ABB HiPerGuard MV UPS User Manual page 24

3.2.5. Transition from Independent to Power Conditioning mode (upstream
generator adaptive walk-in)
Frequently diesel or gas generators are connected to the MV UPS input as secondary power supplies in
case of extended duration power outages. In such cases, the MV UPS assumes change-over to upstream
generator bus in frequency and voltage reference mode, often known as Island Forming.
When supplied by a generator, specific tunings are made. For example, the MV UPS allows separate
energy storage recharge settings depending on whether it is connected to the grid or generator.
HiPerGuard has two options for signaling to it when it is connected to a generator. This is either via a
hard-wired digital input via an I/O interface module or as a written bit input over Modbus TCP
communications.
Figure 3.5: MV UPS during the generator walk-in process
As soon as the generator output connected to the MV UPS input reaches nominal voltage and frequency
level, the MV UPS starts the transition from the Independent to the Power Conditioning mode. The hand-
over process of transferring the load from the MV UPS (using the energy stored in the batteries as a
power source) to the generator is called adaptive walk-in.
The MV UPS achieves maximum generator walk-in speed using a frequency droop & clamp approach.
When the MV UPS detects that the input voltage and frequency are according to the specification, and
following a two second trust period, the input breaker is closed.
The MV UPS attempts to transfer the load to the generator by lowering its frequency at a rate of 1 Hz-
per-second but clamps the frequency droop to an error of 2% (configurable). The load sharing starts by
the MV UPS trying to load the generator. If the generator drops down to the threshold (2% of the system
nominal frequency), then the MV UPS holds the existing power level until the generator can catch up.
The MV UPS understands that the generator can handle the load when it increases the frequency back
towards nominal. The MV UPS will support the load by clamping the frequency at 98% of the nominal
until the generator has completed picked up the load. The UPS does this all in a very smooth manner.
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