Capacitance compensation

The electrical equipment of the power system generates reactive power when it is in use, and it is usually inductive. The efficiency of its power supply capacity is reduced, and the way to increase the capacitance in the system can be improved. Power capacitance compensation is also called power factor compensation,

The role of capacitanceedit
1. The capacitor can maintain the voltage at a higher upper limit in the AC circuit. Near peak value, high charge and low discharge, can improve the stability of increasing circuit voltage.
2. Provide current compensation for the sudden start of a large current load. The power compensation capacitor bank can provide a huge instant current, which can reduce the impact on the power grid.
3. The phase of a large number of inductive loads in the circuit to the power grid is deviated (the phase of the inductive component is biased and the AC current is lagging, and the voltage phase is 90 degrees ahead), and the characteristics of the capacitor in the circuit are exactly the opposite of that of the inductance, which acts as compensation.

Principle of capacitance compensationedit
In an AC circuit, the phase difference of the voltage and current of resistance, inductance, inductance, and capacitance components is that in a pure resistance circuit, the current and voltage are in phase; in a pure capacitance circuit, the current leads the voltage by 90°; in a pure inductance circuit, the current lags behind Voltage 90°. From the perspective of power supply, the ideal load is converted from P to S, and the power factor cosφ is 1. The utilization rate of the power supply equipment at this time is the highest. This is possible because of resistance. Most of the electrical load devices in the circuit are inductive in nature, which causes the total current of the system to lag behind the voltage. As a result, in the power factor triangle, the reactive power Q side increases, the power factor decreases, and the efficiency of the power supply equipment decreases.The power triangle is a right-angled triangle. Cosφ (the cosine of the φ angle) is used to reflect the level of power consumption. A large number of inductive loads make the power equipment from power generation to power consumption not fully applied in the power system. Part of the electric energy is exchanged back and forth between the generation, transmission, transformation, and distribution systems and user equipment.
To understand reactive power from another aspect, reactive power is not useless. It is a necessary condition for inductive equipment to establish a magnetic field. Without reactive power, our transformers and motors cannot work properly. Therefore, trying to solve the problem of reducing reactive power is the correct solution.In practical applications, the phase difference between the capacitor current and the inductor current is 180°, which is called antiphase. This complementary feature can be used to connect a corresponding number of capacitors in parallel in the power distribution system. The reactive capacitive current that is ahead of the voltage is used to offset the reactive inductive current that is lagging behind the voltage, so that the active power component in the system is increased, cosφ is improved, and the reactive current is exchanged between the internal devices of the system. This reduces the capacity of part of the power supply equipment occupied by reactive power, thereby improving the power factor of the system, thereby increasing the utilization rate of electric energy.
The characteristics and changing laws of the voltage and current in the power triangle and RLC hybrid circuit are shown in the figure. In the two branches of the circuit, the resistance and the inductance form the RL branch. Due to the series effect of the resistance R and the inductance L, its current phase obviously lags behind the voltage phase. The existence of the resistance makes this lag no longer 90 °, in the impedance triangle, it depends on the ratio of resistance R to inductive reactance XL.According to the parallelogram method, the impedance triangle can be obtained according to the value of its resistance and inductance, and the angle of φ1 can be obtained. In the other branch, the current phase of the capacitor Ic leads the voltage by 90°. The total current in the system is not the algebraic sum of the above two, but the phasor sum of capacitance, inductance and resistance current. It can be seen that after the compensation capacitor is put in, the angle of φ2 is still obtained according to the parallelogram method after the capacitor current offsets a part of the inductor current. It can be seen that the angle of φ2 is smaller than φ1; the cosine value cosφ is improved, the reactive power is reduced, and the total current of the system is reduced.

