Paralleling (Synchronising) Generators

In facilities supplied by a single large generator, the set runs far below its capacity during low-load periods. For example, when a 1,400 kVA set runs at around 300 kVA on the night shift, oily soot forms in the exhaust, and the facility has no backup while the set is being serviced. If the same facility uses 3 × 700 kVA sets in parallel, a single set runs at low load, and during maintenance the other sets stand by as backup.

Synchronized (parallel) operation means two or more generators connected to the same busbar and supplying the load together. Generators can run in parallel with each other or with the grid. The condition is this: the voltage, frequency, phase sequence and phase angle of the incoming set must match those on the busbar. Once connected, active and reactive power must be shared evenly between the sets. This job is done by the synchronization panel and the generator control units.

Synchronization conditions

Parameter	Why does it matter?	Typical window (approximate, adjustable)
Phase sequence	Connecting in the wrong sequence causes serious damage	Must be the same (L1-L2-L3)
Voltage	A difference produces a reactive balancing current	A few %
Frequency	A difference creates a power surge at the moment of connection	~0.1–0.2 Hz
Phase angle	A large difference creates a mechanical and electrical shock	~±10°

The control unit adjusts the speed and voltage of the incoming set to bring the values closer. When the conditions are met, it closes the breaker. If the busbar is empty (dead bus), the first set ready connects directly; the others synchronize to it. Interlocking logic that prevents two sets from closing onto a dead bus at the same time is essential.

Load sharing

• Active power (kW): Shared by the speed governors. In modern systems the control units communicate with each other; frequency stays constant and the load is distributed in proportion to the sets’ ratings (isochronous load sharing). The other method is droop: speed drops slightly as load increases, and the sets share the load accordingly.
• Reactive power (kVAr): Shared by the AVRs. If reactive sharing is off, circulating current flows between the sets. One set shows high current while another shows low; the alternators heat up for nothing.

Load sharing is never perfect. When calculating total capacity, leaving a margin of a few per cent is common practice. The exact values are in the control system manufacturer’s data.

Why parallel?

• Redundancy (N+1): Even if one set fails or goes in for maintenance, the rest carry the load.
• Running to suit the load: When the load drops, unneeded sets stop automatically. The remaining sets run in their efficient range, and problems caused by low load (wet stacking) are reduced.
• Step-by-step growth: New sets are added as the facility grows.
• Easier maintenance: Sets are taken out for maintenance one at a time.
• High power: Requirements that exceed the rating of a single set are met with several sets; for very large systems, medium voltage solutions are also considered.

There is a price too: more panels, more complex control, more space and usually a higher initial investment.

One large set or parallel sets?

Example: 1,200 kVA critical load.

Option	Installed power	With one set out of service	Note
1 × 1,400 kVA	1,400 kVA	0 kVA	Simple, no redundancy
2 × 1,400 kVA (1+1)	2,800 kVA	1,400 kVA	Fully redundant, high installed power
3 × 700 kVA (N+1)	2,100 kVA	1,400 kVA	Redundant, step-by-step running

The figures are there to illustrate the method. In a real selection, the power rating (standby, prime, continuous), load steps, motor starts and layout are assessed together.

Paralleling with the grid

Typical applications:

• Closed transition: Transferring the load without interruption when the mains returns.
• Peak shaving: Limiting the power drawn from the grid during expensive hours.
• Continuous parallel operation: For example, cogeneration.

Grid paralleling requires reverse power, loss-of-mains (such as vector shift and ROCOF) and synchronization protection. The connection is usually subject to the distribution company’s permission and technical requirements.

Common mistakes on site

• Alternators with different winding pitches. If alternators running in parallel have different winding pitches (e.g. 2/3 pitch), third-harmonic current can circulate through the neutral. The neutral cable heats up.
• Neutral earthing is thought about set by set. A separate neutral–earth link on each set can lead to circulating currents. See generator earthing.
• Communication line failure is not defined. Nobody knows what the system will do if the cable between the control units breaks.
• No load shedding. When one set drops out, the rest are overloaded and a chain of shutdowns follows. Priority loads and load shedding logic must be defined from the start.
• The real scenario is not tried during commissioning. Synchronization, load sharing, loss of a set and load steps should be tested with the real load or a load bank; see load bank testing.

Checklist

• Are the total load, load steps and redundancy target (N, N+1, 2N) known?
• With one set out of service, can the rest meet the load?
• Are the control units and speed governors suitable for parallel operation?
• Are the alternator types and winding pitches compatible?
• Has the neutral–earth arrangement been designed at system level?
• If there is grid paralleling, have the distribution company’s requirements and the protection been defined?
• Are the load shedding and priority logic written down?
• Were synchronization and load sharing tested during commissioning?

Next step

Parallel systems are used most in data centres, hospitals and industrial facilities. To assess the generator and synchronization panel arrangement for your project with the Terex Diesel team, contact us.