Capacitors function similarly to generators in power systems, producing capacitive reactive current. Their compensation principle lies in connecting devices with capacitive power loads and inductive power loads in parallel with the same capacitor, achieving energy conversion between the two. This reduces the load on transformers and transmission lines in the power grid, thereby increasing the output active power capacity. Simultaneously, power system losses are reduced while maintaining a certain output active power. Compared to other methods, capacitors are undoubtedly the simplest and most economical way to reduce the load on transformers, power systems, and industrial distribution. Therefore, capacitors play an indispensable role in reactive power compensation in power systems, and the use of parallel capacitors as reactive power compensation devices has become a common practice.
◉ Advantages and disadvantages of capacitor compensation
Advantages: Power capacitor reactive power compensation devices have many advantages, such as convenient installation, flexible location addition and reduction, minimal active power loss (only about 0.4% of the rated capacity), short construction period, low investment, simple operation and maintenance, and individual capacitor bank failures do not affect the overall operation.
Disadvantages: However, power capacitor reactive power compensation devices also have some shortcomings, such as only being able to perform stepped adjustment, not being able to perform smooth adjustment, poor ventilation may lead to overheating and expansion of capacitors, poor voltage characteristics leading to poor short-circuit stability, insufficient compensation accuracy which may affect the compensation effect, and difficulties in operation and management.
Electrical equipment can be broadly categorized into three types: resistive devices, inductive devices, and capacitive devices. Resistive devices, such as immersion heaters, water heaters, and electric stoves, primarily convert electrical energy into heat. Inductive devices, such as motors, fans, and air conditioners, involve inductance or inductor coils, and their operating principle is related to magnetic fields. Capacitive devices, represented by capacitor-based electrical equipment, are not commonly seen in our daily lives; examples include mosquito killers and compensating cabinets.
Why choose capacitor compensation instead of resistor or inductive compensation? The underlying principle lies in the influence of different electrical loads on alternating current (AC). When AC flows through resistive loads, such as immersion heaters, water heaters, and electric furnaces, the current and voltage remain synchronized because these loads do not generate reactive power. However, with inductive loads, such as motors, fans, and air conditioners, the voltage phase leads the current phase due to the inductance. Similarly, with capacitive loads, such as mosquito killers and compensation cabinets, the current phase leads the voltage phase due to the capacitance.
In everyday electricity use, resistive loads are the majority and do not require reactive power compensation. However, inductive loads do require reactive power, which can cause voltage and current to be out of phase, i.e., voltage leads current. To restore the synchronization of current and voltage, a capacitor needs to be connected in parallel with the inductive load. Since the inductive load causes voltage lead, while the capacitive load causes current lead, these two effects cancel each other out, thus achieving synchronization. Currently, reactive power compensation controllers are widely used in automatic compensation systems to achieve efficient capacitor compensation.
01 Classification of Electrical Equipment and Compensation Principles
◉ Classification of Electrical Equipment
Electrical equipment can be broadly classified into three categories: resistive devices, inductive devices, and capacitive devices. Resistive devices, such as immersion water heaters and electric stoves, primarily convert electrical energy into heat; therefore, they can be simply understood as heating-type electrical equipment. Inductive devices, such as motors, fans, and air conditioners, are named for their inductance or inductance coils and can be considered coil-type electrical equipment. Capacitive devices, such as mosquito killer lamps and compensating cabinets, refer to capacitor-type electrical equipment, which are relatively rare in our daily lives.
◉ Compensation Principle
In AC circuits, resistive loads have zero reactive power, so the current and voltage remain synchronized. However, inductive loads, due to their inductance, cause the voltage phase to lead the current phase. Similarly, capacitive loads cause the current phase to lead the voltage phase. In everyday electricity use, resistive loads do not require reactive power compensation, but inductive loads do. To maintain current and voltage synchronization, we need to connect a capacitor in parallel with the inductive load for compensation. Because inductive loads cause voltage lead, and capacitive loads cause current lead, these two effects cancel each other out, thus achieving synchronization.
02 Capacitor Compensation Methods and Wiring Diagrams ◉ Modern Capacitor Compensation
In modern power systems, reactive power compensation controllers are typically used to achieve fully automatic capacitor compensation. These controllers can monitor the reactive power demand in the circuit in real time and automatically switch the corresponding capacitors on and off to ensure that the current and voltage phases remain synchronized.
Capacitor compensation acts like a helpful assistant in a circuit, making current transmission more efficient. In DC transmission, the inherent resistance of the wires causes voltage drops. In AC circuits, some devices (such as motors) can cause current and voltage to become out of sync, resulting in wasted electricity. A capacitor acts like a small reservoir that charges and discharges rapidly. In AC circuits, it offers minimal resistance to current and responds quickly to changes in current; however, in DC circuits, it acts as a barrier, preventing current from flowing through.
