In the low-voltage power distribution part, there are incoming line cabinets, outgoing line cabinets, and of course capacitor compensation cabinets. So what is the role of capacitor compensation cabinets? As the name implies, they play the role of capacitor compensation. Let's first look at the principle of capacitor compensation. When compensating, the capacitor and the load are connected in parallel. The capacitor is like a battery bank. When the load increases, due to the internal resistance of the power supply, the output voltage of the power supply will drop because both ends of the capacitor need to maintain the original voltage, that is Part of the battery in the capacitor flows out, which delays the downward trend of the voltage. This is the principle of capacitor compensation.
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1. Compensation principle of power capacitor
In principle, a capacitor is equivalent to a generator that generates capacitive reactive current. The principle of reactive power compensation is to connect a device with a capacitive power load and an inductive power load in parallel on the same capacitor, and the energy is converted between the two loads. In this way, the load on the transformers and transmission lines in the grid is reduced, thereby increasing the output active capacity. Under the condition of outputting a certain active power, the loss of the power supply system is reduced. In comparison, capacitors are the easiest and most economical way to reduce the load on transformers, power supply systems and industrial 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 parallel capacitors as reactive power compensation devices.
2. Characteristics of power capacitor compensation
advantage
The power capacitor reactive power compensation device has the characteristics of easy installation and convenient installation location; small active power loss (only about 0.4% of the rated capacity); short construction period; small investment; no rotating parts, easy operation and maintenance; if individual capacitor banks are damaged, It does not affect the operation of the entire capacitor bank and other advantages.
shortcoming
The disadvantages of the power capacitor reactive power compensation device are: it can only perform step adjustment, but cannot perform smooth adjustment; poor ventilation, once the operating temperature of the capacitor is higher than 70 ℃, it is prone to expansion and explosion; poor voltage characteristics, poor short-circuit stability, There is residual charge after removal; the reactive power compensation accuracy is low and easily affects the compensation effect; the operation management of the compensation capacitor is difficult and the issue of safe operation of the capacitor is not taken seriously, etc.
3. Reactive power compensation method
High pressure dispersion compensation
High-voltage dispersion compensation is actually a reactive power compensation capacitor installed on the high-voltage side of a single transformer to improve the quality of the 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 in which capacitors are installed on the 6 kV ~ 10 kV high-voltage bus in the substation or the user's step-down substation; the capacitor can also be installed on the low-voltage bus in the user's main distribution room, which is suitable for applications where the load is concentrated and away from the distribution bus. When the user itself has a certain high-voltage load in a nearby place with a large compensation capacity, it can reduce the reactive power consumption of the power system and play a certain compensation role. Its advantages are that it is easy to implement automatic switching, can reasonably improve the user's power factor, has high utilization rate, less investment, is easy to maintain, and is easy to adjust to avoid overcompensation and improve voltage quality. However, the economic benefit of this compensation method is poor.
Low pressure dispersion compensation
Low-voltage dispersion compensation is based on the reactive power requirements of individual electrical equipment. Single or multiple low-voltage capacitor banks are installed dispersedly near the electrical equipment to compensate for the reactive power of all high and low-voltage lines and transformers in front of the installation site. power. The advantage is that when the electrical equipment is running, reactive power compensation is put in, and when the electrical equipment is out of service, the compensation equipment is also withdrawn, which can reduce the reactive power flow in the distribution network and transformers, thereby reducing active power losses; it can reduce the wire cross-section of the line And transformer capacity, small footprint. The disadvantages are low utilization rate and large investment. It is not suitable for variable speed operation, forward and reverse operation, inching, stalling, and reverse braking motors.
Low voltage centralized compensation
Low-voltage centralized compensation refers to connecting low-voltage capacitors to the low-voltage bus side of the distribution transformer through a low-voltage switch, using the reactive power compensation switching device as a control and protection device, and directly controlling the switching of the capacitors according to the reactive power on the low-voltage bus. Capacitor switching is performed as a whole group and cannot be adjusted smoothly. Advantages of low-voltage compensation: simple wiring, small operation and maintenance workload, balancing reactive power locally, thereby improving distribution transformer utilization, reducing network losses, and being highly economical. It is one of the commonly used methods in reactive power compensation. .
4. Calculation of capacitor compensation capacity
The reactive power compensation capacity should be determined according to the reactive power curve or the reactive power compensation calculation method. The calculation formula is as follows:
QC=p(tgφ1-tgφ2) or QC=pqc(1)
In the formula:
Qc: Compensation capacitor capacity;
P: load active power;
COSφ1: Compensation preload power factor;
COSφ2: 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 should operate at the rated current, the maximum operating current shall not exceed 1.3 times 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 the capacitor is proportional to the square of the voltage. Overvoltage will cause serious heating of the capacitor, and the insulation of the capacitor will accelerate aging, shorten its life, and even cause electrical breakdown. Therefore, the capacitor device should operate at the rated voltage, which generally should not exceed 1.05 times the rated voltage, and the maximum operating voltage should not exceed 1.1 times the rated voltage. When the busbar exceeds 1.1 times the rated voltage, cooling measures must be taken.
