Introduction To Capacitors And Reactors
Reactance includes inductive reactance and capacitive reactance, and reactor includes inductive reactance (inductor) and capacitive reactance (capacitor). Inductance has the function of "blocking AC and blocking DC", that is, in the AC circuit, the characteristics of inductive reactance are used to "pass low-frequency DC and block high-frequency AC". The capacitor has the function of "connecting AC and isolating DC", that is, in the AC circuit, the frequency characteristic of capacitive reactance is used to "connect high-frequency AC and block low-frequency DC".
Capacitors are capacitive loads, mainly used to compensate reactive power and store energy.
Function of capacitance
Capacitor is the most common device in circuit design and is one of the passive components. In short, the active component is the component that needs energy (electricity) source, and the component that does not need energy (electricity) source is the passive component. Capacitors also often play an important role in high-speed circuits. There are many kinds of functions and uses of capacitors. Such as: the role of bypass, decoupling, filtering and energy storage; In the completion of oscillation, synchronization and time constant.
In general, the capacitor we often refer to refers to the parallel capacitor used for reactive power compensation. Generally, we only need to pay attention to three parameters of the power capacitor: the rated voltage, rated capacity and compensation type of the capacitor.
Rated voltage
In general, the rated voltage of the partial compensation capacitor is calculated according to the photographic voltage, and the rated voltage of the common compensation capacitor is calculated according to the line voltage. Generally, capacitors are required to be able to withstand at least 1.1 times the working voltage. Take 400V as an example, 400vx1.1 = 440V. Therefore, the rated voltage of common common common compensation capacitors is 440V or 450V, and the rated voltage of partial compensation capacitors is 250V.
Symbols and wiring of shunt capacitor | Symbols and wiring of common compensation capacitor |
Function of reactor
Reactors are also called inductors. When a conductor is energized, it will generate a magnetic field in a certain space occupied by it. Therefore, all electrical conductors that can carry current have a general sense of inductance. However, the inductance of the long electrified straight conductor is small, and the magnetic field generated is not strong. Therefore, the actual reactor is a wire wound into a solenoid, which is called an air core reactor; Sometimes, in order to make this solenoid have greater inductance, iron core is inserted into the solenoid, which is called iron core reactor.
The common reactors used in the power system are series reactors and parallel reactors. The series reactor is mainly used to limit the short-circuit current, and it is also used in series or parallel with the capacitor in the filter to limit the higher harmonics in the power grid. Reactors in 220kV, 110KV, 35kV and 10kV power grids are used to absorb charging capacitive reactive power of cable lines. The operating voltage can be adjusted by adjusting the number of shunt reactors. EHV shunt reactor has various functions to improve the reactive power related operation of power system.
When the series reactor is used for reactive power compensation, it is mainly used in series with the capacitor. Its main function is to suppress harmonic and prevent harmonic amplification or resonance caused by the capacitor connected to the circuit.
For series reactors, we generally need to pay attention to four parameters, namely, rated voltage of matching capacitor, reactor capacity, reactance rate and reactance type.
Rated voltage of supporting capacitor
When the reactor is connected in series with the front end of the capacitor, the working voltage of the capacitor will be increased, and the increase factor = 1 / (1-reactance rate). Taking 7% reactance rate as an example, under 400V system, the rated voltage of capacitor = 400vx1.1 / (1-7%) ≈ 473V, so the rated voltage of general capacitor is 480v.
Reactor capacity
Reactor capacity = matching capacitor capacity x reactance rate. For example, if 50kvar capacitor is connected in series with 7% reactor, then reactor capacity = 50kvar x 7% = 3.5kvar.
Reactance rate
Reactance ratio refers to the ratio of reactance value of series reactor to capacitance reactance value of capacitor bank. Reactance rate mainly affects the tuning frequency of the system. Tuning frequency = 50Hz * sqrt (1 / reactance rate). 7% reactance tuning frequency is about 189hz, and 14% reactance tuning frequency is about 134hz. When the tuning frequency is less than the harmonic frequency, the harmonic can be suppressed. Therefore, 7% reactance can suppress 5 and more harmonics; 14% reactance can suppress the 3rd and above harmonics.
Function of capacitance compensation cabinet
Function and working principle of the capacitor compensation cabinet the function of the compensation cabinet is: the current is 90 degrees ahead of the voltage, and the parallel connection of capacitors is used to increase the line voltage and reduce the reactive power loss.
1. In the actual power system, most of the loads are asynchronous motors. Its equivalent circuit can be regarded as a series circuit of resistance and inductance. The phase difference between voltage and current is large and the power factor is low. After the capacitor is connected in parallel, the current of the capacitor will offset part of the inductance current, so that the inductance current decreases, the total current decreases, the phase difference between the voltage and the current decreases, and the power factor increases.
2. Generally speaking, the low-voltage capacitance compensation cabinet is composed of cabinet shell, busbar, circuit breaker, isolating switch, thermal relay, contactor, lightning arrester, capacitor, reactor, primary and secondary conductors, terminal strip, power factor automatic compensation control device, panel instrument, etc.
3. In addition to resistive loads, most of the electrical equipment belong to inductive loads (such as fluorescent lamps, transformers, motors and other electrical equipment). These inductive loads change the voltage phase of the power supply (that is, the current lags behind the voltage), so the voltage fluctuates greatly, the reactive power increases, and a large amount of electric energy is wasted.
4. When the power factor is too low, the output current of the power supply is too large and overload occurs. The computer capacitance control system in the capacitance compensation cabinet can solve the above drawbacks. It can automatically set the input of the number of capacitor groups according to the change of the power load to compensate the current, so as to reduce a large amount of reactive current, minimize the power loss of the line, and provide a high-quality power source.
YT Power Quality Solutions
However, the capacitor compensation cabinet cannot cope with fast changing loads or inductive loads. At this time, the latest generation of reactive power compensation products (SVG) must be used.
Based on the principle of voltage source inverter, YTPQC-SVG static var generator adopts insulated gate bipolar transistor (IGBT) to control the amplitude and phase of AC voltage of the inverter, so as to realize reactive power compensation and three-phase load balance. Because the switching frequency of IGBT is very high (up to 25.6khz), SVG can compensate fast reactive load and realize high-precision compensation. SVG is the best product in the field of reactive power control.
We also have another solution for customers with lower budgets: Hybrid reactive power compensation HPFC
To meet the requirement of continuous reactive power compensation with low cost, high reliability and large capacity, a hybrid dynamic reactive power compensation system was proposed based on TSC (thyristor switched capacitor in parallel) and SVG (static var compensator). This system consisting of discrete subsystem TSC and continuous subsystem SVG, coordinated the capacitor switching rating of TSC with dynamic compensation of SVG by using two hybrid control laws based on expert decision making, made full use of their respective advantages. The machine learning approach was applied to avoid repeated movements of capacitors and extend equipment life.
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