to prevent resonant overvoltage

The neutral point of the 35kV system is grounded through an arc suppression coil (with an anti-harmonic resistor installed) and operates in over-compensation mode. Its voltage acts on the zero-sequence circuit.

Try to reduce the number of PTs running in parallel in 6~35kV systems.

For any substation with 6~35kV busbar segmentation, if the busbar is often not operated in segments, a group of PTs should be withdrawn as backup; the primary neutral point of the power customer's 6~10kVPT is always ungrounded. ③ Replace 6~35kVPTs with poor volt-ampere characteristics.

6~35kV: A damping resistor is connected in series with the neutral point of the primary side or a damping resistor or a vibration absorber is connected in parallel with the open triangle winding on the secondary side.

A set of Y-connected capacitors with grounded neutral point is installed on the 6~10kV busbar.

Connect a single-phase PT in series at the neutral point on the high-voltage side of the 10kVPT. In actual work, the occurrence of resonance is often accompanied by a grounding fault, and in many cases it is even caused by grounding. The effective method to eliminate resonance is often to change the system operation mode to change the system parameters and destroy the resonance condition. Changing the system operation mode is often achieved through the following ways:

Throw in and out capacitors.

Add investment lines.

If the substation has more than one main transformer, the transformers that were originally running in parallel (separately) can be separated (in parallel) depending on the specific operating conditions.

The busbars are disconnected.

If the above methods cannot eliminate vibration, the line selection method should be adopted to find the line single-phase grounding fault. After the fault line is selected, it should be cut off immediately. The line selection principle refers to the system single-phase grounding fault processing method. This method is the most effective and can solve the problem, but it is often not necessarily able to accurately and timely determine the grounding line, resulting in delays in vibration elimination time. Therefore, in order to eliminate resonance in time during work, the above four methods are generally considered first.

Summarize

Aiming at a resonance accident in a 110kV substation, the author analyzed the cause of the accident by using the resonance principle and knowledge, and proposed a variety of control measures and methods for the resonance overvoltage in combination with actual work experience, so as to facilitate reference and application in specific work, effectively improve the system operation stability, and improve the power supply safety and reliability.