In the low-voltage power distribution part, there are incoming cabinet, outgoing cabinet, and of course, capacitor compensation cabinet. What is the role of capacitor compensation cabinet? As the name suggests, it is to play the role of capacitor compensation. Let's first look at the principle of capacitor compensation. During capacitor compensation, the capacitance and load are connected in parallel. The capacitance is the same as the power store. When the load increases, the output voltage of the power supply will drop due to the internal resistance of the power supply, Since the two ends of the capacitor need to maintain the original voltage, that is, a part of the electric current in the capacitor will flow out, delaying the downward trend of the voltage, that is, the principle of capacitor compensation.

1. Compensation principle of power capacitor

In principle, capacitors are equivalent to generators that generate capacitive reactive current. The principle of reactive power compensation is that the device with capacitive power load and inductive power load are connected in parallel on the same capacitor, and the energy is converted between the two loads. In this way, the load of transformers and transmission lines in the power grid is reduced, thus increasing the output active power capacity. The loss of the power supply system is reduced when a certain amount of active power is output. By comparison, capacitors are the simplest and most economical way to reduce the load of transformers, power supply systems and industrial power distribution. Therefore, it is imperative for capacitors to be used as reactive power compensation in power systems. At present, it is very common to use shunt capacitors as reactive power compensation devices

2. Characteristics of power capacitor compensation

advantage

The reactive power compensation device of the power capacitor has the advantages of convenient installation and convenient increase and decrease of the installation site; Low active power loss (only about 0.4% of rated capacity); Short construction period; Small investment; No rotating parts, easy operation and maintenance; The damage of individual capacitor bank will not affect the operation of the whole capacitor bank.

shortcoming

The disadvantages of the reactive power compensation device of power capacitor are as follows: it can only be adjusted step by step and cannot be adjusted smoothly; Poor ventilation, once the operating temperature of the capacitor is higher than 70 ℃, it is easy to expand and explode; Poor voltage characteristics, poor stability to short circuit, and residual charge after removal; The reactive power compensation accuracy is low, which easily affects the compensation effect; It is difficult to manage the operation of compensation capacitors and the problem of safe operation of capacitors has not been paid attention to.

3. Reactive compensation mode

High voltage decentralized compensation

High voltage decentralized compensation is actually a reactive power compensation capacitor installed on the high voltage side of a single transformer to improve the quality of power supply voltage. It is mainly used in urban high-voltage power distribution.

High voltage centralized compensation

High voltage centralized compensation refers to the compensation method of installing capacitors on 6 kV~10 kV high-voltage buses in substations or user step-down substations; Capacitors can also be installed on the low-voltage bus in the user's general distribution room, which is suitable for places where the load is concentrated, close to the distribution bus, and the compensation capacity is large. When the user has a certain high-voltage load, it can reduce the reactive power consumption of the power system and play a certain role in compensation. Its advantages are that it is easy to implement automatic switching, can reasonably improve the power factor of users, has high utilization rate, less investment, is convenient for maintenance, is convenient for adjustment, can avoid over compensation, and improves the voltage quality. However, the economic benefits of this compensation method are poor.

Low voltage decentralized compensation

Low voltage decentralized compensation refers to the decentralized installation of single or multiple low-voltage capacitor banks near the electrical equipment according to the reactive power demand of individual electrical equipment to compensate the reactive power of all high and low voltage lines and transformers in front of the installation position. Its advantage is that when the electric equipment is running, the reactive power compensation is put into operation, and when the electric equipment is stopped, the compensation equipment is also withdrawn, which can reduce the reactive power flow in the distribution network and transformer, thus reducing the active power loss; It can reduce the wire section of the line and the capacity of the transformer, with small occupation. The disadvantages are low utilization rate and large investment. It is not suitable for motors with variable speed operation, forward and reverse operation, inching, locked rotor and reverse braking.

Low voltage centralized compensation

Low voltage centralized compensation refers to connecting the low-voltage capacitor to the low-voltage bus side of the distribution transformer through the low-voltage switch, taking the reactive compensation switching device as the control protection device, and directly controlling the switching of the capacitor according to the reactive power on the low-voltage bus. Capacitors are switched on and off as a whole group without smooth adjustment. Advantages of low voltage compensation: simple wiring, small operation and maintenance workload, enabling reactive power to be balanced locally, so as to improve the utilization rate of distribution transformer, reduce network loss, with high economy, it is one of the commonly used means of reactive power compensation at present.

4. Calculation of capacitor compensation capacity

Reactive compensation capacity should be determined according to reactive power curve or reactive compensation calculation method, and its calculation formula is as follows:

QC=p(tg φ 1-tg φ 2) Or QC=pqc (1)

Where:

Qc: capacity of compensation capacitor;

P: Load active power;

COS φ 1: Load power factor before compensation;

COS φ 2: Load power factor after compensation;

Qc: reactive power compensation rate, kvar/kw.

