Function of Voltage Regulator and Parallel Generator Operation

The voltage regulator's primary function is to maintain a stable and precise generator voltage under no-load conditions and as loads fluctuate. When generators operate in parallel, a parallel compensation circuit is required to help voltage regulators manage reactive power distribution among the generators.

Reactive power imbalances between generators can occur when the voltage regulator adjusts the generator’s excitation due to load variations, prime mover speed changes, thermal drift, and other factors. 

These excitation changes may lead to significant circulating currents flowing between generators (Drawing 20).

Drawing 20:  Circulating Currents

This results in the generator with higher field excitation attempting to supply power to the generator with lower field excitation, forcing it to match the output voltage of the more highly excited generator.

The parallel compensation circuit adjusts the voltage regulator by increasing the field excitation on the generator with lower excitation and decreasing it on the generator with higher excitation. By regulating the reactive load, this circuit effectively eliminates unwanted circulating currents.

Drawing 21: A - Equal Pressure. B - Unequal Pressure

An analogy for how one generator attempts to power another through circulating currents can be compared to two water pipes of equal diameter feeding into a single pipe. When both pipes have the same water pressure, they supply an equal amount of water to the common pipe (Drawing 21A). However, if one pipe experiences a slight drop in pressure, the second pipe will compensate by supplying more water to maintain the overall flow (Drawing 21B). Additionally, because the pressure in the second pipe is now higher than in the first, water will start flowing from the second pipe into the first in an attempt to equalize the pressure between them.

Reactive Droop Compensation and Reactive Differential Compensation

There are two primary types of parallel compensation circuits. The most commonly used method is parallel droop compensation, also known by its IEEE designation as reactive droop compensation. The second method is crosscurrent compensation, referred to in IEEE terminology as reactive differential compensation.

Drawing 22:  Reactive Droop Compensation

When reactive droop compensation is used to parallel two or more generators, each droop circuit operates independently (Drawing 22). A typical parallel droop circuit consists of a current transformer and a paralleling module. The module includes a burden resistor and a switch connected across the primary winding of a transformer (Drawing 23).

Drawing 23: Paralleling Module

A switch in the paralleling module's transformer primary is used to short the secondary winding of the current transformer and the burden resistor, allowing the generator to operate independently of the paralleling system.

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The secondary of the current transformer is connected to the paralleling circuit, where a burden resistor is placed across its output terminals (Drawing 24). The secondary current of the transformer induces a voltage across this resistor, which is then vectorially added to the line voltage to generate an error signal for the voltage regulator. The voltage across the burden resistor is directly proportional to the magnitude of the line current and maintains the same phase as the current flowing through the transformer's primary.

Drawing 24:  Burden Resistor

Drawing 25:  Development of Current Transformer Error Signal

The error signal generated across the current transformer’s burden resistor must be applied in a way that ensures no corrective signal is produced when the load has a unity power factor. This prevents unnecessary excitation changes due to the load. By adjusting the burden resistor voltage to be 90 electrical degrees out of phase with the system voltage when the generator operates at unity power factor, an appropriate error signal can be generated. Drawing 25 illustrates the vector relationship between system voltage and burden resistor voltage required to produce this error signal.

In a three-phase system, line-to-neutral voltages—regardless of internal connections—are displaced by 120 electrical degrees from one another. By leveraging this phase displacement, it is possible to sense system voltage from line to line, creating a line voltage that is shifted by 30 electrical degrees relative to the phase voltage. At unity power factor (resistive loads), the voltage signal from the current transformer’s burden resistor will be displaced by 90 electrical degrees from the system or line voltage (Drawing 26A).

Drawing 26: Phase Displacement

At unity power factor, the vector diagram (Drawing 26B) shows that the burden resistor voltage (VB) and the sensing voltage (Vac) are 90 electrical degrees apart. When a reactive load is applied to the generator, the burden resistor voltage vector shifts either clockwise or counterclockwise, depending on the load type — capacitive or inductive.

Drawing 27:  Vector Diagram of Capacitive Load

If the generator operates with a leading power factor (capacitive load), the burden resistor voltage vector will rotate counterclockwise from its unity power factor position (Drawing 27). As a result, the phase angle between the line voltage and the burden resistor voltage increases, causing the regulator sensing voltage to decrease. This reduction in the sensing signal prompts the regulator to increase generator excitation.

Drawing 28:  Vector Diagram of Inductive Load

When the generator operates with a lagging power factor (inductive load), the burden resistor voltage vector rotates clockwise from its unity power factor position (Drawing 28). This brings the phase angle between the line voltage and burden resistor voltage more in phase, increasing the voltage sensing signal to the regulator. In response to this larger signal, the regulator decreases generator excitation.

The paralleling module supplies an error signal to the voltage regulator, which controls the generator's excitation level. When generators are paralleled, the regulator detects this slight increase in sensing voltage and reduces excitation to the generator field, causing a voltage droop in the generator’s output.

