What is the function and principle of low-voltage capacitor compensation cabinet？
1. Compensation principle of power capacitor
In the power system, the compensation principle of capacitors can be simply understood as follows: it is equivalent to a generator that generates capacitive reactive current. By connecting devices with capacitive power loads in parallel with inductive power loads on the same capacitor, energy can be converted between these two types of loads. In this way, the load on transformers and Transmission lines in the power grid is effectively reduced, thereby enhancing the output active power capacity. At the same time, when a certain amount of active power is output, the loss in the power supply system is also correspondingly reduced. Therefore, capacitors have become the simplest and most economical method to reduce the load on transformers, power supply systems, and industrial distribution systems.
2. Characteristics of power capacitor compensation
In terms of advantages, the reactive power compensation device of power capacitors boasts numerous benefits, such as convenient installation, flexible location adjustment, low active power loss (only about 0.4% of the rated capacity), short construction period, low investment, and simple operation and maintenance. Furthermore, the damage of individual capacitor banks will not affect the operation of the entire capacitor bank.
However, the reactive power compensation device of power capacitors also has some shortcomings. For example, it can only perform step-wise adjustment and cannot achieve smooth adjustment; poor ventilation may lead to expansion and explosion of capacitors when the operating temperature is higher than 70°C; its voltage characteristics are poor, with poor short-circuit stability, and there may be residual charge after removal; in addition, the low accuracy of reactive power compensation may also affect the compensation effect. At the same time, the operation management and safe operation of compensation capacitors have not received sufficient attention.
3. Reactive power compensation method
High-voltage dispersion compensation primarily involves installing reactive power compensation capacitors on the high-voltage side of a single transformer, aiming to improve the quality of power supply voltage. It is commonly seen in urban high-voltage power distribution.
High-voltage centralized compensation involves installing capacitors on the 6kV~10kV high-voltage busbar of substations or user step-down substations, or on the low-voltage busbar of the user's main distribution room. This approach is suitable for locations with concentrated loads, proximity to distribution busbars, and large compensation capacity. It can effectively reduce the consumption of reactive power in the power system and provide a certain degree of compensation. Its advantages include ease of automatic switching, improved power factor, high utilization rate, low investment, and convenient maintenance. However, it should be noted that this compensation method yields relatively poor economic benefits.
Low-voltage distributed compensation is a compensation method carried out at the low-voltage side, and its specific implementation details are not described in detail in the given text.
4. Principle of low-voltage distributed compensation
Low-voltage distributed compensation involves installing single or multiple low-voltage capacitor banks near individual electrical equipment based on the reactive power demand of the equipment, in order to compensate for the reactive power of all high and low-voltage lines and transformers upstream of the equipment. Its advantage lies in the fact that when the electrical equipment is running, the reactive power compensation equipment is also in operation, while when the equipment is shut down, the compensation equipment is turned off. This can effectively reduce the reactive power flow in the distribution network and transformers, thereby reducing active power loss. In addition, it can also reduce the cross-sectional area of line conductors and the capacity of transformers, saving space. However, this compensation method has a relatively low utilization rate, requires significant investment, and is not suitable for motors with variable speed operation, forward and reverse operation, as well as inching, locked-rotor, and reverse polarity braking.
5. Principle of low-voltage centralized compensation
Low-voltage centralized compensation involves connecting low-voltage capacitors to the low-voltage busbar side of the distribution transformer through a low-voltage switch, and controlling and protecting them through a reactive power compensation switching device. This device directly controls the switching of capacitors based on the reactive load on the low-voltage busbar, achieving group switching without smooth adjustment. Its advantages include simple wiring, low operation and maintenance workload, effective local reactive power balance, improved utilization rate of distribution transformers, reduced network loss, and significant economic benefits.
6. Calculation of capacitor compensation capacity
The compensation capacity of capacitors should be determined based on the reactive power curve or reactive compensation calculation method. The commonly used calculation formula is QC=p(tgφ1-tgφ2) or QC=pqc(1), where Qc represents the compensation capacitor capacity, P represents the load active power, COSφ1 and COSφ2 represent the load power factors before and after compensation, respectively, and qc represents the reactive power compensation rate.
7. Safe operation of power capacitors
The safe operation of power capacitors is of utmost importance. During operation, close attention must be paid to parameters such as temperature, voltage, and current of the capacitors to ensure they remain within safe limits. Additionally, regular inspection and maintenance of the capacitors are necessary to promptly identify and address potential issues, thereby ensuring their stable and reliable operation.
8. Allowable operating current
Under normal operating conditions, capacitors should operate at their rated current, and their maximum operating current should not exceed 1.3 times the rated current. At the same time, the difference between the three-phase currents should be controlled within 5% to ensure the stable and safe operation of the capacitor.
9. Allowable operating voltage
Capacitors are highly sensitive to voltage fluctuations. Since the loss of capacitors is proportional to the square of the voltage, excessively high voltage can cause severe heating of the capacitors, which in turn accelerates the rate of insulation aging, shortens the service life, and may even lead to electrical breakdown. Therefore, capacitor devices must be operated at rated voltage, and it is generally not recommended to exceed 1.05 times the rated voltage, and the maximum operating voltage should not exceed 1.1 times the rated voltage. Once the bus voltage exceeds this upper limit, cooling measures must be taken immediately to ensure the safety of the capacitors.
