Generator sets are commonly operated in parallel to enhance fuel efficiency and increase the reliability of the power supply. When generators are connected in parallel, fuel economy is optimized by activating only the necessary generators to meet the load demand at any given moment. Operating each generator close to its full capacity ensures efficient fuel consumption.
The reliability of the power system is also improved since backup generators are available when others are in use. Additionally, protective systems can be implemented to detect faults and isolate the affected part while maintaining power to the rest of the system. If one generator fails or develops a problem, it can be shut down, with the remaining generators taking over the load.
Given the benefits of parallel operation, the installation of multiple generators has become standard in applications ranging from standby and prime power to both portable and stationary power, as well as for commercial and military use. This trend continues to grow.
When operating generators in parallel, careful attention must be paid to two main control systems of the generator set: the voltage regulator and the speed governor. This discussion focuses on the voltage regulator.
Parallel Operation and the Voltage Regulator
To illustrate the impact of the voltage regulator on a generator system, consider an example with two batteries and a load. In the scenario shown in Drawing 1, if the two batteries have exactly equal voltages (open circuit), they would divide the load evenly, with each battery supplying its share. However, if the two voltages are not perfectly matched, the load will not be distributed equally, diminishing the advantages of using two batteries.
Additionally, the division of load is highly sensitive to voltage imbalances. In Drawing 2, the battery with an output voltage of 102 volts (B2) will dictate the load voltage, causing the load to experience a higher current. Battery B1, which has a lower terminal voltage, is being charged by B2 at a rate of 5 amperes. This charging current is clearly not proportional to the voltage difference and leads to a load current on B2 that exceeds 6 amperes.
In Drawing 3, when Battery B1 is 2 volts higher in voltage than Battery B2, B1 takes on the responsibility of supplying the load while simultaneously charging B2. As a result, the total load current becomes 6 amperes, with B1 providing both the load current and the charging current for B2.
The voltage shown in the diagrams represents the open-circuit voltage. Once the batteries are connected in parallel, their terminal voltages equalize.
Two generators operating in parallel to supply a common load function similarly. If both generators have identical open-circuit voltages, they will share the load equally (see Drawing 4). However, even a slight voltage difference can lead to an unbalanced load distribution (Drawing 5) or, in extreme cases, circulating currents (Drawing 6).
In practice, precisely matching voltages is not feasible. Therefore, a method must be implemented to facilitate easy load sharing between paralleled generators.
Since circulating current or load imbalance arises from voltage mismatches, the voltage regulator can help mitigate these issues. This is achieved through paralleling compensation circuits, such as reactive droop compensation or reactive crosscurrent compensation, which will be discussed in detail in later papers.
The reactive droop circuit, the simplest and most commonly used method, operates based on the principle illustrated by the curve in Drawing 7.
A regulator designed for precise voltage control includes an additional circuit that receives a current signal from the generator's output. This current signal is combined with the generator's sensing voltage signal to create a vector-summed voltage proportional to the reactive load.
For instance, if the voltage drops from 480V to 458V as the reactive load increases from no load to rated kvar load, the resulting voltage droop is -4.3%. When two generators operate in parallel with matching droop curves and properly adjusted voltage setpoints, any imbalance—where one generator takes on more load while the other takes on less—will prompt the droop circuits to adjust voltage setpoints, restoring balance. With droop compensation, the bus voltage naturally decreases as the reactive load changes.
Incorporating this characteristic into the regulator of each generator operating in parallel ensures straightforward and effective load sharing.
Before closing the breaker to parallel two generators, their voltages should be matched to minimize current surges at the moment of breaker closure.
When voltages are balanced and the total load is 100 Amperes, each generator supplies its proportional share. If the voltage of G2 increases, its output current will also increase, leading to a voltage droop that counteracts the rise. Conversely, G1 will experience a decrease in output current, triggering its droop circuit to raise its voltage. This dynamic creates a self-regulating load balance control mechanism, ensuring balanced loading when multiple generators operate in parallel.
- 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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