The Working Principle and Difference Between Arc Suppression and Harmonic Elimination

The working principles of arc suppression and harmonic elimination are different. Arc suppression means that when the single-phase metal grounding of the bus occurs, the arc suppression device operates to make the metal ground directly ground through the vacuum contactor operated by the arc suppression device, which is beneficial to the bus protection action and avoids the generation of harmonics. Harmonic elimination is mainly to eliminate the second harmonic and higher harmonic, which is conducive to the safe operation of the power grid. During normal operation, no current flows through the arc suppression coil. When the power grid is struck by lightning or a single-phase arc is grounded, the neutral point potential will rise to the phase voltage. After being compensated, the residual current after compensation becomes very small, which is not enough to maintain the arc, thus extinguishing itself. In this way, the grounding can be eliminated quickly without causing overvoltage.

The arc suppression coil is mainly composed of an iron core with an air gap and a winding on the iron core, which are placed in an oil tank filled with transformer oil. The resistance of the winding is small and the reactance is large. The inductance of the arc suppression coil can be adjusted by changing the number of turns connected to the winding. Under normal operating conditions, since the voltage at the neutral point of the system is a three-phase asymmetrical voltage with a very small value, the current passing through the arc suppression coil is also very small, and the arc may be automatically extinguished.

Generally, overcompensation is adopted, that is, the inductor current is slightly larger than the capacitor current. The arc suppression coil is an inductance coil with an iron core. It is connected between the neutral point of the transformer (or generator) and the earth to form an arc suppression coil grounding system. During normal operation, no current flows through the arc suppression coil. When the power grid is struck by lightning or a single-phase arc is grounded, the neutral point potential will rise to the phase voltage. After being compensated, the residual current after compensation becomes very small, which is not enough to maintain the arc, thus extinguishing itself. In this way, the grounding can be eliminated quickly without causing overvoltage.

The working principle and difference between arc suppression and harmonic elimination

For a long time, most of China's 6-35kV (including 66kV) power grids have adopted the neutral point ungrounded operation mode. When a single-phase grounding occurs in the power grid of this type of operation, the fault phase-to-ground voltage drops to zero, and the phase-to-ground voltage of the non-faulted phase will rise to the line voltage (UL), but the line voltage of the system remains unchanged. Therefore, the national standard stipulates that this type of power grid is allowed to operate with faults for a short time (2 hours) after a single-phase ground fault occurs, so the reliability of power supply of this type of power grid is greatly improved. The existing operating regulations stipulate: "After a single-phase ground fault occurs in the neutral point non-effectively grounded system, it is allowed to run for two hours", but the concept of "single-phase ground fault" is not clearly defined in the regulations.

If the single-phase grounding fault is metallic grounding, the voltage of the faulty phase will drop to zero, and the phase-to-ground voltage of the other two healthy phases will rise to the line voltage. The electrical equipment of this type of power grid should be able to withstand this overvoltage under normal conditions. Not damaged. However, if the single-phase ground fault is an arc ground fault, an overvoltage with a maximum value of 3.5 times the phase voltage will be generated in the system. If such a high overvoltage acts on the power grid for several hours, it will inevitably cause the accumulation of insulation in the electrical equipment. If the insulation is broken down to the ground in the weak link of the sound phase insulation, it will cause a major accident of short circuit between phases.

In the high-voltage power grid where the neutral point is not grounded, the hazards of single-phase grounding capacitive current are mainly reflected in the following four aspects:

1) Harm of arc grounding overvoltage

Once the capacitor current is too large, the arc at the grounding point cannot be extinguished by itself. When intermittent arcing occurs to grounding, arcing grounding overvoltage is generated, which can reach 3~5 times of phase voltage or higher, it spreads throughout the entire power grid, and lasts for a long time, up to several hours, it It not only breaks down the weak link of insulation in the power grid, but also has great harm to the insulation of the entire power grid.

2) Cause thermal damage to the grounding point and increase the voltage of the grounding grid

If the current of the single-phase grounding capacitor is too large, the thermal effect of the grounding point will increase, causing thermal damage to cables and other equipment. After the current flows into the ground, due to the grounding resistance, the voltage of the entire grounding network will increase, endangering personal safety.

