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Gas turbines PB4-5

Alstom GT11N2 GT units, with 2x114 MW output, operate in simple cycle. They are adapt to operate with fuel oil, fuel gas or the simultaneous combination of both. Operation began in the year 2000. Gas turbines PB4 and PB5 are environmentally friendly: noise and gas (NOX, CO, SO2) emissions are under legislated limits.
Reliability, operation flexibility and the ability to provide system servicer in the power grid are main features of these units. They differ from standard power production units by being able to:
  • Execute a fast start (t < 15 min) to nominal power
  • Big load gradient (up to 25 MW / min)
  • Operate in remote control for secondary regulation purpose
  • Operate in frequency regulation mode with fast power / frequency response
  • Operate in island mode. In the case of power grid collapse, PB4 and PB5 can be started via PB1 – 3. Thus they can begin to reassemble the power grid
  • Operate with liquid and gas fuel. Fuel transfer is possible during operation. Mix fuel operation is possible for 20% : 80% to 80% : 20%.
  • Operate with high stability. In case of load rejection, (power grid failure, switchyard failure, etc.) turbine remains on nominal rotation speed and supply’s its own electrical consumption. Thus, it can be synchronised to the grid instantaneously.

1. Main turbine unit


Axial compressor with 14 stages supply’s compressed air into the combustion chamber. Three stages of turbine vanes have a hydraulic adjustable pitch, which enables turbine air mass flow regulation. The result is stabilised combustion and high efficiency even at partial loads.

Combustion chamber with burners

Silo type combustion chamber is fitted vertically above turbine housing. Silo type combustion chambers have the advantage of easier and faster access to burners, the first turbine stage and other thermally stressed turbine parts. Thirty seven radially situated burners are fitted on top of the chamber. The centre burner is the fire igniter. Each burner can work on both fuel types, one at a time. Turbine load and fuel type determine number of burners active.

Gas turbine

It transforms the energy of the burnt fuel into kinetic energy that drives the generator. The turbine rotor houses a four-stage turbine and a compressor. The rotor is connected to the generator via gearbox.


Two pole synchronous generator with static excitation system transfers turbine kinetic energy into electrical energy. The generator is air cooled, indirectly via heat exchangers in a closed, combined circuit for generator and lubrication oil cooling. During the turbine start up phase, the static starting device and the synchronous generator perform as a synchronous motor to rotate the turbine, until the turbine itself can propel the generator. The nominal output of the generator is 155 MVA.


The 135 MW gearbox is fitted in-between the gas turbine and the generator. It reduces turbine rotation speed of 3600 min-1, to generator rotation speed of 3000 min-1. It is cooled and lubricated with lubrication oil system that is common also to the turbine and the generator.

2. Other mechanical systems

They insure safety in reliability of the gas turbine:
  • Fuel supply system – fuel gas and fuel oil,
  • Ignition system,
  • Demineralised water supply system for reducing NOx emission while operating with fuel oil,
  • Hydraulic protecting system,
  • Lubrication oil system,
  • Generator and lubrication oil cooling system,
  • Compressor air filtering and supply system,
  • Exhaust system.

3. Electrical and control systems

They perform next functions:
  • Control of rotation speed, bearing temperature rotating parts vibrations,
  • evacuation of produced electrical energy,
  • Gas turbine auxiliary supply,
  • Gas turbine start up with the static starting device,
  • Control and protection

Block transformer

Steps up voltage from 11,5 kV to 110 kV

Auxiliary power supply transformer

Provides electrical energy at 2,1 kV to supply the static starting device and at 0,4 kV for gas turbine auxiliary supply.

Generator protection

Its purpose is to protect:
  • The generator,
  • Generator bus bars,
  • Block transformer,
  • Auxiliary power supply transformer.

Control system

EGATROL 8 is an autonomous gas turbine control system, which includes all control functions needed to ensure safe and effective start, operation and stop of the gas turbines:
Automated start / stop procedure,
Exact turbine process control with short response times,
Automatic load, frequency and temperature control during all of the phases,
Automatic operation at different loads with maximum efficiency.