Theory Of Reactive Power Compensation

sources of reactive power

Reactive power is an indispensable power exchange specific for AC system, whose distribution and flow in the power system has great effect on power quality, voltage levels and line losses.

Most electrical equipment are working under the principle of electromagnetic induction, such as generators, transformers and motors, etc. They transform and transmit electrical power by establishing and alternating electromagnetic field, where power generator and motors can transform between mechanical energy and electric-magnetic energy.

The energy established for alternating electromagnetic field and magnetic flux is called reactive power, which represents a measurement of energy exchange between the load and the power if some storage devices exist. Therefore in addition to the need of active power, reactive power is also needed in the electricity transmission system, both are indispensable.

Definition of Power Factor

In the power triangle, the fundamental power factor PF is calculated as:

, S is the apparent power, P is the active power.

Traditionally, the reactive power of inductive load is defined as positive, capacitive load negative.

Grid power systems and transformers’ transmission capacity are measured by the apparent power, so the power factor can reflect how much capacity of the grid and the transformer is in effective use. Therefore, we hope for a higher power factor, meaning the reactive power can be minimized, and apparent power is dedicated to supply active power, thus increasing the capacity of active power’s transmission.

Main factors affecting power factor

1, inductive device.

Asynchronous motor, induction furnace, AC welding machines and other equipment are major consumer of reactive power. According to statistics, in the industrial and mining enterprises, out of all reactive power required, Asynchronous motor takes up 60% to 70%; out of all reactive power by all motors, Asynchronous motor take up 60% to 70% under no load condition. But Asynchronous motor needs of reactive power are correlated with its load size, generally at full load its power factor can reach 0.7 to 0.9, while at light load, the power factor will be lower.

2, Transformer.

Power transformer’s impedance (leakage resistance) is about 8% to 11% of its base value, distribution transformer’s impedance is about 2% to 4% of its base value. The majority of distribution transformers work close to saturation, making it very sensitive to voltage. Transformer’s reactive power is generally about 10% to 15% of its rated capacity and at no-load about 33%. Therefore, in order to improve the power factor, light-load operation should be avoided, and trip no-load transformer in a timely manner.

3, deflector.

Thyristor controlled deflector, such as HVDC converter station, rectifier in aluminum and other large industrial need large amount of reactive power in the commutation process. In general, rectifier need about 30% to 40% of DC power as reactive power, Inverter costs about 40% to 60% .

4, voltage fluctuations.

If the voltage on power supply grid rise more than 10%, inductive load’s magnetic flux would saturate, thus raising almost 35% of reactive power. So, measures should be taken so that the power supply voltage as remains stable.

Difficulty in Reactive power transmission

Reactive power, different from active power’s transmission conditions, could not be transmitted in long-distance. When the distance reach a limit, it would be impossible to transmit reactive power The difficulties are:

1, Reactive power’s transmission need significant voltage amplitude gradient, because reactive power can only flow from the high-voltage end to the low-voltage end. But the requirements that the line voltage amplitude throughout the distribution maintain within the (1 ± 5%) pu restrict the ability of transmission.

2, Transmission of reactive power will cause drop in voltage, which contradict the requirements that the grid voltage at each node maintaining within standard range.

3, Reactive power will cause line losses in transmission network, so in order to minimize loss, reactive power has to be minimized. Especially, transmitting a large active power makes restricting reactive power more difficult. So compared to transmission of active power, the long-distance transmission of reactive power is difficult in technique and unsound in economy.

4, Transmission of reactive power may lead to "load rejection", resulting in transient over-voltage at the load, whose seriousness is determined by the reactive power transmitted.

5, The capacity of transformers and cables need to be increased accordingly with transmission of reactive power.

Therefore compensating reactive power at each user’s end is enforced by the government. At the same time, because transformers and power transmission lines at modern time needs much more reactive power than the amount electrical generators can generate, compensation of reactive power at user’s end is indispensable.

Effect of reactive power Balancing

1, To maintain the quality of supply voltage, including voltages and waveforms.

Chinese city network and rural power grids have different levels of power quality problems, for example, some loads end have serious voltage harmonics and flicker problem in City, where in rural areas, voltage is typically lower than standard.

Damage can be done if deviation of voltage reaches beyond a limit. When voltage dropped by 10%, the motor torque reduced by approximately 19%, so if the motor’s load maintain the same, then Slip ratio will increase, leading to winding overheat and accelerated aging of insulation, greatly affecting their life-time. When voltage drop too much, motor will stall, and lights in houses will dim a lot, affecting people's vision and work efficiency.

Besides, voltage deviating too much will have an adverse impact on the power system. If voltage drops too much, the reactive power and line losses will increase and may endanger the stability of power system. If voltage raise too much, electrical equipments’ insulation may be damaged.

2, To reduce line losses..

The line loss overall in China is still high, till 2005, China's line loss is about 7.18%, though lower than the 8.93% in 1980, but a few percentage worse than the international advanced level. In rural low-voltage distribution grid, line loss reaches 12% to 20%.

The significance of reactive power compensation

Reactive power compensation equipment are a device that absorbed or given appropriate reactive power to compensate for the reactive power demand on grid. The purpose of it is to balance the reactive power in each and every hierarchical level. The basic requirements are: The equipment’s reactive power capacity must be slightly larger than or at least equal to the required reactive power from the load. In order to guarantee reliability of system and possible increase of load in future, a moderate redundancy in reactive power capacity is recommended.

Achieve the following results:

1, To improve power factor.

2, To maintain voltage levels.

3, To improve power quality.

4, To improve static and dynamic stability of power system.

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