5. Safe Operation of Power Capacitors

1. Allowable operating current
During normal operation, the capacitor shall operate at the rated current, the maximum operating current shall not exceed 1.3 times of the rated current, and the three-phase current difference shall not exceed 5%.
2. Allowable operating voltage
Capacitors are very sensitive to voltage. Because the loss of capacitors is proportional to the square of voltage, overvoltage will cause serious heating of capacitors, and the insulation of capacitors will accelerate aging, shorten life, and even electric breakdown. Therefore, the capacitor device shall operate under the rated voltage, generally not exceeding 1.05 times of the rated voltage, and the maximum operating voltage shall not exceed 1.1 times of the rated voltage. When the bus exceeds 1.1 times the rated voltage, cooling measures must be taken.
3. Harmonic problem
As the capacitor circuit is a LC circuit, it is easy to generate resonance for some harmonics, which is easy to cause higher harmonics and increase the current and voltage. And the harmonic current is very harmful to the capacitor, which is very easy to make the capacitor breakdown and cause inter phase short circuit. Therefore, when the capacitor works normally, a reactor with appropriate inductance value can be connected to the capacitor in series when necessary to limit harmonic current.
4. Relay protection problems
The relay protection is mainly realized by the complete set of relay protection devices. At present, the technology of relay protection devices produced by several well-known domestic electrical manufacturers is very mature, safe, stable and powerful. The relay protection device can effectively remove the fault capacitor, which is an important means to ensure the safe and stable operation of the power system. The main capacitor relay protection measures are: ① Three section overcurrent protection; ② Overvoltage protection set to prevent capacitor damage caused by system steady-state overvoltage; ③ Low voltage protection is set to avoid overvoltage damage caused by capacitor instantaneous reclosing due to system power supply transient shutdown; ④ Unbalanced voltage protection, unbalanced current protection or three-phase differential voltage protection configured to reflect the internal breakdown fault of capacitors in the capacitor bank.
5. Closing problem
Live reclosing is prohibited for capacitor banks. The main reason is that it takes a certain time for the capacitor to discharge. If the switch of the capacitor bank trips, if it is re closed immediately, the capacitor will not have time to discharge, and there may be charges in the capacitor that are opposite to the polarity of the reclosing voltage, which will generate a large impact current at the moment of closing, resulting in the expansion of the capacitor shell, oil injection and even explosion. Therefore, when the capacitor bank is closed again, it must be conducted 3 min after the circuit breaker is disconnected. Therefore, it is not allowed to install automatic reclosing device on the capacitor, and the phase reaction device is equipped with automatic tripping device without voltage release.
Some terminal substations are often equipped with automatic switching device of standby power supply. The device acts to cut off the fault power supply, and then the standby power supply is switched on after a short delay. In this process, if the capacitor bank has the low-voltage automatic switching function, the capacitor bank will be switched on again in a short time, which will lead to the above-mentioned fault. Therefore, the system with automatic switching device for standby power supply and the switching of capacitor banks should be fully emphasized.
 

6. Allowable operating temperature
When the capacitor works normally, its ambient rated temperature is generally 40 ℃~- 25 ℃; The temperature of the internal medium shall be lower than 65 ℃, and the maximum temperature shall not exceed 70 ℃, otherwise it may cause thermal breakdown or bulge. The temperature of capacitor shell is between the medium temperature and the ambient temperature, and shall not exceed 55 ℃. Therefore, the capacitor room shall be well ventilated to ensure that its operating temperature does not exceed the allowable value.

7. Discharge sound in operation

There is generally no sound when the capacitor is running, but in some cases, there will be the problem of discharge sound when it is running. If the capacitor bushing is placed in the open air for a long time, once the rainwater enters between the two bushings, the discharge sound may be generated after the voltage is applied; When the capacitor is short of oil, it is easy to expose the oil surface at the lower end of its bushing, and then it may emit a discharge sound; If there is faulty soldering or desoldering in the capacitor, it will flash over and discharge in the oil; When the core of the capacitor is in poor contact with the shell, floating voltage will appear, causing discharge sound.

In case of discharge sound in the above situations, treatment shall be carried out according to each situation, that is, after the capacitor is stopped and discharged, the outer sleeve shall be removed, wiped and reinstalled; Add capacitor oil of the same specification; In case of continuous discharge, it shall be disassembled for repair; After the capacitor is stopped and discharged, it shall be treated to make its core and shell contact well.

8. Explosion problem

During the operation of the capacitor, if the internal components of the capacitor are broken down, the insulation of the capacitor to the shell is damaged, the sealing is poor and oil leaks, the bulge and the internal free, the bulge and the internal free, the charged switch on or the temperature is too high, the ventilation is poor, the operating voltage is too high, the harmonic component is too large, the operating overvoltage and other conditions occur, the capacitor may be damaged and exploded. In order to prevent capacitor explosion accidents, under normal circumstances, a fast fuse can be equipped according to the current passing through each group of phase capacitors by 1.5~2 times. If the capacitor is broken down, the fast fuse will melt and cut off the power supply to protect the capacitor from continuing to generate heat; Install an ammeter on each phase of the compensation cabinet to ensure that the current difference of each phase does not exceed ± 5%. If imbalance is found, immediately exit the operation and check the capacitor; Monitor the temperature rise of capacitor; Strengthen the patrol inspection of the capacitor bank to avoid oil leakage and bulge of the capacitor to prevent explosion. (Source Power Partner)