Load Sharing: Balancing kW and kVAR Between Generators

When low-voltage diesel generators operate in parallel, balancing the load does not happen automatically. Each generator must contribute its share of real power (kW) and reactive power (kVAR) to share the system load properly.

These two types of load sharing rely on separate systems: speed control for kW and voltage control for kVAR.

Real power (kW) sharing is all about prime mover speed. Each generator's governor adjusts its engine speed, directly affecting how much real power it delivers to the system. If one generator's prime mover shaft is physically spinning slightly faster than the others, it will pick up more kW load. Conversely, if it slows down, it carries less. The key to proper kW sharing is ensuring that the governors across all generators coordinate, allowing them to distribute the load proportionally to their capacities.

Reactive power (kVAR) sharing, on the other hand, is managed by voltage. Each generator's exciter system adjusts the field current to control its output voltage. If one generator's excitation is too high, it will take on more reactive power, potentially causing an imbalance. Proper kVAR sharing requires careful adjustment of each exciter so that reactive load is distributed evenly across the system.

This balance is tricky because kW and kVAR sharing are interdependent yet separate. You can have perfect kW sharing but still encounter problems with kVAR distribution—or vice versa. This means both systems need attention to ensure smooth operation.

To share kW effectively, the governors need to compare the real power output of each generator to the total system load. Depending on the system's design, this can be done using analog signals or digital communication. Analog systems are often faster and more straightforward, making them suitable for mixing equipment from different manufacturers. Digital systems provide more precision and flexibility but usually require matched controllers. Whichever method you use, the goal is to align each generator's speed and real power contribution.

Sharing kVAR requires a similar approach but focuses on voltage regulators instead of governors. The exciter of each generator measures the reactive load it's handling and adjusts to match the system's overall demand. Cross-current compensation is a common method for ensuring that kVAR is shared properly. The system balances excitation levels by linking current transformers from each generator, ensuring no single generator carries more than its share of reactive power.

The challenge in kW and kVAR sharing is aligning everything under changing loads. Sudden load changes can cause transient imbalances, so the control systems must respond quickly. This is where system design plays a critical role. Ensuring that the governors and voltage regulators are tuned to work together—and that the communication between them is reliable—helps maintain stability during steady-state operation and load transitions.

An AVR tries to maintain a set terminal voltage by adjusting the field excitation. A governor tries to maintain a set speed (frequency). These are separate control loops but interact through the electrical and mechanical systems. For example, if the reactive load suddenly shifts, requiring your AVR to boost excitation, the resulting current surge can affect the mechanical dynamics of the generator (due to magnetic coupling and incremental changes in torque) and cause slight deviations in speed. While the effect on speed might be small, it can become noticeable in sensitive systems or those operating near stability limits, requiring a compensatory governor action.

Increasing the excitation on a single generator tends to push its own terminal voltage upward relative to the common bus. Whether this raises the entire system voltage depends on the size of that generator relative to the network and the system's stiffness (how strongly other sources and the transmission grid support it).

If the generator is connected to a large, robust system with many other generators and stable voltage control elsewhere, you won't see a significant shift in the overall system voltage. Instead, by raising its internal excitation, that generator effectively elevates its internal voltage compared to the bus. This voltage difference drives reactive current out into the system. As a result, that machine starts supplying more kVAR to the network rather than significantly increasing the entire system voltage. The increased kVAR output is basically the generator stepping up to inject more reactive power, meeting the reactive demands of loads or compensating for other generators that might be supplying less kVAR.

On the other hand, if the system is relatively small or weak, and the generator is a major contributor, raising its excitation can nudge the overall bus voltage upward. In such cases, the whole local voltage profile may shift. When the voltage across loads increases, their reactive draw can change (e.g., inductive loads will draw more reactive current at higher voltages). This allows the generator to feed that added reactive demand. Effectively, the generator's higher internal voltage stimulates more reactive power flow into the grid, causing it to supply more kVAR.

In the end, proper load sharing involves understanding the interplay between kW and kVAR. By managing speed to balance real power and voltage to balance reactive power, you can ensure that all generators in the system work together efficiently. Whether you're dealing with similar or dissimilar machines, the fundamentals remain: align speed and voltage controls and let the system do the rest.