10. Harmonic wave problem
Since the capacitor circuit constitutes an LC circuit, specific harmonics are prone to cause resonance, leading to the generation of higher-order harmonics, which in turn increases current and voltage. These harmonic currents are extremely destructive to capacitors, potentially causing capacitor breakdown and resulting in phase-to-phase short circuits. Therefore, during normal operation of the capacitor, to prevent the impact of harmonics, a reactor with an appropriate inductance value can be connected in series with the capacitor to limit harmonic currents.
11. Relay protection issues
Relay protection is a crucial aspect for the safe and stable operation of power systems, primarily achieved through relay protection complete sets of equipment. Currently, relay protection devices produced by several well-known domestic electrical manufacturers have reached a highly mature level, being not only safe and stable but also comprehensive in functionality. These devices can effectively remove faulty capacitors and prevent the escalation of accidents. To ensure the safety of capacitors, commonly adopted relay protection measures include: ① three-stage overcurrent protection, which provides segmented protection based on current magnitude; ② overvoltage protection to prevent system steady-state overvoltage damage to capacitors; ③ undervoltage protection to avoid transient reclosing overvoltage of capacitors caused by transient system power outages; and ④ unbalanced voltage and current protection, as well as three-phase differential voltage protection, to reflect internal breakdown faults of capacitors in capacitor banks.
12. Switch-on issue
Reclosing of the capacitor bank is prohibited when it is live. This is because capacitors require time to discharge. If reclosing occurs immediately after a switch trip, the capacitors may not have had time to discharge, potentially retaining charges with a polarity opposite to the reclosing voltage. This can result in a massive inrush current at the moment of closing, leading to expansion of the capacitor casing, oil spray, and even explosion. Therefore, reclosing of the capacitor bank must be performed after waiting for 3 minutes for the circuit breaker to disconnect. For safety reasons, capacitors should not be equipped with automatic reclosing devices, but rather with automatic tripping devices with voltage-free release.
In some terminal substations, automatic switching devices for backup power supplies may be equipped. This device will disconnect the faulty power supply in case of a fault and switch on the backup power supply after a brief delay. During this process, if the capacitor bank has a low-voltage automatic switching function, the capacitor bank may be closed again in a short period of time, causing the aforementioned fault. Therefore, when using such systems, the switching of the capacitor bank should be fully considered and given sufficient attention.
13. Allowable operating temperature
Under normal operating conditions, the rated ambient temperature around the capacitor is typically set within the range of 40℃ to -25℃. Meanwhile, the internal dielectric temperature of the capacitor must be controlled below 65℃, with a maximum of 70℃, to prevent thermal breakdown or bulging. The temperature of the capacitor housing is between the dielectric and ambient temperatures, and should not exceed 55℃. To ensure the safe and stable operation of the capacitor, it is necessary to maintain good ventilation in the room where it is located, so as to ensure that its operating temperature always does not exceed the specified allowable value.
14. Problem of discharge sound during operation
Capacitors are typically silent during operation. However, under certain specific conditions, capacitors may produce a discharge sound. For instance, when the casing of a capacitor is left exposed for an extended period in the open air and rainwater infiltrates between the two layers of casing, applying voltage may trigger a discharge sound. Additionally, a lack of oil inside the capacitor may cause the lower end of its casing to protrude above the oil surface, thereby emitting a discharge sound. Furthermore, issues such as poor soldering or solder detachment within the capacitor, as well as poor contact between the core and the shell, may lead to flashover discharge and floating voltage within the oil, ultimately generating a discharge sound.
In response to these issues related to discharge sounds, appropriate measures should be taken. Specifically, handling should be carried out according to different situations: Firstly, the capacitor should be shut down and discharged, then the outer bushing should be removed, wiped dry, and reinstalled; secondly, capacitor oil of the specified specification should be added; if the discharge sound persists, the capacitor needs to be disassembled for repair; finally, after the capacitor is shut down and discharged, the contact between the core and the shell should be adjusted to ensure it is in good condition.
15. Explosion problem
During operation, capacitors may face various risks, including internal component breakdown, damage to the shell insulation, oil leakage due to poor sealing, bulging and internal dissociation, closing with charges, excessive temperature, poor ventilation, excessively high operating voltage, excessive harmonic components, and switching overvoltage. These situations can lead to capacitor damage or even explosion. To prevent such accidents, we need to take a series of preventive measures. For example, according to the amount of current passing through each phase capacitor, a 1.5 to 2 times faster fuse is equipped to ensure that the power supply can be quickly cut off in case of capacitor breakdown, thereby protecting the capacitor. At the same time, an ammeter is installed on the compensation cabinet to monitor the current of each phase, ensuring that the current difference does not exceed ±5%. Once an imbalance is detected, the capacitor should be immediately shut down and inspected. In addition, the temperature rise of the capacitor should be closely monitored, and its inspection should be strengthened to prevent phenomena that may lead to explosion, such as oil leakage and bulging.