Generator Excitation System
Generator Excitation System
Obviously, most power plant generators have an excitation system. Starting a generator gives a cause for concern. The excitation system is a device that supplies asynchronous machine (here the generator) with excitation current. It is comprised of power suppliers (feeders), regulator, controls and protection devices.
The system produced by MECO for customers, is a static one that is in fact a controllable thyristor rectifier bridge that is powered via an external source such as a transformer. This transformer can be connected to the generator terminals or powered by an auxiliary bus in the power plant. In the static excitation system, the current is transmitted to the field windings through coals and slip rings. A static excitation system outdoes dynamic excitation system in several ways.
In addition to using the excitation system THYRIPOL 520/1800 R Siemens and Unitrol6000 of ABB Switzerland, MECO has also developed its home-grown excitation system called MAPEX and has many capabilities compared to its European counterpart.
General system description:
The THYRIPOL 520/1800 R excitation system is one of the main strategic systems in power plants used in gas and steam units. It is manufactured by MECO under SIEMENS license for medium and large-scale generators. To achieve redundancy (2×100%) and increase the system reliability, two separate working channels including two AVR controllers and two rectifiers have been used. In case of an error or at operator command, while maintaining the normal system operation and without changing the machine terminal voltage, one channel replaces the other immediately and without interruption. In the power circuits, two AC switches are provided to isolate the excitation system panel from the input which results in easy system maintenance. In implementation of excitation control system based on the generator physical limitations and customer needs, many control functions and limits such as over-excitation and under-excitation limiters, along with stator current limiter, voltage per Hz, PSS power system stabilizer, reactive power controller and power coefficient controller are taken into account.
- Easy and cheap set up, and maintenance via compact and modular design incorporating the installation of excitation system components in separate panels
- Automatic voltage regulator (AVR) and manual (FCR)
- OP touch panel with a set of analog meters to enable system monitoring and run all operator operations in local mode
- Air Force system including fan to prevent heat loss
- advanced equipment such as S7-SIMATIC control racks and DC motor drive (SIMOREG) incorporating features including:
- Modular design with system control board technology
- PC / PG connectivity
- Appropriate software package to adjust system parameters
- Control and operator panel
- A recorder logging events in the system memory by connecting the system to PC / PG and drawing the corresponding diagrams
- Over voltage protection to strengthen system security with AC switches and circuit breaker protection
- Full redundancy (2*100%) design to ensure increased system reliability
- Achieving the control loops required sample values via current and voltage transformers at the generator output
|Excitation system specifications|
|Excitation voltage at rated load||296V|
|System response time||1/3S|
|Excitation current at rated load||1288A|
|Impulse current test||2250A|
|Maximum input voltage||520V|
|Thyristors type||5ST P27 H180|
|Air force system||Forced air cooling|
|IP protection degree||32|
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