Basic principles of compensationedit
1. Under-compensated
The compensated capacitor current must be smaller than the compensated inductor current. After compensation, there is still a certain amount of inductive reactive current, so that cosφ is less than 1 but close to 1.
2. Full compensation
Configure the capacitor according to the inductive actual load current, IC=IL cancels all the inductive current with the capacitive current, making cosφ equal to 1.
3. Overcompensation
A large amount of capacitors are put in, after all the inductive current is cancelled, there is still a part of the capacitive current, and the original inductive load is transformed into a capacitive load. The power factor cosφ is still less than 1.
In the above three cases, after analyzing the circuit rules, the basic principle of compensation is determined to be the most reasonable under compensation. Full compensation In the RLC hybrid circuit, if the inductor current is equal to the capacitor current, current resonance will occur in the system, and the equipment will generate an inrush current several times the rated value, endangering the safety of the system and equipment.
Overcompensation is neither economical nor reasonable. When the system load is converted to capacitive, the power factor will decrease after the power factor exceeds 1. Moreover, it may cause circuit current resonance when it exceeds l. The above two compensation methods are obviously undesirable.
The basic principle of compensation is that undercompensation must be used, and the compensated power factor is required to be less than 1, and as close as possible to 1. In order to prevent resonance, the upper limit is generally determined at 0.95.

Design reference editor
Generally speaking, the low-voltage capacitor compensation cabinet is composed of cabinet shell, bus bar, circuit breaker, isolating switch, thermal relay, contactor, lightning arrester, capacitor, reactor, primary and secondary wires, terminal block, power factor automatic compensation control device, panel Instrument and other components.
The presence of nonlinear components (harmonics) in the load will cause other high-frequency (high-order harmonics) currents to flow through the capacitor circuit in addition to the power frequency fundamental current in the capacitor circuit, causing the capacitor to generate overvoltage, Damage due to over-current, over-capacitance, over-temperature, etc., or failure of the capacitor bank to be put into operation; in this case, in addition to using a special "filter capacitor" to increase its own resistance (the price is higher); By selecting and matching appropriate reactors to form a filter circuit, filter out certain strong higher harmonics; choosing a capacitor with a higher rated voltage is also one of the ways to reduce harmonic accidents.
For a load with a capacity of 700KW, you can first measure its natural power factor value, which is the value of the power factor when the load is started without a capacitor. If there is no way to accurately measure, it is estimated that most of your loads are motors. Estimate with the power factor cosφ1=0.70. If you want to increase the power factor to 0.90 in the rated state, you need to compensate the capacitor capacity:Before compensation: cosφ1=0.70, φ1=0.7953, tgφ1=1.020
After compensation: cosφ2=0.90, φ2=0.451, tgφ2=0.483
Qc=Pe·(tgφ1-tgφ2)=700×(1.020-0.483)=375.9(Kvar)
Rounding, about 378Kvar capacitors need to be compensated, if you choose a single 14Kvar capacitor bank, you need 27.

Technical features
1. Voltage priority
Automatically switch capacitors according to the voltage quality requirements. When the voltage exceeds the highest setting value, the capacitor bank will be gradually removed until the voltage is qualified. When the voltage is lower than the minimum setting value, the capacitor bank shall be gradually put into operation under the condition of not overloading, so that the bus voltage is always in the specified range.
2. Reactive power automatic compensation function
Under the voltage priority principle, the capacitor bank is automatically switched on and off according to the load's reactive power, so that the system is always in a state of minimum reactive power loss.
3. Intelligent control function
Before automatically issuing an action command, first probe all possible over-limit values ​​after the action, reducing the number of actions.
4. Abnormal alarm function
When the capacitor control loop relay protection action fails and the controller automatically blocks the automatic control of the regrouped capacitor.

5. Fuzzy control function
How to implement the comprehensive control principle when the system is at the high end of the qualified voltage range and in a specific environment is a difficult point in the design of this series of products. Due to many factors on site (such as configuration environment, power status, operating time, user restrictions on the number of actions, etc. The frequent actions caused by) are the most worrying for users. The application of fuzzy control takes into account the above factors so that this "blind zone" can be reasonably resolved.
6. Comprehensive protection function
Each device has switch protection (optional), overvoltage, loss of voltage, overcurrent (short circuit) and zero sequence relay protection, double star unbalance protection, fuse overcurrent protection, zinc oxide arrester, grounding protection, quick-break protection Wait.

