Conversion of Capacity of Reactive Power Compensation Cabinet Capacitors

In the field of reactive power compensation, a common question we often ask is: What is the capacity of a capacitor?

The term 'capacity' here also refers to the additional capacity of a capacitor, specifically the power of the capacitor, expressed in KVAR (kilovolt ampere) per unit.

Professional knowledge is widespread

From the above formula, we can see the relationship between the power and voltage of a capacitor:

Q=2πfCU2

Q represents the power of the capacitor, unit var

F represents the system frequency, 50Hz/60Hz

C is the capacitance of the capacitor, unit uF (microfarads)

U represents system voltage, unit kV (kilovolts)

From the above expression, it can be seen that the power of a capacitor is directly proportional to the square of the voltage applied to both ends of the capacitor.

Each capacitor has a parameter called additional voltage, which corresponds to an additional power.

For example, choose a capacitor with a voltage of 450V and an additional power of 30kvar.

Question 1: What is the input power of a capacitor with an additional voltage of 450V and an additional power of 30kvar used in a 400V system?

This is the result we often encounter, where the additional voltage of capacitors is higher than the additional voltage of the system.

By using the above formula, we can quickly calculate:

Q400=Q450×（4002/4502）

=30×（4002/4502）

≈23.7kvar

Question 2: Why choose capacitors with additional voltage higher than the system voltage?

Capacitors must not be quickly damaged when subjected to overvoltage persecution. In order to ensure the safe operation of capacitors, it is necessary to choose capacitors with additional voltage greater than the system voltage.

At this stage, we understand that if the reactive power compensation circuit is designed as a pure capacitor, the input power of the reactive power compensation circuit needs to be converted based on the additional voltage of the capacitor and the system voltage.

This is also what we often refer to as assembly power (assembly capacity) and input power (input capacity).

Assembly power often refers to the additional power of capacitors;

Input power often refers to the actual input power of a capacitor at the system voltage.

Referring to the example above, we can know that if a capacitor with an additional voltage of 450V and 30kvar is applied to a 400V reactive power compensation system, the installed capacity of this system is 30kvar, and its input capacity is 23.7 kvar.

Question 3: What is the power of the compensation circuit composed of capacitors and reactors when capacitors are connected in series with reactance?

It is necessary to consider the following achievements: 1) The reactor raises the terminal voltage of the capacitor; 2) The reactive power of reactors and capacitors tends to be the same.

The change in reactive power compensation branch power after connecting the reactor in series is calculated according to the following formula:

QL+C=QC/(1-reactance rate)

In this

QL+C represents the power of capacitors and reactors connected in series in a branch circuit

QC shows the power before connecting capacitors in series with reactors

Example: The compensation circuit adopts capacitors with an additional voltage of 480V and an additional power of 60kvar, which are applied in the 400V system.

At this point, the pure capacitor compensates for the input power of the branch circuit

Q400=Q480×（4002/4802）

=60×（4002/4802）

≈41.667kvar

When this capacitor branch is connected in series with 7% reactance, the compensation income of the capacitor+reactor is input power

QL+C=QC/(1-reactance rate)

=41.667/(1-7%)

≈44.80kvar

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