3) AC stray current hazard

After the capacitive current flows into the ground, a stray current is formed in the ground. This current may generate sparks, ignite gas explosions, etc., which may cause the detonator to fire in advance, and corrode the water pipes and air pipes.

4) Grounding arc causes gas and coal dust explosion

The role of the arc suppression coil:

After the arc suppressing coil is installed in the power grid, the arc suppressing coil generates inductive current when single-phase grounding occurs, and the inductive current compensates the capacitive current formed due to single-phase grounding, so that the grounding current is reduced, and at the same time, the recovery voltage speed of the faulty phase is reduced. Harm caused by excessive capacitor current. At the same time, due to the clamping effect of the arc suppression coil, it can effectively prevent the occurrence probability of ferromagnetic resonance overvoltage.

Some problems in the arc suppression coil grounding method:

1) When a single-phase ground fault occurs, the non-fault phase-to-ground voltage rises above the 3-phase voltage, which lasts for a long time and affects the entire system equipment, which may cause insulation breakdown at the second point and cause the accident to expand.

2) The arc suppression coil cannot compensate the harmonic current. In some cities, the harmonic current accounts for 5%-15%. The harmonic current alone may be far greater than 10A, and arc grounding overvoltage may still occur.

3) For a distribution network with a large capacitive current, if the single-phase ground fault current Ijd<10A is to be achieved through compensation, the system must maintain a small degree of detuning. The neutral point displacement voltage caused by capacitance asymmetry will have a strong amplification effect, so that the neutral point voltage deviation exceeds the allowable value of the regulations (<15%Un), and the protection will send a ground fault signal.

In addition, if the detuning degree is too small, the system will run close to the resonance compensation state, which will bring great potential danger to the system operation (resonance overvoltage); to ensure that the neutral point displacement voltage does not exceed the allowable value of the regulations, it is necessary to increase the detuning However, if the detuning is too large, the residual grounding current will be too large (Ijd>10A), which may cause intermittent arcing grounding overvoltage. It is difficult to ensure that the residual ground current Ijd<10A and the neutral point displacement voltage do not exceed the allowable value of the regulations, which are two mutually restrictive conditions.

4) The adjustment range of the arc suppression coil is limited by the adjustment capacity, and the ratio of the adjustment capacity to the rated value is generally 1/2. If it is selected according to the final requirements, the system capacitance current is small in the initial stage of the project, and the minimum compensation current of the arc suppression coil is too large. If it is selected according to the requirements in the initial stage of the project, the capacitive current of the system in the final stage of the project is large, and the maximum compensation current of the arc suppression coil is too small, which cannot meet the requirements of reasonable compensation.

5) During operation, there will be a large error between the nominal current and the actual current of each tap of the arc suppression coil. During operation, resonance has occurred due to the large error between the actual current and the current of the famous brand.

6) Due to the frequent changes of the system's operation mode and system voltage, the capacitive current of the system often changes, and it is difficult to track and compensate. The current automatic tracking compensation device is in full bloom, the actual operation test time is relatively short, and the operation condition is not ideal. Moreover, the price is high, the structure is complex, and the amount of maintenance is large, which does not meet the requirements of unattended substations.

7) Due to the above reasons, the grounding of the neutral point through the arc suppression coil can only reduce the probability of the arc grounding overvoltage, but cannot eliminate the arc grounding overvoltage, nor can it reduce the amplitude of the arc grounding overvoltage, and the arc overvoltage multiple is also very high.

8) It is difficult to find a single-phase ground fault line. At present, the success rate of line selection for many small-current grounding line selection methods is not ideal, and the trial pull method is often used.

9) When the test pull method is used, it will not only cause short-term power outages on non-fault lines, but also cause operating overvoltage.

10) The system resonance overvoltage is high, and the resonance overvoltage lasts for a long time and affects the entire system equipment, often causing PT burnout or PT fuse blown. Wugao Institute and Guangzhou Power Supply Bureau measured 1/2 frequency division resonance overvoltage up to 2PU in the test of Quzhuang substation, measured 3 times high frequency resonance overvoltage excited by the closing operation up to 4PU, and measured A phase wire broken When the line is grounded on the load side, the resonant overvoltage value is 3.8PU. .