Specifically, adding a capacitor to an AC circuit can counteract the reverse force (reactive power) generated by some devices (such as coils), thereby allowing the current and voltage to be better synchronized, reducing power waste and improving power efficiency.
So why do low-voltage switchgear need capacitor compensation? This is because many devices in low-voltage switchgear are inductive loads (such as motors and transformers), which can lower the power factor of the circuit, causing power waste and equipment overheating. By adding capacitors for compensation, the power factor of the entire system can be improved, energy loss reduced, equipment lifespan extended, and electricity costs saved.
◆ Capacitor Compensation Cabinet Principle
Capacitors act as generators in power systems, producing capacitive reactive current. The core concept of reactive power compensation is to achieve energy conversion between capacitive and inductive loads by connecting devices with capacitive power loads in parallel with the same capacitor. This strategy helps reduce the load on transformers and transmission lines in the power grid, thereby increasing the output active power capacity. Simultaneously, it reduces power supply system losses, especially when outputting a certain amount of active power. Compared to other methods, capacitors are undoubtedly the simplest and most cost-effective choice for reducing the load on transformers, power supply systems, and industrial distribution. Therefore, the use of capacitors for reactive power compensation in power systems has become particularly necessary. Currently, parallel capacitors have become the mainstream choice for reactive power compensation devices.
◆ Reactive power compensation calculation method
Next, we will discuss the calculation method for capacitor compensation capacity. The reactive power compensation capacity should be determined based on the reactive power curve or a specific calculation method, and its calculation formula is QC=p(tgφ1-tgφ2) or QC=pqc(1). Where Qc represents the compensation capacitor capacity, P represents the active power of the load, COSφ1 and COSφ2 represent the load power factors before and after compensation, respectively, and qc represents the reactive power compensation rate. Using this formula, we can accurately calculate the required capacitor compensation capacity.
02 Characteristics and Methods of Capacitor Compensation ◆ Characteristics of Compensation Devices
Reactive power compensation devices for power capacitor banks have many advantages, such as convenient installation, flexible location, low active power loss, short construction period, low investment, and simple operation and maintenance. Furthermore, even if individual capacitor banks are damaged, the operation of the entire capacitor bank will not be affected. However, they also have some drawbacks, such as being limited to stepped adjustment, the possibility of expansion and explosion due to poor ventilation, and low reactive power compensation accuracy.
◆ Reactive power compensation methods
High-voltage distributed compensation: This is a reactive power compensation capacitor installed on the high-voltage side of a single transformer, designed to improve power supply voltage quality, and is mainly used in urban high-voltage power distribution.
High-voltage centralized compensation: This method involves installing capacitors on the high-voltage busbar of a substation or user's step-down substation, or on the low-voltage busbar of the user's main distribution room. It is suitable for locations with concentrated loads, close to the distribution busbar, and with large compensation capacity, effectively reducing reactive power consumption in the power system and providing compensation. Its advantages include easy automatic switching, improved power factor, high utilization rate, low investment, and ease of maintenance.
Low-voltage distributed compensation involves installing one or more low-voltage capacitor banks near electrical equipment to compensate for the reactive power of all high- and low-voltage lines and transformers upstream of the equipment. Its advantage is that the compensation equipment is activated when the equipment is running and deactivated when the equipment is shut down, thus reducing reactive power flow in the distribution network and transformers and lowering active power losses. Furthermore, it reduces the cross-sectional area of line conductors and transformer capacity, saving space. However, this method has disadvantages including lower utilization rates, higher investment costs, and unsuitability for variable-speed operation.
Low-voltage centralized compensation involves connecting low-voltage capacitors to the low-voltage busbar side of the distribution transformer via a low-voltage switch, and using a reactive power compensation switching device for control and protection. Its advantages include simple wiring, low operation and maintenance workload, local reactive power balance, improved transformer utilization, and reduced network losses, resulting in significant economic benefits. Therefore, it is one of the commonly used reactive power compensation methods.
03 Safe Operation of Capacitors ◆ Operating Current and Voltage
Under normal operating conditions, capacitors should operate at their rated current, and the maximum operating current should not exceed 1.1 times the rated current. Simultaneously, the difference between the three-phase currents should be controlled within 5%. Capacitors are highly sensitive to voltage fluctuations. Since capacitor losses are proportional to the square of the voltage, excessively high voltages can cause severe overheating, accelerate insulation aging and shorten lifespan, and may even lead to electrical breakdown. Therefore, capacitors should operate at their rated voltage, generally not exceeding 1.05 times the rated value, and the maximum operating voltage should not exceed 1.1 times the rated voltage.
◆ Harmonic and Relay Protection
Because capacitor circuits form LC circuits, they are prone to resonance with certain harmonics, leading to the emergence of higher harmonics, which in turn increases current and voltage. These higher harmonic currents are very harmful to capacitors and may cause capacitor breakdown and phase-to-phase short circuits. Therefore, when capacitors are operating normally, it is necessary to connect a reactor with an appropriate inductive reactance in series with them to limit the impact of harmonic currents. Relay protection is a key measure to ensure the safe and stable operation of power systems. Currently, the relay protection devices produced by well-known domestic electrical manufacturers have mature technology and can effectively disconnect faulty capacitors.
