Gas Turbine Handbook : Principles and Practices

Accessories 91
about 1.5 - 2 times per second with an electrical energy pulse of 4 to
30 joules (depending on combustor size, fuel, etc.). 1
“One joule is the unit of work or energy expanded in one
second by an electric current of one ampere in a resistance of
one ohm. One joule/second equals one watt.”
Once the gas generator has been started the function of the igniter
is fulfilled and any further exposure to the hot gases of combustion
shortens its life. To eliminate this unnecessary exposure, some
igniter designs include a spring loaded retracting mechanism that
permits the igniter to move out of the gas path as combustion pressure
increases.
To achieve ignition consistently a high voltage and a high heat
intensity spark are required. Ignition systems include both high and
low energy systems, and energy sources comprise both AC and DC
sources. Typical electrical potential at the spark plug is approximately
25,000 volts. Due to this high voltage the electrical wiring (also
referred to as the ignition harness) to each igniter plug is shielded
and the ignition exciter is hermetically sealed.
Several types of ignition systems have been developed: they are
the capacitive AC and DC, high and low tension systems and inductive
AC and DC systems. Of these the capacitive systems generate the
hottest spark.
The capacitive AC high tension ignition system (Figure 6-2) consists
of the power source, on/off switch, radio interference filter, low
tension AC transformer, rectifier, storage capacitor, high frequency/
high tension transformer, high frequency capacitor, and spark plug.
The power source may be either AC or DC. The function of the ignition
system components is as follows:
• The radio interference filter eliminates ignition energy from affecting
local radio wave signals
• The AC transformer (or transistorized chopper circuit transformer)
boosts the voltage to approximately 2,000 volts
• The rectifier is an electrical one-way valve allowing the flow of
current into the storage capacitor but preventing return flow
• The high tension transformer charges the high tension capacitor.