This guide explains the start sequence and the main types of starting system, with the points owners should consider when choosing or maintaining them. Details vary by turbine model; the manufacturer’s documentation governs.
Why a turbine needs a starter
The compressor in a gas turbine absorbs a large share of the power the turbine produces. At low speed, combustion cannot produce enough power to drive the compressor and accelerate the shaft. The starter turns the rotor up to a speed where combustion can begin, then helps accelerate it until the turbine is self-sustaining.
Once that point is reached, the starter disengages or switches off, and the turbine accelerates to operating speed under its own power.
The start sequence
A typical start begins with the rotor on turning gear or at rest. The starter accelerates the shaft to purge speed, and air is blown through the turbine and exhaust to clear any fuel or gas that may have collected. After the purge, speed is adjusted for ignition, fuel is admitted and igniters light the combustors.
Flame detectors confirm ignition. The starter and combustion then accelerate the unit to self-sustaining speed, after which the starter drops out and the unit reaches full speed, ready for synchronising.
Electric motor starters
Many industrial gas turbines, especially smaller and older frames, use an electric motor, often coupled through a torque converter or clutch, to crank the shaft. The motor needs a substantial electrical supply during the start, which must be available from the grid, auxiliary transformer or a black start generator.
Electric starters are relatively simple and robust, but their supply must be designed for the motor’s starting current.
Static frequency converters
Large heavy-duty turbines commonly use a static frequency converter, sometimes called a load commutated inverter. It feeds variable frequency power into the main generator, which then runs as a motor to turn the shaft. This avoids a separate starting motor but needs a power supply sized for the start and adds power electronics that must be maintained.
Some plants share one converter between several units, which saves cost but means only one unit can start at a time.
Hydraulic and pneumatic starters
Aeroderivative and smaller turbines often use hydraulic starters, driven by an electric or diesel-powered hydraulic pump, or pneumatic starters driven by compressed air. Pipeline compressor turbines may use gas expansion starters that use pipeline gas pressure, with vented gas managed according to the site’s safety and environmental rules.
Hydraulic and pneumatic systems are compact and can support fast starts. Our article on what an aeroderivative gas turbine is explains why those units suit rapid starting.
Diesel engine cranking
Some older heavy-duty units were started by a diesel engine coupled through a torque converter. This arrangement can start a turbine without an external electrical supply, which made it attractive for black start duty. It requires maintenance of the diesel engine and its fuel supply alongside the turbine.
When refurbishing older units, owners sometimes replace diesel starters with electric or hydraulic systems to reduce maintenance.
Power for the start
Every starting system needs energy from somewhere: the grid, an auxiliary transformer, a black start diesel, batteries or compressed gas. Auxiliaries such as lube oil pumps, ventilation and controls also need power during the start. The total demand must be available whenever a start is required.
If a plant must start without the grid, the black start supply must be sized for the starter plus all auxiliaries. Our guide to black start scope for new turbines covers that design.
Common start failures
Failed starts often come from the starting system and its supports rather than the turbine core: a starter that cannot reach purge speed, a clutch that fails to engage or disengage, low supply voltage, hydraulic leaks, igniter faults or flame detector problems. Fuel supply issues and control faults also cause starts to abort.
Our article on common gas turbine trip causes covers faults after the unit is running.
Start reliability and testing
For units that must start on demand, such as peaking or backup plants, start reliability is a key measure. Track each start attempt, its outcome and the cause of any failure. Test starts at regular intervals, and include the starting system in the maintenance programme with inspection of clutches, couplings, motors, converters and hydraulic or air systems.
Recording start time from command to synchronising helps spot slow degradation before it becomes a failed start.
Choosing a starting system
For new turbines, the starting system is usually defined by the manufacturer for each model, sometimes with options. Owners should check the system’s supply needs, start time, how many consecutive starts it allows and whether it can support black start. These points matter especially for data center and peaking plants that must start quickly and reliably.
Ask for the starting system’s maintenance requirements and spare parts list as part of the purchase.
About the authors
This article draws on our gas turbine engineers, who install, commission and maintain turbines and their auxiliaries. USP&E delivers that work under its own EPC and O&M contracts and is not a broker, with 24+ years of turbine projects, 150+ power projects in 45+ countries and 25,000 MW contracted since 2002. Refurbishment of turbines and their systems is described on the refurbishment page.
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Gas turbine starting questions
Why can a gas turbine not start on its own?
At low speed, combustion cannot produce enough power to drive the compressor. A starter must turn the shaft until the turbine becomes self-sustaining.
What is a static frequency converter?
Power electronics that feed variable frequency power into the main generator so it runs as a motor and turns the turbine during start-up. It is common on large heavy-duty units.
Why is the turbine purged before ignition?
Purging blows air through the turbine and exhaust to remove any fuel or gas that may have collected, reducing the risk of an explosion at ignition.
What causes gas turbine start failures?
Common causes include starter or clutch faults, low supply voltage, hydraulic or air leaks, igniter or flame detector problems and fuel supply issues.
Talk to a turbine engineer
Send the turbine model, starting system type and recent start records, and our engineers will review reliability and maintenance with you.
You can reach the engineering desk on +27 10 822 2324, send drawings or data sheets to info@uspeglobal.com, or leave the details on the turbine enquiry page. New and surplus units are listed in our gas turbine inventory.