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Overview of shunt capacitor banks

Public number : Shuangsi Power Transformation Operation and Maintenance Class Source: Time: 2020-02-21 01:31:12

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1. Classification and use of power capacitors

1 )并联电容器 ( 1 ) shunt capacitor     

Shunt capacitor: used to compensate inductive reactive power, improve power factor and voltage quality, reduce line losses, and increase the output power of the system or transformer.

2 )串联电容器 ( 2 ) Series capacitor

It is connected in series in the power frequency high voltage transmission and distribution line to compensate the inductance of the line, reduce the line voltage drop, increase the transmission capacity and transmission distance, and improve the stability of the line voltage.

3 )交流滤波电容器 ( 3 ) AC filter capacitor

Connected with the reactor and resistor to form an AC filter, which is used to provide a low-impedance channel for one or more harmonic currents, reduce the network harmonic level, and improve the power factor of the system.

Figure 1.1 Parallel capacitor bank device

4 )耦合电容器 ( 4 ) Coupling capacitor

In the power line carrier system, the high-voltage terminal is connected to the transmission line, and the low-voltage terminal is grounded through the coupling coil, so that the high-frequency carrier device is coupled with the high-voltage line at low voltage to realize carrier communication and measurement, control and protection.

5 )直流滤波电容器 ( 5 ) DC filter capacitor

Used in high-voltage rectification and filtering devices and high-voltage DC transmission. Filter out residual AC components, reduce ripple in DC, and improve the quality of DC transmission.

6 )电热电容器 ( 6 ) Electric heating capacitor

Used to improve power factor or improve loop characteristics.

7 )断路器电容器 ( 7 ) Circuit breaker capacitor

Connected in parallel on the break of the AC high voltage circuit breaker to improve the voltage distribution.

8 )脉冲电容器 ( 8 ) Pulse capacitor

For impulse voltage and impulse current generators, impulse voltage dividers, oscillating circuits, and continuation pulse devices.

9 )标准电容器 ( 9 ) Standard capacitor

As a standard capacitor or as a capacitive voltage divider for measuring high voltages.

10 )防护电容器 ( 10 ) Protective capacitor

Connected between line and ground to reduce the wavefront steepness and peak value of the atmospheric overvoltage, and cooperate with the arrester to protect the generator and motor.

Figure 1.2 Parallel capacitor bank device

2. Use of shunt capacitor bank

的不足,提高功率因数, 提高母线电压质量,降低电能损耗,改善供电质量 ,达到系统稳定运行目的。 A parallel capacitor bank is installed on the low-voltage side of the substation to compensate for the lack of reactive power , improve the power factor, improve the voltage quality of the bus bar, reduce power loss, improve the quality of power supply , and achieve the purpose of stable system operation.

3. Wiring mode of parallel capacitor bank

The shunt capacitor components are: star (including double star), delta (double triangle) wiring.

,爆炸起火的事故大多发生在三角形接线的电容器组。 Operating experience shows that the damage rate of delta-connected capacitor banks is much higher than that of star-connected capacitors. Most explosion and fire accidents occur in delta-connected capacitor banks.

,较大的路电流流过故障电容器会造成较大的冲击波而使电容器外壳爆破而起火。 Because a delta-connected capacitor bank will cause a phase-to-phase short-circuit of the power supply when the capacitor has an inter-electrode breakdown , a large short-circuit current flowing through the faulty capacitor will cause a larger shock wave, which will cause the capacitor shell to burst and catch fire.

Figure 3.1 Wiring method of shunt capacitor bank

,当电容器极间发生击穿不会形成相间短路。 For star-connected capacitor banks , a phase-to-phase short circuit does not occur when breakdown occurs between the capacitor poles. 发生电容器的极间击穿,其故障电流只有电容器组相电流的 3 ,比起相间短路时故障电流要小得多。 Even if the capacitor's inter-electrode breakdown occurs, its fault current is only three times the phase current of the capacitor bank, which is much smaller than the fault current when the phase-to-phase short circuit occurs.

