Lift Station Generator Operation

The sequence

1. 1

   Utility fails

   The transfer switch senses loss of voltage on the utility source, waits a short delay to ride through momentary dips, typically one to three seconds, then signals the generator to start.

2. 2

   Generator starts

   The set cranks, runs up, and reaches rated voltage and frequency. Ten to fifteen seconds is typical for a diesel set; the transfer switch waits until the generator source is within limits.

3. 3

   Transfer to generator

   The switch opens the utility contacts and closes the generator contacts. Where the switch is open-transition, the station is dark for a fraction of a second. Where it is closed-transition, which is rare at lift stations, the sources are briefly paralleled.

4. 4

   Loads return

   Control power is restored, the PLC boots or resumes, and the pump sequence restarts. The pumps that were running before the outage are called again, staggered.

5. 5

   Utility returns

   The switch senses utility voltage, waits a return delay of several minutes so a brief return does not cause a transfer back and forth, then retransfers. The generator runs unloaded for a cooldown period and stops.

Every step has a timer, and every timer is a setting on the transfer switch or the generator controller that should be recorded. A retransfer delay that is too short causes repeated transfers during a flickering utility restoration; a start delay that is too long lets the well rise before the pumps come back.

Starting pumps on a generator

A generator has a limited ability to supply the inrush current of a motor start. A set sized for the station running load will stall, or trip on underfrequency, if two motors start together or a large motor starts across the line without margin. The controls therefore stagger the restarts.

• After transfer, the PLC delays the first pump start a few seconds to let the generator settle, then starts the lead, waits for it to reach speed and for the generator to recover, and only then allows the lag if the level calls for it.
• The delay between starts on generator power is longer than on utility power, and the program knows which source it is on from the transfer switch position input.
• Soft starters and drives reduce the inrush and let a smaller generator start a larger pump; the generator manufacturer sizing tool accounts for the starting method.
• Loads that are not needed during an outage, such as heaters and receptacles, can be shed by the PLC or by the transfer switch load management when on generator.
• On a generator with limited capacity, the program may hold the station to one pump and accept a higher level until utility returns. That decision is in the control narrative and is alarmed.

The controls during the transfer

Between the utility failing and the generator carrying the load, the station has no power for ten to twenty seconds. What the PLC and the instruments do during that gap is a design choice.

What SCADA should see

• Utility power available, from the transfer switch or a voltage monitor.
• Transfer switch position: normal or emergency. This is the alarm that says the station is on standby power.
• Generator running, from the set controller.
• Generator fault or fail to start, from the set controller, as a high priority alarm because the station is now on the utility clock with no backup.
• Fuel level, for diesel sets, with a low alarm that gives time for delivery.
• Battery charger and starting battery voltage, because a dead starting battery is the most common reason a generator does not start.
• Not in auto: the generator controller or the transfer switch left in a manual or off position after service.

Many generator controllers speak Modbus and provide all of this on one connection; the hardwired minimum is transfer position, generator running, and generator fault.

Exercise and maintenance

Standby generators are run on a schedule so that they start when needed. A weekly or monthly no-load exercise proves starting, and a periodic exercise under load, either by transferring the station to the generator or by a load bank, proves the set can carry the pumps. Diesel sets exercised only at no load suffer wet stacking, where unburned fuel accumulates in the exhaust. The transfer switch usually provides the exercise timer, and the PLC can log the run and alarm a missed or failed exercise. Fuel age, battery condition, coolant heaters, and the transfer switch contacts are the maintenance items that decide whether the set works on the day.

Stations without a permanent generator

Small stations often have no generator and rely on a portable set brought by the crew. The design then includes a manual transfer switch that isolates the utility and connects a generator receptacle, sized and labeled for the set the utility owns, and a documented time to overflow at typical and wet weather inflow, so the utility knows how long it has. The high-level alarm and the power failure alarm at such a station are what start the clock, and both must reach someone through a path that does not depend on station power: a cellular dialer with its own battery, or the SCADA radio on a UPS.