In short. Running a generator in parallel with the grid raises questions the standby case never asks: how much fault current it adds, whether it can export, what happens if the grid trips, and who must approve it. The study answers each one with calculations, settings and tests the utility can accept.
Interconnection modes
| Mode | Description |
|---|---|
| Standby, open transition | No parallel. Simplest. |
| Closed transition | Short parallel during transfer |
| Peak shaving or export-limited parallel | Continuous parallel, no export to grid |
| Export parallel | Exports energy to the grid |
Study elements
- Utility requirements. Obtain the distribution company’s rules, application steps and test requirements.
- Fault contribution. The generator adds fault current at the point of common coupling. Check utility equipment ratings. See generator fault contribution.
- Protection. Over- and under-voltage and frequency, reverse power or export limit, directional over-current, vector shift or rate-of-change-of-frequency for loss of mains.
- Synchronising. Check-sync and breaker interlocks. See synchronising studies.
- Islanding. Detect loss of grid quickly and either trip or transition to island mode in a controlled way.
- Voltage and power quality. Effect on voltage regulation, flicker and harmonics at the point of common coupling.
- Earthing. Consistent neutral arrangement. See earthing architecture.
- Metering and communication. Utility metering and any required telemetry.
- Commissioning tests. Witnessed protection and trip tests.
Loss of mains
If the grid disconnects and the generator keeps feeding a local island, the island can drift out of control or reconnect out of phase. Anti-islanding protection must detect this and act within the time the utility specifies.
Illustrative example
A factory wants to run a generator in parallel to reduce peak demand and not export. The study sets a reverse-power relay and a power controller to hold import above a minimum, adds loss-of-mains protection with a trip time agreed with the utility, and confirms the added fault current stays within the utility switchgear rating.
Why interconnection is a formal study
Connecting a generator in parallel with the utility network affects the utility’s fault levels, voltage regulation and protection, and creates the possibility of islanding, where the generator keeps supplying a section of the network after the utility has opened. The utility will require evidence of safe interconnection.
Typical study content
| Topic | Purpose |
|---|---|
| Fault contribution | Effect on utility breakers and customer equipment |
| Voltage impact | Rise or flicker at the point of common coupling |
| Protection | Over/under voltage and frequency, reverse power or export limit, anti-islanding |
| Synchronising | Closing criteria, sync-check |
| Grounding | Compatibility of neutral earthing with the utility |
| Metering and communications | Billing and remote tripping if required |
Anti-islanding methods
- Rate of change of frequency or vector shift.
- Under/over voltage and frequency with fast settings.
- Transfer trip from the utility where the requirement exists.
Process
- Check the distribution company’s current rules and application process; requirements differ.
- Submit single-line diagram, protection settings and study results.
- Obtain written approval before parallel operation.
Operating modes
Import only, momentary parallel, or export. Each has a different protection set. See utility plus DG synchronisation.
Common mistakes
- Starting parallel operation before approval.
- No loss-of-mains protection.
- Ignoring added fault level on utility equipment.
What to send us
Send the utility connection details and your intended mode. Send your application details.
Questions engineers ask
Do I need permission to parallel?
Usually yes. Apply to the utility and follow its rules.
What is loss-of-mains protection?
Relays that detect grid loss and disconnect or island the generator safely.
Can I avoid export?
Yes, with reverse-power or export-limit control.
Does paralleling raise fault level?
Yes. Add the generator contribution to the utility fault level and check equipment ratings.
This guide gives general principles. Follow the utility’s requirements and applicable regulation.