At present, the wiring of high-voltage shunt capacitor banks only uses star wiring .

高压并联电容器组分类 4. Classification of high-voltage shunt capacitor banks

  • Shell-type unit capacitors

  • Box-type high-voltage shunt capacitor

  • Integrated high-voltage shunt capacitor

  • Inflatable collective shunt capacitor

  • Self-healing capacitor

  • SVC Static type dynamic reactive power compensation device ( SVC )

密集型(集合式)高压并联电容器装置使用较多。 Among them: decentralized (framework) and dense (collection) high-voltage shunt capacitor devices are used more.

5. Composition and function of parallel capacitor device

(1) Composition of parallel capacitor device:

(2) The role of each component is as follows:

Discharge coil: the energy storage of the bleeder capacitor, providing a relay protection signal.

Series reactor: limit closing inrush current and suppress grid harmonics.

Zinc oxide arrester and over-voltage protection device: suppress operation over-voltage.

Single capacitor protection fuse: Provides fast protection for short circuit between poles without internal fuse capacitors.

Grounding knife: used for safety grounding during maintenance.

Conductor, pillar insulator, frame, etc .: load-bearing system and current loop constituting the device.

并联电容器组可能出现的故障及异常运行方式 6. Possible failures and abnormal operation of parallel capacitor banks

  • Phase-to-phase short-circuit and ground faults on the connection lines between the capacitor bank and the circuit breaker and on the internal wiring of the capacitor bank.

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  • Capacitor bank failure refers to the short circuit between the capacitors in the capacitor and the failure of multiple capacitors in the capacitor bank.

  • Capacitor bank is overloaded.

  • The supply voltage of the capacitor bank increases.

  • Capacitor bank is out of voltage.

Figure 6.1 Parallel capacitor bank device

并联电容器组的保护配置 7. Protection configuration of shunt capacitor bank

(1) Short-time overcurrent protection

For short-to-phase faults on the connection lines between the capacitor bank and the circuit breaker and on the internal wiring of the capacitor bank, overcurrent protection with a short time limit shall be installed. 0.1-0.3s , 保护动作于跳闸。 When the current is larger than the setting value, after a short delay ( 0.1-0.3s ) , the protection action will trip.

(2) Overvoltage protection

反映电容器组的供电电压升高 Reflects the increase in the supply voltage of the capacitor bank

1.1 倍额定电压长期运行,当供电母线稳态电压升高时过电压保护应动作,带时限发信号或跳闸。 The capacitor bank is only allowed to run for a long time at 1.1 times the rated voltage. When the steady-state voltage of the power supply bus rises, the over-voltage protection should be activated, with a time limit to signal or trip. 3-5min When the overvoltage protection acts on the signal, there can be no delay; when the overvoltage protection acts on the trip, the delay is 3-5min .

反映电容器组中多台故障电容器切除后引起过电压保护 Reflect the over-voltage protection caused by the removal of multiple faulty capacitors in the capacitor bank

When the internal faults of multiple capacitors are removed by a special fuse, the capacitors that continue to operate will have an unallowable overload or overvoltage.

零序电压保护、电压差动保护、零序电流保护。 The common protections for capacitor banks are: zero-sequence voltage protection, voltage differential protection, and zero-sequence current protection.

2.1 Zero sequence voltage protection commonly used in single star connection

PT 开口三角形上的电压反映的是电容器组端点对中性点 N 的零序电压。 The voltage on the open triangle of the voltage transformer PT reflects the zero-sequence voltage of the capacitor bank terminal to the neutral point N. PT 的一次绕组兼作电容器组的放电线圈。 The primary winding of the voltage transformer PT doubles as the discharge coil of the capacitor bank.

1.1 倍额定电压时保护应动作。 When 1.1 times the rated voltage is exceeded , the protection shall act.

