Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA

In a DC analogy with batteries in parallel, only voltage needs to be controlled for proper load sharing. Similarly, in a mechanical AC analogy, only torque requires control. However, when paralleling AC generators, both variables — torque and excitation — must be properly managed.

• Torque control regulates the division of real power (kilowatts, kW).
• Excitation control manages the division of reactive power (kilovolt-amperes reactive, kVAR).

Both must be precisely controlled to ensure stable and efficient generator operation.

Real power represents the actual work performed by the electrical energy generated. It is supplied by the prime mover in the form of torque, which the generator converts into electrical energy. This energy is then delivered to a load, where it is transformed into useful forms such as heat, light, or mechanical motion (e.g., in a motor).

Reactive power, on the other hand, is the power required by inductive or capacitive loads to store energy during each half-cycle. In the example shown in Drawing 10, the load is purely resistive, such as a heating element. Since no reactive power is needed at any point on the sine wave, the current remains directly proportional to the voltage at all times. Real power can be calculated using Ohm’s Law in this scenario.

When an inductance (Drawing 11) is connected to the generator output, the voltage-current relationship changes. An inductor resists immediate current flow because it must first establish magnetic flux lines in its magnetic circuit. As a result, while the current remains proportional to the voltage, it lags behind the voltage by 90 electrical degrees (Drawing 12).

Drawing 10: Resistive Load

Drawing 11: Inductive Load

Drawing 12: Current Lags Volts

A capacitor behaves differently. It does not allow voltage to develop across its terminals until charge has accumulated on its plates (Drawing 13). Because of this, current must flow before voltage can appear. As a result, capacitor current leads voltage by 90 electrical degrees (Drawing 14).

Drawing 13: Capacitive Load

Drawing 14: Current Leads Volts

For a load with all three elements connected in parallel (Drawing 15), it is convenient to calculate the real and reactive power of each element individually. Using the Law of Superposition (Drawing 16), the three separate calculations can be combined vectorially to determine the total load.

Drawing 15:  Combined RLC Load

Drawing 16: Law of Superposition

Examining I₁ and I₂ reveals that these currents are always opposite in polarity. If they are equal, their net effect is zero. This is why inductive loads are considered to absorb reactive power, while capacitors supply it. Power factor correction capacitors utilize this principle to offset inductive loading. The flow of reactive power is illustrated in Drawing 17.

Drawing 17: LC Loads

The reactive power, measured in VARs, can be calculated using:

The real power, measured in watts, can be determined using:

Additionally, a third parameter can be calculated using the generator voltage and the current I_L in Drawing 18. This parameter is known as "apparent power".

Drawing 18:  Combined RLC Load

These three parameters — real power, reactive power, and apparent power — are interconnected. Given any two, the third can be determined using the Pythagorean Theorem:

Alternatively, they can be determined using vector arithmetic (Drawing 19).

Drawing 19A: Power Triangle

Drawing 19B: Real Power - kilowatts

Drawing 19C: Apparent Power - KVA

Metering is typically used to measure voltage, current, and frequency in standalone generators. In parallel applications, monitoring current provides insight into loading but does not indicate the type of current flow, making load sharing adjustments difficult.

To address this, a meter that measures real power along with current allows power balancing via governor adjustment, followed by current balancing through voltage regulator adjustment. A more effective approach includes an additional meter for reactive power — the Var meter. With both kW and Kvar meters on each generator, optimal governor and voltage regulator adjustments can be made, facilitating efficient load sharing.

Parallel Operation of three generators TAIYO (PMS JACOM JRCS) (video)

Automatic load sharing between parallel generators TAIYO (PMS JACOM JRCS) (video)

Another metering option is the power factor meter, which is often used in place of the Kvar meter. Power factor is defined as the ratio of real power to apparent power:

Drawing 19D: Reactive Power - KVAR

Power factor, influenced by both real and reactive power levels, can be challenging to interpret when monitoring load-sharing performance. Proper instrumentation significantly enhances the operation of systems with parallel generators.

Articles about Automatic Voltage Regulator:

1. Automatic Voltage Regulators. What is a generator AVR or Automatic Voltage Regulator?
2. Automatic Voltage Regulator and Parallel Operation of generators. Voltage droop
3. Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA
4. Function of Voltage Regulator and Parallel Generator Operation
5. The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators
6. Checking and Troubleshooting a Reactive Compensation Circuit for the AVR in an Isolated AC Bus
7. ACB Trouble. The generator does not connect to the main busbars. Troubleshooting