Advanced power electronics can yield considerable efficiency gains in the operation of combined cycle and gas turbine based machines through their use in variable speed drives, frequency converters and excitation systems.

Power electronics have become a key technology in all industrial fields, superceding many traditional products, including the replacment of fixed speed drives and drives with gear boxes, or liquid couplings for those with variable speed.

In gas and combined-cycle gas fired turbine (CCGT) stations a number of power electronics applications are related to variable speed drives for pumps and fans, and increasing power plant efficiency is a prime topic in this respect. In the case of plant modernization, where replacement of existing drives is due, the option to apply variable speed drives must be evaluated and the investment in such modern drives is typically returned within few years thanks to lower operation costs.

Besides variable speed drives, static frequency converters (SFC) used as starters for gas turbines and static excitation systems (SES) are other widely used power electronics applications.

Static excitation systems

The excitation system has a major impact on the performance and availability of a generator. This statement is valid for both SES, usually supplied from the machine terminals, and for exciter machines, today preferably equipped with rotating diodes and controlled by an automatic voltage regulator (AVR). Both excitation systems are commonly used in gas and CCGT plants. The closed loop control functionality is pretty much the same for both systems, however SES have undisputed advantages in terms of response time, as well as direct access to the generator field.

Schematic of the SES and SFC systems
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The prime task of an excitation system is to supply direct current with adequate safety margins to the field winding, and to provide the accurate and stable control of the generator terminal voltage. However, for modern SES with a microprocessor controller this is just one of many control functions. Closed loop limiter circuits for field current, stator current, load angle and so on, prevent the machine from being operated outside the permissible area. In order to get the most out of the generator, the limiter curves are adapted based on the temperature of the machine’s cooling media. Of course, it is also important to coordinate the limiter characteristics with the set values of the generator protection, so that the limiters intervene whenever possible before the protection relay might shut down the machine.

When the machine is connected to the network, it might be more appropriate to control the power factor or reactive load instead of the terminal voltage. Such controllers are part of any UNITROL SES, and can easily be activated if needed.

Improved network stability

Network blackouts analyses have proved that properly adjusted power system stabilizers (PSS) could have prevented such incidents, and this additional control function is becoming increasingly important. In many countries equipping excitation systems for larger generators with PSS is compulsory and in new systems this function is part of the firmware package, while older systems can easily be updated with a stand-alone microprocessor based PSS.

In the past, all major suppliers of AVR and SES followed their own philosophy with respect to the transfer function and the features of the PSS. Currently PSS type 2A or 2B, according to IEEE Standard 421.5 (2005), are widespread. However, the multi-band PSS (named PSS4B in the IEEE standards) developed by Hydro Quebec might well become the new future standard for power stations operated under critical network conditions.

The system availability is also an important aspect to take into consideration when selecting an excitation system as any shutdown results in production losses. This can largely be prevented by providing redundant controllers and power converters. In the case of a gas or CCGT plant investments in SES with redundant circuits provide a return on investment due to the fact that shut downs and consequently new starts caused by equipment failures can be significantly reduced if not excluded.

Thus, careful selection of the excitation system is important not only for new installations but also for replacement of old exciter machines in existing plants.

SFC and start-up

A gas turbine compressor does not supply enough air for the machine to be self-sustaining until it reaches approximately 60 per cent of its rated speed. Therefore the gas turbine must be accelerated to the speed at which it can produce enough power itself to continue acceleration. When it comes to larger machines, it is common to use SFC, using the synchronous machine as a motor.

Modern SFC are controlled by microprocessors, which provides the flexibility needed to adjust the electromagnetic torque developed by the machine to the load torque, allowing “soft starting” of the gas turbine. The resulting benefits are that the starting current is limited to the rated current or less, the starting impact on the network and machine is minimized, and that controlled acceleration of the gas turbine rotor from standstill to the self-sustaining speed is possible.

SES and SFC systems, with common control cubicle installed in a container
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At the beginning of the start-up phase, the excitation current is switched on and the load commutated inverter (LCI) controller calculates the rotor position (thus, there is no need for a rotor position sensor to be mounted on the shaft). Then, the machine current is switched on and controlled to rated current or less. The machine starts rotating, and the speed controller asks for a current/torque so that the speed follows the required acceleration ramp. The terminal voltage produced by the machine increases proportional to the rotational speed, attaining its full level at about 40 per cent of the rated speed. In order to maintain the machine voltage at this level, the excitation current is gradually reduced while the machine speed rises further. The advantage of this mode of operation, known as field weakening, is that the frequency converter does not have to be designed for the full machine voltage. Therefore, for ratings up to 4 MW, series-connection of thyristors is not required.

The high, non-repetitive surge current capability of modern thyristors allows the design of fuseless converters resulting in higher reliability, lower parts count and less spare parts. In case of a failure, a fast overcurrent protection immediately blocks the thyristor firing and initiates the opening of the main breaker.

Benefits of SFC

SFC provides several advantages over the use of so-called pony motor starting systems as the high reliability and availability exhibited by modern SFC permits a single system to be used to start two or more gas turbine-generator units. Furthermore, less maintenance work is required and it can be accomplished while the turbine is running, avoiding loss of revenues. Other static starting configurations are equally easy to implement; for example, a redundant configuration with two static starting systems for several gas turbine units. The system redundancy results in an extremely high starting reliability for such configurations as failure of one of the starting systems will not prevent the turbine operation.

Common power electronics control

ABB has developed the AC 800PEC – a controller dedicated to power electronics applications, including SFC and SES – which is fully compatible with the 800xA control platform. The application programming is based on IEC61161, while the same control architecture for SFC and SES contributes to simpler commissioning and training.

UART, CAN and Ethernet supporting the fieldbus protocols CANOpen, Modbus-RTU and Profibus are the main options for communication with the plant control system. An additional possibility is Ethernet using OPC protocol. Independent of the configuration of the SFC systems in the power plant, each generator has its own SES. Thus, the SES control acts as single control port to the power plant control. It also takes care of the coordination with and among the SFC.

LCD touchscreen provides a range of selectable screens – trending (top left), operation (bottom left) and generator power chart (right)
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The controller contributes to shorter commissioning time and easier maintenance while the connectivity of field equipment to the internet is the base for remote diagnostics and system data collection. It enables service specialists to remotely assist the maintenance engineer in troubleshooting reducing the outage time of the machine considerably.

The control terminal, a powerful industrial PC with a user-friendly interface that runs independently from the systems’ control platform, is used for monitoring and controlling SFC and SES. It can be located at the system’s door panel for local control and/or in the central control room for remote operation. The LCD touch screen provides the operator and the maintenance engineer with a range of selectable screens showing information on the actual status of the systems in graphical and numerical form.

The power to control

The development of power electronics allows sophisticated control of numerous aspects of a gas or CCGT-based power station. With such technologies installed turbines can be monitored, started and controlled with greater ease than ever before and even from remote locations. This both improves reliability and makes operations more efficient. With gas prices becoming increasingly significant on the bottom line of power plant owners and operators, the use of the most advanced power electronics becomes a question not of if, but when.