◆ Circuit breaker issues and temperature control
Because capacitor discharge takes time, reclosing is prohibited while the capacitor bank is energized. Otherwise, it may cause the capacitor casing to swell, leak oil, or even explode. Therefore, reclosing must be performed 3 minutes after the circuit breaker has opened, and the capacitor bank should not be equipped with an automatic reclosing device. Under normal operating conditions, the rated ambient temperature of the capacitor bank is typically between 40℃ and -25℃, and the internal dielectric temperature should be maintained below 65℃, and should not exceed 70℃, to prevent thermal breakdown or bulging.
Compensation principle of power capacitors
In a power system, the compensation principle of a capacitor is similar to that of a generator producing capacitive reactive current. By connecting a device with capacitive power load and an inductive power load in parallel to the same capacitor, energy can be converted between these two types of loads. This reduces the load on transformers and transmission lines in the power grid, thereby increasing the output power capacity. Simultaneously, while maintaining a certain level of active power output, losses in the power supply system are also reduced. Therefore, capacitors play an indispensable role as reactive power compensation devices in power systems. Currently, using parallel capacitors for reactive power compensation has become a common practice.
Characteristics of power capacitor compensation
Advantages: Power capacitor reactive power compensation devices offer numerous advantages, such as convenient installation, flexible location additions and subtractions, low active power loss (only about 0.4% of rated capacity), short construction period, low investment, and simple operation and maintenance. Furthermore, even if individual capacitor banks are damaged, it will not affect the operation of the entire capacitor bank.
Disadvantages: However, power capacitor reactive power compensation devices also have some shortcomings. For example, they can only perform stepped regulation and cannot achieve smooth regulation; in poorly ventilated environments, capacitors are prone to expansion and explosion when the operating temperature exceeds 70°C; their voltage characteristics are poor, their short-circuit stability is poor, and residual charge is generated after disconnection. Furthermore, low reactive power compensation accuracy may affect the compensation effect, and difficulties in operation and management, as well as insufficient attention to capacitor safety, also contribute to the problem.
reactive power compensation method
High-voltage distributed compensation: This is a reactive power compensation capacitor installed on the high-voltage side of a single transformer, designed to improve power supply voltage quality, and is mainly used in urban high-voltage power distribution.
High-voltage centralized compensation: This method involves installing capacitors on the 6kV to 10kV high-voltage busbars of substations or user-run step-down substations. It can also be installed on the low-voltage busbars of the user's main distribution room, suitable for locations with concentrated loads, close proximity to the distribution busbars, and large compensation capacity. When a user has a certain high-voltage load, this compensation method can reduce reactive power consumption in the power system and provide some compensation. Its advantages include ease of automatic switching, reasonable improvement of the user's power factor, high utilization rate, low investment, and ease of maintenance and regulation, but its economic compensation benefits are relatively poor.
Low-voltage distributed compensation: This method will be described in detail in subsequent paragraphs.
The principle and characteristics of low-voltage distributed compensation
Low-voltage distributed compensation is a method of distributing one or more low-voltage capacitor banks near electrical equipment based on the reactive power demand of individual devices. It aims to compensate for the reactive power of all high- and low-voltage lines and transformers upstream of the installation location. This method activates reactive power compensation when the equipment is running and deactivates it when the equipment is shut down, thereby reducing reactive power flow in the distribution network and transformers, and thus reducing active power losses. Furthermore, it can reduce the conductor cross-section and transformer capacity, and requires less space. However, low-voltage distributed compensation also has some drawbacks, such as lower utilization rate, higher investment, and it is not suitable for motors with variable speed operation, forward and reverse operation, or inching, stall, and reverse braking.
The principle and advantages of low-voltage centralized compensation
Low-voltage centralized compensation involves connecting low-voltage capacitors to the low-voltage busbar side of the distribution transformer via a low-voltage switch, and using reactive power compensation switching devices for control and protection. This compensation method directly controls the switching of capacitors based on the reactive load on the low-voltage busbar, and operates on a group-by-group basis, making smooth adjustment impossible. Nevertheless, low-voltage centralized compensation has advantages such as simple wiring and low operation and maintenance workload. It can effectively achieve local reactive power balance, improve transformer utilization, reduce network losses, and has significant economic benefits, thus becoming one of the commonly used methods for reactive power compensation.
Calculation method for capacitor compensation capacity
The reactive power compensation capacity should be determined based on the reactive power curve or a specific calculation method. The calculation formula is: QC=p(tgφ1-tgφ2) or QC=pqc(1), where Qc represents the capacity of the compensation capacitor, P represents the active power of the load, COSφ1 and COSφ2 represent the load power factor before and after compensation, and qc represents the reactive power compensation rate.
