26-07-2012, 03:30 PM
EXCITATION SYSTEMS
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1.1INTRODUCTION:
Excitation control systems have been undergoing improvements/modifications keeping in step with requirements of larger generating units and complexity of inter connections in power systems. Various basic types of excitation systems which are in operation are
1. DC Excitation system using DC exciter and voltage regulators.
2. AC Excitation systems using Ac exciter, static rectifier and voltage regulator.
3. Brush less Excitation systems using Ac exciter, rotating diode rectifier system& Voltage regulator.
4. Static Excitation system.
All these are in operation and as such cannot be called obsolete. But for most present day applications in power plants, Static excitation systems sing shunt connected, completely static with thyristor control system OR Brush less excitation systems are being called as such we can presently confine to these two systems.
1.1 (I) MAJOR IMPROVEMENTS:
1. Based upon collaboration agreement with ABB, Switzerland which existed from 1976 to 1980, basic manufacturing of excitation systems was established at BHHL-EDn, Ban galore.
2. Subsequent to that a number of additional modules/modifications in design and established by conducting special tests and quality improvements have been done.
3. Modifications as required were introduced and a new design for SEE for synchronous motors applicators WD introduced.
4. Based upon the customer request for checking and ensuring of electronic module at site a Test JIG equipment was designed and the same has been supplied since 5 years.
5. Cubical design for various protection classes have been developed and are been supplied depending upon the location of the equipment.
6. Type tests for various environmental and vibration/seismic conditions have been carried out to ensure reliability of design and manufacture.
7. As per the state of art technology Microprocessor based DVRs have been introduced for Sheravathi HEP Renovation contract, based on ABB’s technology.
8. Power system stabilized having input of both power and frequencies have been introduced. This improved PSS will more effectively cater to the damp down oscillations of both local inter area mode.
1.1 (II) AUTOMATIC VOLTAGE REGULATORS:
For hydro sets mostly SEEs are preferred as these are comparatively slow speed machines connected with fairly long transmission lines. As such contribution to stability and fastness of excitation response are more essential.
For thermal and industrial power plants both static as well as indirect excitation having exciter machines and automatic voltage regulators are used. Quality improvements in design and manufacturing have been gradually strengthened. For ex., routine tests like temperature cycling on electronic modules, type tests viz. environmental tests, SWC tests etc.
1.2 FUNCTIONS OF EXCITATION:
1. Provide variable DC current with short time overload capability.
2. Control terminal voltage with suitable accuracy.
3. Ensure stable operation with network and / or other machines.
4. Keep machine within permissible operating range.
1.3 BASIC REQUIREMENTS OF EXCITATION:
1. Excitation Current up to 10’000 amps• Input frequency range from 16 Hz to 400 Hz.
2. Adaptable to different redundancy requirements for controls and Converters.
3. State of the art man machine interface.
4. Compatibility with most applied power plant control systems.
5. Remote diagnostics.
6. Comfortable commissioning tools.
1.4 APPLICATION DEVELOPMENT:
Excitation control equipment is the equipment providing excitation power, regulation, control and protection for a synchronous machine. The primary function of the equipment is to provide field current. Equally important is the regulation, control and protection aspects of a modern excitation system. These functions are accomplished automatically by appropriate changes in the level of machine excitation. A modern excitation system not only Maintains steady-state response but also maintains continuity of service during a severe power system disturbance or equipment malfunction that would threaten to damage the generating unit or system. The voltage regulator and excitation system functions to control the output of an ac generator or synchronous motor by varying the machine’s field voltage. Voltage regulation is accomplished by control of thyristor power amplifiers. The excitation may be controlled either manually by a base or dc regulator adjuster or automatically by the ac voltage regulator in response to the generator terminal voltage. The system may be configured to provide var or power factor control.
1.5 COMPARISION B/W DIFFERENT TYPES OF EXCITATION SYSTEM:
At present various types of excitation systems, such as, conventional DC, High frequency AC, Static & Brushless are being adopted in India and abroad. The conventional DC exciter was the unchallenged source of Generator Excitation for nearly fifty years till the rating of turbo-generators reached around 10OMW. In the last three to four decades, alternative arrangements have been widely adopted because of limitations of the DC exciters. With increase in generator ratings, it is no longer enough to consider the exciter used as earlier. Instead, the performances of the whole excitation system including the automatic voltage regulator and the response of the main generator have to be considered. Techno econ6mic considerations, grid requirements, reliability and easy maintenance have become prime considerations.
1.6 COMPARISION OF EXCITER CHARACTERISTICS:
Features Exciter Performance Characteristics
Potential Controlled Rectifier Brushless Exciter (rotating rectifier exciter)
High Initial Response Yes No
Sustained Fault Current Support No No
Online Rectifier Maintenance Possible Yes No
Spare Exciter User Yes No
Field Monitoring Ground Relaying Yes Yes, if Aux slip rings or OPTO/EM/RF Couplings used.
Rapid De-Excitation Yes, for half wave control field breaker discharge resistor is required. No
General Maintenance Brushes and Collectors Exciter Diode Check.
CONCLUSION:
By this time we know how an Excitation System & an Exciter work for Alternator & how much essential. If it is looking so simple yet it is very much essential for production of electricity by an Alternator.
UNIT-II
STATIC EXCITATION SYSTEM
2.1 INTRODUCTION:
In order to maintain system stability it is necessary to have fast excitation systems for large synchronous machines which means the field current must be adjusted extremely fast to changing operational conditions, besides maintaining the field current and steady state stability limits. It is because of these reasons the static excitation system is preferred to conventional excitation systems.