3. Harmonic problem
Since the capacitor circuit is an LC circuit, it is easy to resonate with certain harmonics, which can easily cause high-order harmonics, causing an increase in current and voltage. Moreover, this harmonic current is very harmful to capacitors and can easily cause capacitor breakdown and cause phase-to-phase short circuit. Therefore, when the capacitor is working normally, a reactor with appropriate inductance value can be connected in series with the capacitor to limit the harmonic current when necessary.
4. Relay protection issues
Relay protection is mainly realized by complete sets of relay protection devices. At present, the relay protection device technology produced by several well-known domestic electrical manufacturers is very mature, safe, stable, and powerful. Relay protection devices can effectively remove faulty capacitors and are an important means to ensure safe and stable operation of power systems. The main capacitor relay protection measures include: ① three-stage overcurrent protection; ② overvoltage protection set up to prevent capacitor damage caused by steady-state overvoltage of the system; ③ to avoid overvoltage caused by instantaneous reclosure of capacitors caused by a brief shutdown of the system power supply. Low voltage protection set due to voltage damage; ④ Unbalanced voltage protection, unbalanced current protection or three-phase difference voltage protection configured to reflect the internal breakdown fault of the capacitors in the capacitor bank.
5. Closing problem
Capacitor banks are prohibited from reclosing when charged. The main reason is that it takes a certain amount of time to discharge the capacitor. When the switch of the capacitor bank trips, if it is reclosed immediately, the capacitor will not have time to discharge. There may remain charges with the opposite polarity to the reclosing voltage in the capacitor, which will cause the closing A large surge current is generated instantly, causing the capacitor shell to expand, spray fuel or even explode. Therefore, when the capacitor bank is closed again, it must be done 3 minutes after the circuit breaker is disconnected. Therefore, capacitors are not allowed to be equipped with automatic reclosing devices, and instead should be equipped with pressure-free automatic tripping devices.
Some terminal substations are often equipped with automatic backup power switching devices. The device operates to cut off the faulty power supply, and then switches on the backup power supply after a short delay. During this process, if the capacitor bank has a low-voltage self-switching function, the capacitor bank will be switched on in a short time. If it is closed again within a certain period of time, the above-mentioned failure will occur. Therefore, the switching issues of systems and capacitor banks equipped with automatic backup power switching devices deserve full attention.
6. Allowable operating temperature
When the capacitor is working normally, the rated ambient temperature around it is generally 40 ℃ ~ -25 ℃; the temperature of the internal medium should be lower than 65 ℃, and the maximum should not exceed 70 ℃, otherwise it will cause thermal breakdown or bulging. The temperature of the capacitor shell is between the medium temperature and the ambient temperature and should not exceed 55°C. Therefore, the capacitor room should be kept well ventilated to ensure that its operating temperature does not exceed the allowable value.
7. Discharge sound problem during operation
Capacitors generally have no sound when they are running, but in some cases, they may also have the problem of discharge sounds when they are running. For example, if the casing of a capacitor is left in the open for too long, once rainwater enters between the two casings and a voltage is applied, a discharge sound may occur; when there is a lack of oil in the capacitor, the lower end of the casing will easily be exposed to oil. surface, then a discharge sound may be emitted; if there is soldering or desoldering inside the capacitor, flashover discharge will occur in the oil; when the core of the capacitor is in poor contact with the shell, a floating voltage will appear, causing a discharge sound. .
Once the above discharge sound conditions occur, each situation should be dealt with, that is, the treatment methods are as follows: stop the capacitor and discharge it, remove the outer casing, dry it and reinstall it; add the same specifications Capacitor oil; if the discharge sound does not stop, it should be disassembled and repaired; the capacitor should be out of service and discharged, so that the core and shell are in good contact.
8. Explosion problem
During the operation of the capacitor, if there is breakdown of the internal components of the capacitor, damage to the insulation of the capacitor shell, poor sealing and oil leakage, bulging and internal dissociation, bulging and internal dissociation, charged closing or excessive temperature, and poor ventilation, , operating voltage is too high, harmonic components are too large, operating overvoltage, etc. may cause capacitor damage and explosion. In order to prevent capacitor explosion accidents, under normal circumstances, 1.5 to 2 times the amount of current passing through each group of phase capacitors can be equipped with a quick fuse. If the capacitor is broken down, the quick fuse will melt and cut off. power supply to protect the capacitors from continuing to generate heat; install an ammeter on each phase of the compensation cabinet to ensure that the current difference between each phase does not exceed ±5%. If an imbalance is found, exit the operation immediately and check the capacitors; monitor the temperature rise of the capacitors; strengthen monitoring Inspect the capacitor bank to avoid oil leakage and bulging of the capacitor to prevent explosion.
