Synchronising onto a grid

Reference notes

Use Next → on the narrator above to walk through the four conditions that must hold to connect an alternator to a live grid, and the classical instruments operators use to verify them.

The four conditions for paralleling

Before closing the breaker that ties an incoming machine to the bus, four conditions must all be satisfied — otherwise large transient currents and torques will damage windings, couplings, and the breaker itself.

1. Same voltage magnitude (within ~1 %): adjusted by raising/lowering the incoming machine's field current.
2. Same frequency (within ~0.1–0.2 Hz): adjusted by the prime-mover speed governor.
3. Same phase sequence: a-b-c on both sides must match. Set once when the machine is wired; verified by a phase-sequence indicator.
4. Same instantaneous phase: the two systems' voltage waveforms must coincide at the moment of closure (Δφ ≈ 0).

Conditions 1 and 3 are static checks (verified once and left). Conditions 2 and 4 are dynamic — and you can't make Δf = 0 exactly, so you keep it small and wait for Δφ to drift through zero, then close.

Method 1: Three dark lamps

Connect a lamp between each pair of corresponding phases (incoming "A" to bus "A", etc.). Each lamp sees the difference voltage v_A,inc − v_A,bus:

• If Δφ = 0 and ΔV = 0 → all three lamps go dark simultaneously.
• If Δf ≠ 0, the difference voltage beats at frequency Δf → lamps brighten and darken together at Δf cycles per second.
• Operator: tune the prime mover until the beat frequency is just a fraction of a Hz, watch the lamps dim and brighten slowly, close the breaker exactly at the dark instant.

If phase sequence is wrong, the three lamps brighten and darken in a sequential pattern (not together) — easy to spot, fix by swapping any two leads.

Method 2: Two-bright-one-dark (rotating brightness)

Cross two of the three lamps' phase connections (e.g. incoming-A to bus-B, incoming-B to bus-C, incoming-C to bus-A). Now:

• Each lamp sees a different phase difference, so they brighten/darken in sequence — like a rotating "lighthouse".
• Direction of rotation indicates whether the incoming machine is too fast or too slow → operator knows which way to nudge the governor.
• Close at the instant when the designated lamp is dark and the other two are equally bright.

Method 3: Synchroscope

An analog meter with a single rotating pointer. The pointer's angular position is Δφ; its rotational speed is Δf. Operator's task: adjust the speed governor until the pointer creeps slowly, then close the breaker as it passes the 12-o'clock mark (Δφ = 0). The direction of rotation tells you fast/slow.

Method 4: Automatic synchroniser

Modern installations have a relay that monitors Δf, ΔV, Δφ continuously and issues the close command itself when all four conditions are within tolerance. Operators initiate the sequence; the relay does the timing. Far less error-prone than human-eye sync, especially on islanded systems where multiple machines need to come online quickly after an outage.

What happens if you close at the wrong moment

• Δφ ≠ 0: a huge current flows from the lagging system to the leading one. Mechanical torque on the rotor can shear couplings.
• ΔV ≠ 0: reactive current circulates between the two systems, dissipating in winding resistance.
• Wrong phase sequence: the machine acts as a reverse-direction load. Burns windings within seconds.
• Δf too large: even with Δφ = 0 at the instant of closure, Δf means Δφ grows immediately afterward and you may pull out of step. Beats above ~0.5 Hz are typically rejected.

Keyboard shortcuts

• → next step  ·  ← previous step
• R replay narration  ·  M mute / unmute  ·  F fullscreen