In short. Running a generator in parallel with the utility brings extra obligations: written approval, protection that disconnects the generator if the utility is lost, control of export, and a safe synchronising method. It is used for closed-transition transfer, peak shaving and testing, and must be designed with the utility’s rules.
Why parallel operation is different
- The utility is a very strong source that sets voltage and frequency, so the generator runs in base-load or export mode, not setting frequency.
- Faults on the utility side can feed from the generator, and faults on the site can feed from the utility.
- If the utility trips and the generator keeps feeding the local network, it becomes an unintended island.
Operating modes
| Mode | Description |
|---|---|
| Momentary parallel (closed transition) | Parallel for seconds to transfer load |
| Peak shaving / base load | Generator supplies a share of site load |
| Export | Power flows to the grid. Needs approval and strict protection |
| Zero export | Generator parallels but a reverse power or export-limit control stops any flow to the grid |
Protection and control
- Sync-check and synchroniser at the paralleling breaker.
- Loss-of-mains protection: rate of change of frequency, vector shift, voltage and frequency elements. See voltage and frequency protection.
- Reverse power or export limit where export is not allowed.
- Fault level review: the generator adds to fault current at the connection point.
- Earthing: avoid circulating neutral currents. See neutral-earth architecture.
Process
- Confirm with the utility what parallel operation is allowed and the technical requirements.
- Design the interface breaker, protection and control.
- Submit documents and settings for approval.
- Witness testing of protection and synchronising.
- Operate within the approved limits.
Illustrative example
A hospital wants closed-transition testing so critical loads are never interrupted during monthly generator tests. The design adds sync-check, a loss-of-mains relay and a limit on parallel time of a few seconds. The utility reviews and approves the settings, and the commissioning includes a trip test showing the generator disconnects if the utility supply is lost during parallel.
Why parallel with the utility
Paralleling can allow peak shaving, soft (no-break) transfer, and export where permitted. It also brings risks: the generator sees the utility fault level, and faults on the utility side feed from the generator.
Operating modes
| Mode | Description |
|---|---|
| Momentary parallel (soft transfer) | Parallel for seconds during transfer, no outage |
| Continuous parallel, import only | Generator supports load, no export |
| Continuous parallel with export | Needs utility agreement and protection |
Protection required
- Reverse power or export limiting if export is not permitted.
- Loss of mains (islanding) detection, such as vector shift or rate of change of frequency.
- Under and over voltage and frequency protection.
- Sync-check on closing.
Fault level impact
With the utility connected, switchgear sees the utility fault contribution plus the generator. Confirm ratings. See fault level engineering.
Utility approval
Utilities have requirements for interconnection protection and metering. Confirm the current rules with the distribution company before design and obtain written approval.
Transfer back
When returning to utility supply, synchronise the utility and generator, ramp load across, then open the generator breaker. See synchronising switchgear.
Common mistakes
- Paralleling without utility approval.
- No loss-of-mains protection.
- Ignoring the added fault level.
What to send us
Send site, utility and generator details. Send your application details. See also ATS engineering.
Questions engineers ask
Do I need utility permission?
Usually yes. Rules differ by region and utility.
What is anti-islanding?
Protection that disconnects the generator if the utility supply fails, so it does not energise the network unexpectedly.
Can I export power?
Only with approval and the required protection and metering.
Is closed transition worth it?
For sensitive loads and tests, it avoids interruption, but adds complexity and requirements.
This guide gives general principles. Utility rules and local regulations govern.