Generator load sharing principle

Load sharing control methods and systems

Load sharing control methods determine how parallel generators distribute active power (kW) and reactive power (kVAR) among units. These methods range from simple mechanical systems to advanced digital platforms.

1. Droop control

Droop control is the most commonly used method for active power sharing in parallel generator systems. It allows a small, intentional reduction in generator speed and frequency as the load increases. This speed-load relationship enables multiple generators to share kW automatically without communication between units.

Droop % = (f no-load − f full-load) / f full-load × 100

Most systems use a droop setting of 3% to 5%. The same principle applies to reactive power through voltage droop, where generator voltage decreases slightly as reactive load increases, allowing AVRs to distribute kVAR proportionally.

Droop control uses the engine governor for active power and the AVR for reactive power. Traditional systems use mechanical governors and analog AVRs; modern sets use electronic governors and digital AVRs with programmable droop settings.

Droop control offers a simple, dependable, and configurable solution that operates independently without inter-generator communication in large systems. However, its limitations include frequency variations under changing loads, potential unevenness during rapid transitions, and insufficient regulation for facilities requiring strict frequency stability.

2. Isochronous control

Isochronous control keeps speed and frequency constant as load changes. The governor continuously adjusts fuel to hold the rated speed. For reactive power, isochronous kVAR sharing uses a load-sharing controller that monitors each generator’s kVAR output and adjusts AVR excitation so the units carry equal reactive proportions.

Isochronous control needs a load-sharing controller (LSC) to coordinate the generators; without one, two isochronous generators compete for load and become unstable. Modern systems use microcontroller-based controllers, PLCs, or dedicated load-sharing controllers. In a master-slave setup, one generator sets the frequency reference while the others follow its load commands.

It maintains constant frequency, provides accurate and equal load sharing, improves fuel efficiency through optimized loading, and suits facilities with sensitive loads. However, it requires reliable communication and compatible controllers, is more complex and costly to set up than droop, and communication problems can affect stability.

3. Cross-current compensation

Cross-current compensation shares a small signal from each generator’s current transformer with all AVR units, letting them correct kVAR differences while keeping bus voltage constant. All generators must have identical or compatible AVR models.

4. Power management system (PMS)

A power management system (PMS) is an advanced control platform that coordinates multiple generators and electrical loads across an entire power system. It automatically starts and stops generators to match real-time demand, runs them at efficient load levels, distributes kW and kVAR, prioritizes critical loads during shortages, and manages alarms and shutdown sequences. Modern PMS platforms use digital networks such as CAN bus, Modbus, and Ethernet/IP to exchange real-time data; SCADA integration adds remote monitoring, centralized control, and data logging.

5. Choosing the right control method

Choosing the appropriate load sharing method depends on application requirements, load sensitivity, and budget. Droop control is the most economical and suits general industrial and standby use where minor frequency variations are acceptable. Isochronous control suits frequency-sensitive loads and offers more accurate sharing at a higher cost. A PMS is best for large facilities with multiple generator sets and complex load priorities. For dealers, the key questions are how frequency-sensitive the loads are and how much automation is needed.