Generator Synchronising Panel: Parallel Operation and Load Sharing
generator synchronising panel: A generator synchronising panel matches voltage, frequency, phase sequence and phase angle before connecting two or more machines to a common busbar; the acceptable window is typically 5 percent on voltage, 0.2 hertz on frequency and 10 degrees on phase angle, and the breaker is closed only inside that window.
Contents
Where a single generator is not enough, paralleling several machines onto a common busbar provides both redundancy and staged load management. Paralleling without satisfying the synchronising conditions, however, applies a mechanical shock at short circuit level to the machine and the busbar. This article covers the architecture of a synchronising panel, its control chain, load sharing methods and the requirements of parallel operation with the utility grid.
Why Parallel Operation Is Preferred
Running several generators in parallel allows the load to be carried in the efficient range by using smaller units instead of one large machine, since a diesel generator operates most efficiently and with the least sooting between 70 and 90 percent load. It also keeps the site running while one machine is taken out for maintenance and lets additional capacity be brought on in stages. Data centres, hospitals and continuous process plants consider that flexibility critical. In exchange, the panel has to handle the added complexity of synchronising control, load sharing and protection.
Synchronising Conditions and the Control Chain
Four conditions must be satisfied simultaneously before two sources can be paralleled: the voltage magnitudes must be close, the frequencies must match, the phase sequence must be identical and the phase angle difference must be small. The practical window is typically 5 percent on voltage, around 0.2 hertz on frequency and about 10 degrees on phase angle, and letting the incoming machine run marginally above the bus frequency ensures it picks up load immediately after closing. These conditions are supervised by a synchronising check relay (ANSI 25) and the closing command is issued only inside the window. If manual synchronising is provided, a synchroscope and dual voltage and frequency indication are required.
Load Sharing: Droop and Isochronous Control
Balanced division of active power between parallel machines comes from the governor characteristic. In droop mode the speed falls slightly as load increases, so machines share load proportionally by themselves, but the frequency varies with load. In isochronous mode the frequency is held constant and load sharing is coordinated through a communication line or a load sharing module. Reactive power sharing is handled by the reactive droop setting of the voltage regulator; an incorrect setting produces circulating reactive current between machines and unexplained alarms.
Protection Functions and Reverse Power
| Parameter | Typical acceptance window | Consequence if exceeded |
|---|---|---|
| Voltage difference | Below 5 percent | Reactive current surge |
| Frequency difference | Around 0.2 hertz | Mechanical torque shock |
| Phase angle difference | Below 10 degrees | Shaft and coupling damage |
| Phase sequence | Exactly identical | Fault at short circuit level |
| Reverse power threshold | 5 to 10 percent of rating | Machine driven as a motor |
| Load sharing error | Below 5 percent | One machine overloaded |
Parallel operation makes several protections mandatory that a single machine does not need. Reverse power protection (ANSI 32R) detects a machine being motored when fuel is lost or the governor misbehaves, and typically trips at a threshold of 5 to 10 percent of rated power. Loss of field (ANSI 40), loss of synchronism protection and voltage and frequency limit protection should also be included. On the busbar side, a breaker with check synchronising, interlocking logic and a bus section device are arranged, and the neutral earthing scheme must be resolved so that it exists at a single point with parallel machines.
Parallel Operation with the Grid and Island Mode
Running a generator in parallel with the public network is only possible with the permission of the distribution company and with additional protection. In that scenario anti-islanding detection, a voltage and frequency window and a fast tripping chain must disconnect the generator when the network fails, otherwise a lethal back-feed threatens the utility maintenance crew. Load shedding and load recovery sequences are staged according to the critical loads of the site. These scenarios should be captured in a logic diagram during design and verified step by step during factory acceptance testing.
Key takeaways: generator synchronising panel
- Close only inside the synchronising window — The check synchronising relay (ANSI 25) must interlock the closing command in hardware.
- Never omit reverse power protection — A machine motored by a fuel or governor failure suffers serious damage.
- Decide the load sharing mode early — Droop is simple; constant frequency requires isochronous control and a load sharing line.
- Anti-islanding is mandatory with the grid — Without fast disconnection on network failure, back-feed endangers utility crews.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
What happens if two generators are paralleled out of synchronism?
With a large phase angle difference, a current close to short circuit level flows between the two sources, stressing the windings thermally and hitting the shaft and coupling mechanically. A single incorrect closure can break a coupling or permanently damage an alternator winding. That is why a synchronising check relay is not an option but a mandatory safety element.
What is the difference between droop and isochronous mode?
In droop mode the governor lets the frequency fall slightly as load increases, so machines share load naturally; the setup is simple but frequency varies with load. In isochronous mode the frequency is held constant and load sharing is managed over a dedicated communication line. Installations needing precise frequency choose isochronous, while simple standby systems remain on droop.
What approvals are needed for parallel grid operation?
Parallel operation with the network requires technical approval from the distribution company, and the connection agreement usually defines protection settings, anti-islanding requirements and the maximum export power. Protection setting reports and commissioning test records are normally requested as well. Paralleling without approval is unacceptable both legally and from a life safety perspective.
Related articles
- Automatic Transfer Switch (ATS) and Generator Changeover Panels
- Protection Relays and ANSI Codes: 50/51, 27/59 and 87 Functions
- Energy Monitoring in Switchgear: Meters, Modbus and SCADA Integration
- All technical articles
Devpan designs generator synchronising and paralleling panels together with the synchronising, load sharing and protection logic and verifies them step by step in factory testing. Let us shape your redundant power architecture together. Related Devpan solution: DevPanel main distribution board.
