Gas Turbine Handbook : Principles and Practices

Microturbines 177
turbine engine control. This controller must not only manage the fuel
valve during Start, Stop and Governing sequences, provide machine
protection against high vibration, over-speed and over-temperature,
but must also condition the high frequency, high voltage power produced
by the generator to grid quality.
Also, the controller must be able to communicate with other
systems {such as the plant Distributed Control System (DCS), and
the Human Machine Interface (HMI)}. Communication protocols such
as Modbus®, Ethernet TCP/IP, Ethernet UDP, OPC (Ethernet), DDE
(Dynamic Data Exchange), EGD (Ethernet) must be available so that
the user/operator can easily interface the control to existing or new
plant systems and to maintenance systems.
To achieve suitably fast response the fuel valve must be sized
for the microturbine. It is also helpful if the fuel valve can handle
two flow paths—one for starting flow (pilot flow) and another for
running flow (primary flow).
For power generating applications a power conditioning control
must manage load (KW) control, frequency control, synchronizing,
load sharing and KW droop.
For mechanical drive applications (compressor or pump drives)
the controller must manage suction and discharge pressure & temperature
control, bypass or recirculation flow control & cooling, surge
control (for centrifugal compressors) and miscellaneous plant valves.
Generator
Microturbine manufacturers are currently employing two design
variations: high speed single shaft and split shaft. The single shaft
type drives the high-speed synchronous (external field coil excitation
or permanent magnet) or asynchronous (induction) generator at the
same speed as the gas turbine. This generator produces very high
frequency AC power that must be converted first to DC power and
than to 60 Hz AC power. This is accomplished using a rectifier and
an inverter. The inverter rectifies the high frequency AC voltage produced
by the alternator into unregulated DC voltage. It then converts
the DC voltage into 50 Hz or 60 Hz frequency and 480 volts AC
With the split shaft design the electric generator is driven by
the free power turbine, usually through a speed reduction gearbox,
at 3600 rpm. In this design the generator produces 60 Hz power and
does not require a rectifier and an inverter.