12-01-2011, 05:19 PM
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Introduction
Microturbine generator systems are considered as distributed energy resources which are interfaced with the electric power distribution system. They are most suitable for small to medium-sized commercial and industrial loads. The microturbine provides input mechanical energy for the generator system, which is converted by the generator to electrical energy. The generator nominal frequency is usually in the range of 1.4-4 kHz. This frequency is converted to the supply frequency of 50 Hz by a converter .The electrical energy, passing through the transformer, is delivered to the distribution system and the local load. A mathematical model of a microturbine generator system includes electromechanical sub-system, power electronic converter, filters, interface transformer, local load, distribution system, turbine-generator control and converter control.
In recent years, modern precision manufacturing techniques and design methods have substantially improved the performance of micro-turbine generators (MTG). Compared to conventional generators, micro-turbine power sources are much smaller and portable. Microturbine generators are also proving to be more efficient, easier to maintain, and more environmentally friendly with fewer emissions. Although power generators running on microturbines can use various types of energy sources, Micro-turbine Generators brings together a wide range of engineering experience to describe the emergence of micro-turbine technology, its viability and its future potential.
Classification
Recuperated Micro-Turbine:- Recuperated microturbines , which recover the heat from the exhaust gas to boost the temperature of combustion and increase the efficiency.
Uncuperated Micro-Turbine:- Unrecuperated (or simple cycle) microturbines, which have lower efficiencies, but also lower capital costs.
Technical Background
MTG ‘s are small, high speed power plants that usually include the turbine, compressor, generator and power electronics to deliver the power to the grid. These small power plants typically operate on natural gas. Future units may have the potential to use lower energy fuels such as gas produced from landfill or digester gas. The generic MTG can be divided into three primary sub-systems :
Mechanical : including turbine, generator ,compressor and recuperator.
Electrical : including main control software, inverter and power firmware.
Fuel : including fuel delivery and combustion chamber.
WORKING
Mechanically the MTG is a single shaft ,gas turbine with compressor ,power turbine and permanent magnet generator being mounted on the same shaft. The MTG incorporates centrifugal flow compressors and radial inflow turbine.
MTG ‘s have a high speed gas turbine engine driving an integral electrical generator that produces 20-100 KW power while operating at a high speed generally in the range of 50,000-120,000 rpm. Electric power is produced in the range of 10KHz converted to high voltage dc and then inverted back to 60 Hz, 480 V ac by an inverter.
During engine operation, engine air is drawn into the unit and passes through the recuperator where temperature is increased by hot exhaust gases. The air flows into the combustor where it is mixed with fuel , ignited and burnt. The ignitor is used only during start up and then the flame is self-sustaining.
The combusted gas passes through the turbine nozzle and turbine wheel converting the thermal energy of the hot expanding gases to rotating mechanical energy of the turbine. The turbine drives the generator. The gas exhausting from the turbine is directed back through the recuperator and then out of the stack.
Electrical Components
A. ENGINE CONTROLLER : The features of an engine controller includes
automated start sequence
Battery or utility start
Gas or liquid fuel algorithm
Recuperated or simple cycle engines
Fault detection and protection
Advanced user interface
The design is fully digital to give it the flexibility of adaptation to different engine types and makes it more precise .
B. Power Conditioning System
The power conditioning system converts the unregulated, variable-frequency output of the generator into a high quality ,regulated waveform and manages the interaction with any applied load both in stand-alone and utility connect modes. The waveform quality surpasses general utility standards and is suitable for supplying sensitive equipment. Output voltage and frequency are software adjustable between 380-480 V and 50-60 Hz ,allowing the system to be easily configured for operation anywhere. The system can be selected to operate as a stand-alone power source [island mode] and in parallel with a site utility supply [utility mode]. Emergency power and back start are also possible.
C. Power Controller
The overall power conversion process is managed by an advanced microprocessor-based control system. Unique control algorithms and active filtering techniques are used which allows the system to maintain voltage distortion levels under 3% even with severe non-linear loads [crest factor of 3]. The control system optimizes the capability of the power conditioning electronics and achieves a robust and tolerant supply which surpasses any UPS performance. An intelligent fault-clearing feature permits the supply of sufficient, short duration overload current to operate as appropriately sized circuit breaker. This feature prevents interruption to the remaining site load in the event of localized load faults. Other real time intelligent algorithms are used to cope with a variety of overload conditions commonly experienced in island mode. These include dc-offset control and current limiting which optimizes transformer energization and motor start capability. Utility mode protection includes over and under voltage and frequency, incorrect phasing and loss of supply.
