The Four Synchronising Conditions
Before closing a generator breaker onto a live busbar, four parameters must match simultaneously:
- Voltage magnitude – incoming generator voltage equals busbar voltage
- Frequency – incoming frequency equals busbar frequency
- Phase angle – the incoming waveform is in phase with the busbar waveform at the moment of closure
- Phase sequence – the rotational order of phases (L1–L2–L3) is identical
Phase sequence is verified once at installation or after rewiring; it does not change during normal operation. The remaining three conditions are managed every time a generator is brought online. A synchroscope or automatic synchroniser confirms phase angle alignment — closing off-phase causes a current transient that can trip protection or damage windings.
Droop vs Isochronous: Knowing Which to Use
These two governor modes are frequently confused because both govern frequency, but they do so for different purposes.
Droop control allows frequency to fall slightly as load increases. Each generator on the bus has its own droop characteristic, and because load naturally distributes to whichever machine is running at higher frequency, droop is self-stabilising when two or more generators run in parallel. Without droop, two isochronous governors fight each other for load, causing hunting and instability.
Isochronous control holds frequency constant regardless of load. It is appropriate for a single generator running alone, or when a PMS load-sharing controller actively manages kW distribution across machines — the controller itself replaces the passive droop mechanism.
The practical rule: parallel machines → droop; single machine or PMS-managed load sharing → isochronous.
Reverse Power Protection
A generator that loses its prime mover — fuel cut, governor fault — but remains connected to the bus will begin to motor: the bus drives current into it, spinning the alternator as a synchronous motor. This loads the other generators, can damage the prime mover, and is an abnormal condition the crew may not immediately notice.
A reverse power relay detects current flowing back into the machine and trips its breaker. A trip at around 10% of rated power in the reverse direction is cited as a common setting, though the exact figure on any given switchboard must be confirmed from the ship's documentation.
PMS Context
The power management system coordinates these functions automatically: it starts or stops generators based on load demand, initiates synchronising sequences, manages load sharing mode, and executes preferential tripping — shedding non-essential loads such as air conditioning and galley equipment in timed stages — when a machine is lost unexpectedly. Understanding the underlying electrical principles lets the ETO intervene intelligently when automation fails.