1.Learn about power cosφ

In the power grid, there are two types of capacity:

• Effective power P (kW) is the power that produces useful work in the loads. P = S*Cosφ.
• Reactive power Q (kVAr) is useless power, caused by the inductance of loads such as electric motors, transformers, voltage converters… Q = S*Sinφ.

To evaluate the effect of reactive power on the system,  cos power factor is used.

Describe the relationship between types of electrical power, reactive power compensation

2. Why reactive power compensation?

Reactive power Q does not produce work but causes negative economic and technical effects:

• About the economy: we have to pay for the amount of reactive power consumed.
• Technical: Reactive power causes voltage drop on the line and power loss on the transmission line.

Therefore, we need to have measures to compensate for reactive power Q to limit its influence. That is, we increase the coefficient cosφ.

3.Benefits of improving power factor cosφ:

• Reduce power loss on elements of the power supply system (transformers, lines…).
• Reduce voltage loss on transmission lines.
• Increase power transmission capacity of lines and transformers.

According to the regulations of the Ministry of Industry and Trade:  “Circular Regulations on the purchase and sale of reactive power” effective from 10/12/214, electricity users will be fined if the capacity cosφ is below the allowable level.

4. How to calculate reactive power to compensate

To calculate the reactive power that needs to be compensated to select a capacitor to compensate for a certain load, we need to know the power (P) and the power factor (Cosφ) of that load: Suppose we have the load’s capacity P, the coefficient the capacity of the load is Cosφ1 → tgφ1 (before compensation), the power factor after compensation is Cosφ2 → tgφ2. The reactive power to be compensated is:

Qb = P*(tgφ1 – tgφ2)

To make the calculation process fast, people often use the lookup table to calculate the capacitance of the capacitor.

Coefficient lookup table to calculate the capacity of capacitors.

5. Methods of improving reactive power factor

5.1. Method to improve the natural coefficient of cosφ

Raising the cosφ naturally means finding ways for electricity consumers to reduce the amount of reactive power they need in the supply.

• Change and improve the technological process so that electrical equipment works in the most reasonable mode.
• Replacing under-loaded motors with motors with smaller capacity.
• Limit the engine to idling.
• Where technology allows, use synchronous motors instead of asynchronous motors.
• Replace the transformer that works under load with a transformer with a smaller capacity.

5.2. Method to raise the artificial cosφ coefficient

This method is implemented by placing  reactive power compensator  devices in electricity consuming households. Reactive power compensation devices include:

5.2.1 Synchronous compensator

It is the synchronous motor working in no-load mode.

Advantage: Synchronous compensator has the ability to both produce reactive power and consume reactive power of the electrical network.

Disadvantages: synchronous compensators have rotating parts, so assembly, maintenance and operation are complicated. Synchronous compensators are usually for centralized compensation with large capacity.

5.2.2. Power capacitors

Making the current phase earlier than the voltage, thus, can generate reactive power supply to the electrical network.

• Advantages:

• Small capacity, no rotating part, so it is easy to maintain and operate.
•  Capacitor capacity can be changed according to load development.
• Low cost compared to the synchronous compensator.

• Disadvantages:

• Sensitive to voltage fluctuations and less reliable, especially susceptible to damage when short circuit or voltage exceeds rated. Capacitor life is limited, will be damaged after many years of service.
• When the capacitor is closed to the electrical network, there will be an impulse current, and when the capacitor is cut off from the network, there is still a residual voltage that can be dangerous for the operator.
• Use capacitors in small and medium reactive power consumers (under 5000 kVAr).

5.3.Methods of compensating reactive power with capacitors

There are two methods of compensating reactive power by compensating capacitors:

5.3.1.Static compensation (background compensation)

The compensation arrangement consists of one or more compensating capacitors providing a constant amount of compensation. Control can be done in the following ways:

• Manual: use CB or LBS (load – break switch).
• Semi-automatic: use contactor.
• Connected directly to the load energizing the compensator circuit while closing the load.

• Advantages: simple and low cost.
• Disadvantages: when the load fluctuates, it is likely to lead to overcompensation. This is quite dangerous for systems using transmitters. –> Therefore, this method applies to loads that vary little

5.3.2.Dynamic compensation

(Using automatic compensating capacitor controller): using automatic compensating capacitors, also known as automatic compensating capacitor cabinets, capable of changing the capacitance capacitor capacity to ensure that the power factor achieves the desired value. desired value.+ Advantage: does not cause excess compensation and ensures the desired power factor.

Disadvantage: higher cost than static compensation. –> Therefore, this method is applicable at locations where the active and reactive power varies over a very wide range.

Diagram of the location of compensating capacitors in the power network

5.3.3 Individual compensation (Qc3, Qc7, Qc9)

Separate compensation should be considered when the motor power is significant compared to the mains capacity;

Compensating capacitors connected directly to the terminals of inductive electrical equipment;

The power of the condenser must be limited to the power (kW) of the motor.

Advantages:

• Reduced fines due to reactive power consumption problem.
• Reduce reactive current to the motor.
• Reduced conductor size and loss for all conductors.

Disadvantages:

• Difficult to operate.
• Capacitors only work when the motor is working.
• Causing self-excitation to the motor.

5.3.4 Group compensation (Qc6, Qc8)

Advantages:

• Reduced electricity bills due to reduced reactive power consumption.
• Reduce current to dynamic cabinets, distribution cabinets.
• Reduce cable cross-section to distribution cabinets.
• Reduced power loss on conductors.

Disadvantage:

• when there is a significant change in the load, there is a risk of residual compensation and accompanying overvoltage.

5.3.5 Focus compensation (Qc1, Qc2, Qc4, Qc5)

Applicable when the load is stable and continuous;

The compensating condenser is connected to the low-voltage busbar of the main distribution board and is closed during load operation.

Advantages:

• Reduce fines due to reactive power consumption problem.
• Simple in operation and installation.
• Lightens the transformer load and thus has the ability to develop additional loads as needed.

Disadvantage:

• The reactive current continues to flow all the way to expose the main distribution cabinet of the low voltage network.
• The size of the conductor, the power lost on the wire of the power network after the position of the capacitor is not improved.