Figure 7.1 Zero sequence voltage protection wiring of capacitor bank

2.2 Voltage differential protection

When multiple capacitors of a certain phase are cut off, the voltages on the two series sections are not equal, and a differential voltage appears in this phase to protect the operation. 0.2s The operating time limit of the protection device can be taken as 0.2s .

Figure 7.2 Voltage Differential Protection of Capacitor Bank

(3) Imbalance protection

The principle of protection is to reflect the difference (current or voltage) between the healthy and faulty parts of a group of capacitors .

Different capacitor banks are connected in different ways to form unbalanced protection. Commonly used: zero sequence voltage protection (open triangle voltage protection), neutral point unbalanced voltage or current protection, voltage differential protection, bridge differential current protection.

When multiple capacitors are cut, there is current in the neutral point, and the protection can be activated.

The neutral point unbalanced current protection is as follows:

Figure 7.3 Wiring of neutral point unbalanced current protection for capacitor bank

(4) Overload protection

Overload is caused by system overvoltage and higher harmonics. 1.3 倍额定电流下长期运行 ,对电容量大的电容器,过电流允许达到 1.43 倍额定电流。 According to regulations, capacitors should be able to run for a long time at 1.3 times the rated current . For capacitors with large capacitance, overcurrent is allowed to reach 1.43 times the rated current.

The capacitor bank must be equipped with over-voltage protection that responds to the steady-state voltage rise, and a large-capacity capacitor is assembled with a series reactor that suppresses higher harmonics. In this case, overload protection may not be installed . Install overload protection only when the system's high harmonics are too high or the loop current exceeds the allowable value.

The protection delay action is on the signal, and the inverse time characteristic overload protection should be adopted. Generally, overload and overcurrent protection are combined.

Figure 7.4 Parallel capacitor bank device

(5) Low voltage protection

When the supply voltage disappears, the capacitor bank loses power and begins to discharge, and the voltage on it gradually decreases. 0.1 倍额定电压就恢复供电,则电容器组上将承受高于 1.1 倍额定电压的合闸过电压,导致电容器组的损坏,因而需装设低电压保护。 If the residual voltage is not discharged to 0.1 times the rated voltage before power is restored, the capacitor bank will withstand a closing overvoltage higher than 1.1 times the rated voltage, which will cause damage to the capacitor bank. Therefore, low voltage protection will be required.

止所接 PT 二次空开误跳造成 PT 二次失压引起电容器低电压保护误动,保护经电流闭锁。 Only when the three-phase voltage decreases to the low-voltage operating value at the same time, the protection can be activated; at the same time, in order to prevent the PT secondary voltage loss caused by the PT secondary air-open tripping, the low-voltage protection of the capacitor will malfunction and the protection will be blocked by current. .

0.15-0.4 倍额定电压,保护的动作时间与本侧出线后备保护时间配合。 The low voltage protection is set to 0.15-0.4 times the rated voltage, and the action time of the protection matches the backup protection time of the outgoing line on this side.

Figure 7.5 Low voltage protection logic diagram of capacitor bank

(6) Other protection

In addition to the above-mentioned protection functions, it also generally has an automatic switching function or a low-voltage automatic switching function.

For series reactors used in capacitor banks, protection is generally not installed due to their small capacity.

电容器组断路器跳闸后的处理 8. Treatment after capacitor bank circuit breaker trips

After the capacitor bank circuit breaker has tripped, forced power is not allowed.

所以在接触故障电容器前,应戴绝缘手套,用短路线将两极短路,方可动手拆卸。 The cause of the overcurrent protection trip should be identified, otherwise it is not allowed to be put into operation; before checking and handling the capacitor failure, the circuit breaker and the knife breaker should be opened, and then the electricity should be grounded; due to the faulty capacitor, the lead may have poor contact and internal disconnection. The wire or fuse is blown, so some of the charge may not be discharged. Therefore, before touching the faulty capacitor, you should wear insulating gloves and short-circuit the two poles with a short-circuit wire before you can disassemble it.

For double star-connected capacitor banks, they should also be discharged separately.

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