The main technical parameters
1. Rated voltage (AC) 6KV, 10KV
2. System voltage sampling (AC) 100V (PT secondary line voltage)
3. AC current sampling 0～5A (if PT takes the secondary A and C phase line voltage of 10KV side, CT should take B phase current)
4. Voltage setting value 6～6.6KV 10～11KV adjustable
5. The action interval is adjustable from 1 to 60 minutes
6. The power factor setting value is adjustable from 0.8 to 0.99
7. Current transformer changes 50～5000/5A adjustable
8. The action requires system stabilization time 2～10 minutes adjustable
Use environment
1. Ambient temperature -15℃～+45℃
2. Relative humidity ≤85%
3. Altitude ≤2000m (above 2000m adopts high prototype)
4. There is no explosive and flammable dangerous goods in the surrounding medium, no gas that can damage insulation and corrode metal, no conductive dust, no severe vibration and no bumps in the installation site.
5. The power supply complies with national standards and has no strong harmonic components.

Low-voltage reactive power dynamic device
I. Overview
The WDB-K low-voltage reactive power dynamic compensation device adopts high-power thyristor switching switches. The controller can control the thyristor switches to quickly switch the multi-level capacitor banks according to the system voltage, reactive power and two-phase criteria. The thyristor switch adopts the zero-crossing trigger mode, which can realize the capacitor input without inrush current and impact, and achieve the purpose of stabilizing the system voltage, compensating the reactive power of the grid, improving the power factor, and improving the load-carrying capacity of the transformer. It can be widely used in electric power, metallurgy, petroleum, ports, chemicals, building materials and other industrial and mining enterprises and residential power distribution systems.
2. Device structure and technical performance of main components
1. Device structure
The WDB-K low-voltage reactive power dynamic compensation device is composed of a controller, a non-contact switch bank, a parallel capacitor bank, a reactor, a discharge device and a protection circuit. The whole machine is designed as a mechatronics.

2. Technical performance of main components
(1) Controller
The WDB-K low-voltage reactive power dynamic compensation device controller is a new digital design, software and hardware modularization, high integration, electromagnetic compatibility, and strong anti-interference ability. There are 12 output terminals, which can realize phase separation, balance, and phase addition. Balance compensation in three ways. It has a wide range of applications and can meet the compensation needs of different loads. It can control the switching of the non-contact switch group according to the system voltage and reactive power. It has two operation modes, manual and automatic, and has protection functions such as overvoltage removal, overvoltage lockout, undervoltage removal, and overtemperature alarm.
(2) Non-contact switch group
The non-contact switch group is the main executive element of the device. It consists of a thyristor switch, a radiator, a fan, a temperature control switch, a zero-crossing trigger module and a resistance-capacitance absorption circuit. The integrated design single group can control the maximum capacity of 90kvar, and the thyristor switch Imported components, high power, high safety factor.
(3) Parallel capacitor bank
Choose high-quality self-healing shunt capacitors, which can be flexibly coded and combined according to different capacities, with many switching stages, and large-capacity compensation can be done at one time.

3. Basic working principle
When the device is working, the controller monitors the changes of system voltage and reactive power in real time. When the system voltage is lower than the power supply standard or the reactive power reaches the set capacitor bank switching threshold, the controller will give a switching command. The zero-crossing circuit quickly detects the voltage across the thyristor (that is, the voltage difference between the capacitor and the system), and when the voltage at both ends is zero, the thyristor is triggered, and the capacitor bank realizes no inrush current input or no inrush current removal.
 the main technical parameters
1. Rated voltage AC220V/380V±10% 50Hz
2. Wiring mode three-phase four-wire
3. Switching based on system voltage and reactive power
4. Response time ≤20ms
5. Switching delay 0.1～30s (continuously adjustable)
6. Switching precision average ≤+2%
7. Compensation capacity 60kvar～1080kvar
8. The number of switching stages 1～18
Five, use environmental conditions
1. Working environment temperature -25℃～+45℃
2. Relative air humidity ≤85%
3. Altitude ≤2000m (above 2000m adopts high prototype)
4. Installation environment No flammable, explosive, chemical corrosion, flooding and severe vibration places
5. Installation method: indoor screen type, outdoor box type
6. Installation conditions The harmonic content in the power grid meets the requirements of the 0.38kV clause in GB/T14549

 protection function
It has multiple protections for overcurrent, overvoltage, undervoltage, and temperature overrun. The device can automatically exit operation in the event of external failure and power failure, and automatically restore after power is supplied.