The extent of voltage droop in a generator system can be adjusted using the burden resistor and the current transformer ratio. A typical burden resistor has a resistance of one ohm and features an adjustable slide tap to vary the voltage across it. The error signal sent to the voltage regulator depends on both the magnitude and vector angle of the voltage across the burden resistor. This voltage is determined by the secondary (output) current of the current transformer. Standard current transformers are designed for a 5-ampere secondary current with a 25VA maximum burden rating. According to Ohm’s Law (Voltage = Current × Resistance), the maximum voltage across a one-ohm burden resistor is 5 volts. This means that at full load, the voltage developed across the burden resistor is typically about 5% of the system’s output voltage (e.g., 120/208V, 240/416V, 480/600V) to minimize circulating currents.

When generators are paralleled with reactive droop compensation, most systems are set to operate at maximum droop. The burden resistor is adjusted for maximum resistance, ensuring maximum voltage across it. Operating at maximum droop allows for optimal control of circulating currents. If the system voltage droop is set to less than 3%, the regulator and paralleling circuit may struggle to effectively manage circulating currents.

Voltage regulators with single-phase sensing typically provide up to 8% maximum droop, whereas three-phase sensing regulators offer about 6% droop. Single-phase sensing results in greater droop because the average error signal is proportionally larger in comparison to the single-phase sensing voltage than to the three-phase sensing voltage. When paralleling generators on the same bus with different sensing types, adjustments to the burden resistor must be made to compensate for sensing differences.

Since the burden resistor voltage depends on the generator’s line current through the current transformer, any changes in load power factor will affect the burden resistor voltage. As a result, when a reactive lagging power factor load increases, the bus voltage experiences a greater droop. Conversely, an increase in a capacitive leading power factor load causes the bus voltage to rise. The magnitude of these voltage changes depends on both the load size and its power factor.

To prevent voltage fluctuations due to power factor changes, an alternative circuit can be used where the current transformers of individual regulators are interconnected. This method, known as crosscurrent compensation (or reactive differential compensation), enables generators to operate in parallel without voltage droop caused by the error signal.

Drawing 29 illustrates two generators operating in parallel with reactive differential compensation. The interconnection of their current transformers (CTs) is shown:

• On Generator 1, the polarity-marked terminal of CT1 is connected to the non-polarity terminal of CT2 on Generator 2.
• On Generator 2, the polarity-marked terminal of CT2 is connected to the non-polarity terminal of CT1 on Generator 1.

This configuration ensures proper compensation, allowing stable voltage levels across the system.

Drawing 29:  Reactive Differential Compensation (Two Generators Paralleled)

Although the voltage involved is AC, a clearer understanding of the closed crosscurrent loop's operation can be achieved through DC voltage analysis. First, the polarity markings on the generator line and current transformer indicate the current direction. The line current always flows into the polarity mark on the generator line, while the current flows out of the polarity mark on the current transformer (Drawing 30).

Drawing 30:  Burden Resistor

The current leaving the polarity marks of the current transformer splits into two paths (Drawing 29). One path flows through the burden resistor in the paralleling module, while the other flows through the crosscurrent loop. The current in the crosscurrent loop enters the burden resistor in the adjacent parallel circuit and opposes the current produced by that circuit’s own current transformer. As a result, the opposing currents cancel each other out, leading to zero net current flow through the burden resistor. Consequently, no voltage develops across the burden resistor, preventing any droop in line voltage.

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If one generator begins to take on more reactive load than the other, the line current increases, causing a rise in the secondary current of the current transformer. This results in a higher voltage across the burden resistor in the paralleling circuit, prompting the voltage regulator to reduce excitation in that generator, thereby decreasing the line current. The increased current in the crosscurrent loop, caused by the first generator’s imbalance, induces a voltage across the second generator’s paralleling burden resistor that opposes the normal voltage generated by its own current transformer. Instead of causing a voltage droop, this opposite polarity increases the line voltage. This self-regulating effect allows the parallel generating system to maintain balance.

Parallel Operation of three generators TAIYO (PMS BEMAC UGS-21) (video)

Automatic load sharing between parallel generators TAIYO (PMS BEMAC UGS-21) (video)

For reactive differential compensation to function correctly, all paralleling current transformers from generators supplying power to the bus must be connected into the crosscurrent loop. These transformers must have identical ratios to ensure each contributes the same current, effectively canceling the voltage across the burden resistor. When paralleling generators of different sizes, the current transformer ratios must be adjusted to provide similar secondary current levels. Otherwise, improper cancellation will lead to circulating currents between generators, causing an imbalance.

Additionally, all generators must have the same burden resistor settings in their voltage regulator paralleling circuits. Consistent burden resistor settings ensure that when an imbalance occurs, the current flowing in the crosscurrent loop establishes a proportional voltage across each burden resistor, maintaining balanced reactive loads across the generator system.

Drawing 31:  Reactive Differential Compensation

For proper operation, all generators must be connected to the crosscurrent loop. Paralleling with an infinite bus (utility) is virtually impossible. If a generating system using crosscurrent compensation were to be paralleled with a utility outside the loop, opening a switch contact anywhere in the loop would force all generators to operate in droop mode.

To comply with most safety codes, a ground connection must be placed on the secondary side of the current transformer (CT). However, only one ground connection should be installed; otherwise, the current transformers will be shorted out (Drawing 31).

Isolation between the regulator sensing circuit and the current transformer circuit is necessary for crosscurrent compensation circuits. AVRs and paralleling circuits achieve this by using isolation transformers, which effectively separate the crosscurrent loop from the regulator's input voltage circuit.

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