11) When single-phase grounding occurs on cables in cable ducts or cable tunnels, if the faulty line is not disconnected in time, it may cause a fire. A 35KV system cable in Shanghai caused a fire after single-phase grounding occurred one hour later, burning 40 in the cable tunnel. Major accident with multiple cables.

12) It takes a long time to find the faulty line, and it is easy to cause personal electric shock accidents during operation with ground faults.

13) When the single-phase is grounded, the non-faulty phase voltage rises to the line voltage or higher. If the fault point cannot be detected in time, the gapless metal oxide (MOA) surge arrester operates under the line voltage for a long time, which is easy to be damaged or even explode. Arc light grounding overvoltage, resonant overvoltage has high amplitude and long duration. Due to the action load problem of MOA, overvoltage in WGMOA system is generally not required, and the excellent characteristics of MOA cannot be effectively used, which is not conducive to the promotion and use of MOA in distribution network.

The characteristics of the distribution network dominated by cable lines:

1) The capacitive current of the cable line per unit length is more than 10 times larger than the capacitive current of the overhead line, and the urban power grid dominated by cables has a large capacitive current to the ground.

2) The cable line is less affected by external environmental conditions (lightning, external force, trees, strong wind, etc.), and there are few instantaneous ground faults, and the ground faults are generally permanent faults.

3) When a ground fault occurs in the cable line, the grounding arc is a closed arc, and the arc is not easy to extinguish by itself. If it is not tripped in time, it is easy to cause a phase-to-phase short circuit and expand the accident.

4) The cable is a weak insulation device. For example, the one-minute power frequency withstand voltage of 10kV XLPE cable is 28KV, while the insulation level of general 10kV power distribution equipment is 42kV. In the arc suppressing coil grounding system, due to the long time to find the fault point, the cable is subjected to power frequency or transient overvoltage for a long time, and it is easy to develop into a phase-to-phase fault, causing one or more lines to trip.

5) In the cable line, the amplitude of the high-frequency oscillating current is large and the attenuation is slow. The high-frequency oscillating current is much larger than the power frequency current. Due to the attenuation speed of the high-frequency oscillating current and the power frequency current, the arc-suppression coil cannot compensate the high-frequency oscillating current, and because the residual current after compensation by the arc-suppression coil in the cable line is large, the arc-suppression coil cannot extinguish the arc in the cable line. the

PT resonance:

(1) PT resonance PT resonance For yo/yo electromagnetic PT, under normal circumstances, single-phase grounding of the line will not cause ferromagnetic resonance overvoltage, but under the following conditions, ferromagnetic resonance may be caused.

1) For the neutral point ungrounded system, when the system is single-phase grounded, the fault point flows through the capacitive current, and the ungrounded two-phase phase voltage increases by 3 times. However, once the ground fault point is eliminated, the line voltage charge charged by the non-ground phase during the ground fault period can only flow into the ground through the PT high-voltage coil through its own ground point. During this instantaneous voltage mutation process, the non-ground voltage of the PT high-voltage coil The excitation current of the grounded two phases will suddenly increase, or even saturate, thus forming a series resonance between phases.

2) Ferromagnetic resonance occurs in the system. In recent years, due to the increase in the number of distribution line users PT, electronically controlled electric welding machines, and speed-adjusting motors, the electrical parameters of the 10kV power distribution system have undergone great changes, resulting in frequent occurrence of resonance. When the system resonates, the PT will generate an overvoltage to increase the current sharply. At this time, in addition to causing the fuse on the primary side to blow, it will also cause the PT to burn out. In some cases, it can also cause flashover or explosion of the bushings of lightning arresters, transformers and circuit breakers.

3) For line maintenance, do not apply to the dispatching department for power outage procedures in advance, and open the branch line isolation switch with load or the high-voltage drop switch of the distribution transformer with load, causing arc short circuit between switches and causing resonance.

4) When a single-phase ground fault occurs inside the distribution transformer, the fault current will discharge to the ground through the insulating oil with strong electric resistance, and an unstable arc will also be generated to stimulate the grid resonance.

5) The operator's power transmission operation procedure is incorrect, and the PT is directly sent to the empty bus without opening the PT high-voltage side switch, causing PT ferromagnetic resonance.

HZ-5100 Contact Resistance Tester