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).
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).
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.
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.
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 IL in Drawing 18. This parameter is known as "apparent power".
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.
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:
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.
- Automatic Voltage Regulators. What is a generator AVR or Automatic Voltage Regulator?
- Automatic Voltage Regulator and Parallel Operation of generators. Voltage droop
- Automatic Voltage Regulator. Real power, Reactive power, Apparent power. KW, KVAR, KVA
- Function of Voltage Regulator and Parallel Generator Operation
- The Current Transformer Unit - Parallel Switch. Paralleling Different Size Generators
- Checking and Troubleshooting a Reactive Compensation Circuit for the AVR in an Isolated AC Bus
- ACB Trouble. The generator does not connect to the main busbars. Troubleshooting
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