In this system, the AC power is tapped off from the generator terminal, stepped down and rectified by fully controlled thyristor bridges and then fed to generator field, there by controlling the generator voltage output. A high controlled is achieved by using an inertia free control and power electronic system. Any deviation in the generator terminal voltage is sensed by an error detector and causes the voltage regulator to advance or retard the firing angle of thyristors there by controlling the field excitation of alternator. Fig. Shows the block diagram of static excitation system.
Static excitation system can be designed without any difficulty to achieve high response ratio, which required by the system. The response ratio of the order 3 to 5 can be achieved by this system
This equipment controls the generator terminal voltage and hence reactive load flow by adjusting the excitation current. The rotating exciter is dispensed with SCRs are used which directly feed the field of the alternator.
The S.E.E consists of
1. Rectifier transformer
2. SCR output stage
3. Excitation start up and field discharge equipment
4. Regulator and operational control circuits
2.2 DESCRIPTION OF THE BLOCK DIAGRAM:
2.2.1 RECTIFIER TRANSFORMER:
The excitation power is taken from generator output and fed through the excitation,(rectifier) transformer which steps down to the required voltage ,for he SCR bridge and then fed through field breaker to the generator field. The rectifier transformer used in the SEE should have high reliability, as failure on this will cause shut down of power system. Dry type cast coil transformers are suitable for static excitation applications. The transformer is selected such that it supplies rated excitation current at rated voltage continuously and is capable of supplying ceiling current at the ceiling excitation for short period of 10 seconds.
2.2.2 SCR OUTPUT STAGE:
The SCR output stage consists of a suitable number of bridges connected in parallel. Each thyristor bridge comprises of 6thyristors working as a 6 pulse fully controlled bridge. Current carrying capability of each bridge depends on the rating of individual thyristor. Thyristors are designed such that their junction temperature rise is well within its specified rating. By changing the firing angle of the thyristors is cooled by fan.
The bridges are equipped with protection devices and failure of the bridge causes alarm. It there is failure of one more thyristor bridge the excitation current will be limited to a predetermined value lesser than the normal current. However, failure of the third bridge results in tripping and rapid de-excitation of the generator.
2.2.3 EXCITATION STARTS UP AND FIELD DISCHARGE EQUIPMENT:
Excitation starts up and field discharge equipment For the initial built-up of the generator voltage, a field flashing equipment is required. The rating of this equipment depends on the no-load excitation requirement and field time constant of the generator. From reliability point of view, provisions for the AC & DC field flashing are made.
The field breaker is selected such that it carries the full load excitation current continuously and also it breaks the max. Field current when the three phase short circuit occurs at the generator terminals. To protect the field winging of the generator against over voltages, and over voltage protector along with a current limiting resistor is used to limit the over voltage across the field winding. The voltage level at which OVP should operate is selected on insulation of field of the generator.
2.2.4 CONTROL ELECTRONICS:
Regulator is the heart of the system. This regulates the generator voltage by controlling the firing pulses to the thyristors.
2.2.4a) ERROR DETECTOR & AMPLIFIER:
The generator terminals a voltage is stepped down a three phase PT and fed to the AVR. The i.e. input thus obtained is rectified. Filtered and compared against a highly stabilized reference value and the difference is amplified in different stages of amplification. The AVR is designed with highly stable elements so that variation in ambient temperature does not cause any drift or change in the output level. Three CTs sensing the output of the generator feed proportional current across variable resistors in the AVR. The voltage thus obtained across the resistors, can e added vector ally either for compounding or for transformer drop compensation.
2.2.4b) GRID-CONTROL UNIT:
The output of the AVR is fed to a grid control unit, it gets its synchronous a.c. reference through a filter circuit and generates a double pulse spaced 600 el. Apart whose position depends on the output of the AVR, i.e. the pulse position varies continuously as a function of the control voltage. Energizing which provides two relays, the pulses can be either blocked completely or shifted to inverter mode of operation.
2.2.4c) PULSE-AMPLIFIER:
The pulse output of the “Grid control unit” is amplified further at intermediate stage amplification. This is also known as pulse intermediate state. The unit sets D.C. power supply, form a AC/DC or Dc converter unit built in relay is provided which can be used for blocking the 6 pulse channels. In a two-channel system (like Auto and Manual), the changeover is effected by energizing/ reenergizing the relay.
2.2.4d) PULSE FINAL STAGE:
This unit receives input pulses pulse from the pulse amplifier and transmits them through pulse transformers to the gates of the thyristors. A power supply, unit provides the required D.C. supply to the final pulse amplifier. Each Thyristor bridge has its own final pulse stage. Therefore, even if a thyristor bridge fails with its final pulse stage, the remaining thyristor bridges can continue to cater to full load requirement of the machine and thereby ensure (n-1) operation.
2.2.4E) MANUAL CONTROL CHANNEL:
A separate manual control channel is provided where the controlling d.c. Signal is taken form a stabilized d.c. Voltage through a D/A. Converter. The d.c. Signal is fed to a separate grid control unit whose output pulses after being amplified at an intermediate stage can be fed to the final pulse stage, when one channel is working generation the required pulses, the other remains blocked Therefore, a changeover from “Auto” to Manual” control or vice versa is effected by blocking or releasing the pulses of the corresponding intermediate state.A pulse supervision unit detects spurious pulses or loss of pulses at the Bus bar and transfers control form “Automatic” channel to “Manual” Channel