CHAPTER - 1 INTRODUCTION 1.1 AIM & OBJECTIVES: The main aim of this project is to discuss about the classification and functions of excitation system and study of brushless excitation system adopted in JPL 600MW unit. The main topics focused in this project are components of brushless excitation system, monitoring and supervisory of excitation system, excitation cooling arrangement and automatic voltage regulator. 1.2 SCOPE & LIMITATIONS: There are different types of excitation system (as per above classification) and ratings of excitation system also varies according to capacity of unit. The MW rating of excitation system is generally 0.5 to 1% of alternator MW rating (for 600MW unit it is 3 to 4 MW). In this project the discussion is limited to brushless excitation system of 600MW BHEL unit and ratings are also related to that unit. 1.3 FUTURE LINKAGES: The advances in excitation control systems over the last 20 years have been influenced by developments in solid state electronics. The latest development in excitation systems have been the introduction of digital technology. The control, protection and logic functions have been implemented digitally, essentially duplicating the functions previously provided by analog circuitry. The digital controls have become cheaper and possibly more reliable alternative to analog circuitry. They have the added advantage of being more flexible, allowing easy implementation of more complex control strategies and interfacing with other generator control and protective functions.
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1.4 CONCLUSION: From the power system view point, the excitation system should contribute to effective control of voltage and enhancement of system stability. It should be capable of responding rapidly to a disturbance so as to enhance transient stability of the power system.
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1.4 CONCLUSION: From the power system view point, the excitation system should contribute to effective control of voltage and enhancement of system stability. It should be capable of responding rapidly to a disturbance so as to enhance transient stability of the power system.
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CHAPTER – CHAPTER – 2 2 LITERATURE SURVEY
2.1 INTRODUCTION: In literature survey the functions and classification of excitation systems will be discussed. The dynamic performance measures of excitation control system (i.e. large signal and small signal performance parameters) are also described in this chapter. The small signal performance indices associated with time response and frequency response are considered for open circuit generator system only (i.e. generator supplying an isolated load). 2.2 EXCITATION SYSTEM: The Equipment for supply, control and monitoring of D.C supply to the field winding of alternator is called as “Excitation System”. Flux in the alternator rotor is produced by feeding D.C Supply in the field coils, thus forming required number of poles on the rotor (2 or 4 for Turbo Alternator). Excitation system performance is controlled by excitation control system which consists of automatic voltage regulator (AVR). AVR will control the excitation system output such that it will supply sufficient field current to alternator rotor to have constant terminal voltage. The AVR will generate control signal by considering other generator and exciter parameters along with alternator terminal voltage such that the operation of alternator is within its capability limits. 2.3 FUNCTIONS OF EXCITATION SYSTEM:
Provide stable reactive load sharing between generators running in parallel.
Automatically regulate the output voltage of synchronous generator by providing rotor with a controlled field supply over the entire load range.
Improve Dynamic and Transient Stability, thereby increasing Availability.
Ensure safe operation of generator within its capability limits.
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2.4 TYPES OF EXCITATION SYSTEM: EXCITATION SYSTEM
ROTATING EXCITATION
Conventional D.C Excitation
High Frequency Excitation
STATIC EXCITATION
Brushless Excitation
Fig 2.1 Classification of Excitation system 2.5 DYNAMIC PERFORMANCE MEASURES: The performance of the excitation control system depends on the characteristics of excitation system, the generator and the power system. Since the system is nonlinear, it is convenient to classify its dynamic performance into large signal performance and small signal performance. For large signal performance the nonlinearities are significant, for small signal performance the response is effectively linear. 2.5.1 LARGE SIGNAL PERFORMANCE MEASURES: Large signal performance measures provide a means of assessing the excitation system performance for severe transients such as those encountered in the consideration of transient, mid-term and long term stability of the power system. Such measures are based on the quantities define below. Some of the performance measures are defined under specified conditions, these conditions may be specified as appropriate for the specific situation.
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a) Ceiling Voltage: The maximum direct voltage that the excitation system is able to supply from its terminals under specified conditions. Ceiling voltage is indicative of field forcing capability of the excitation system. Higher ceiling voltages tend to improve transient stability. For excitation systems whose supply depends on the generator voltage and current (static excitation), the ceiling voltage is defined at specified supply voltage and current. For excitation systems with rotating exciters (brushless excitation), the ceiling voltage is determined at rated speed. b) Ceiling Current: The maximum direct current that the exci tation system is able to supply from its terminals for a specified time. When prolonged disturbances are a concern the ceiling current may be based on the excitation system thermal duty. c) Voltage Time Response: The excitation system output voltage expressed as a function of time under specified conditions. d) Voltage Response Time: The time in seconds for the excitation voltage to attain 95% of the difference between the ceiling voltage and rated load field voltage under specified conditions. The rated load field voltage is the generator field voltage under rated continuous load conditions o
with the field winding at (i) 75 c for windings designed to operate at rating with temperature rise o
o
60 c or less, or (ii) 100 c for windings designed to operate at rating with a temperature rise o
greater than 60 c. e) High initial response Excitation system: An excitation system having a voltage response time of 0.1 sec or less. It represents a high response and fast acting system. f) Excitation system Nominal response: The rate of increase of the excitation system output voltage determined from the excitation system voltage response curve, divided by the rated field voltage. This rate, if maintained constant, would develop the same voltage time area as obtained from the actual curve over the first half second interval (unless a different time interval is specified). The nominal response is determined by initially operating the excitation system at the rated load field voltage (and field current) and then suddenly creating the three phase terminal voltage input 5
signal conditions necessary to drive the excitation system voltage to ceiling. It should include any delay time that may be present before the excitation system responds to the initiating disturbance.
Fig 2.2 Excitation system nominal response Referring to Fig 2.2, the excitation response is illustrated by line „ac‟. This line is determined by establishing area „acd‟ equal to area „abd‟.
Where, oe = 0.5 sec ao = rated load field voltage The basis for considering a nominal time span of 0.5 sec in the above definition is that, following a severe disturbance the generator rotor angle swing normally peaks between 0.4 sec to 0.7 sec. The excitation system must act within this time period to be effective in enhancing transient stability. It is not a good figure of merit for excitation systems supplied from the generator or the power system, due to reduced capability of such systems during a system fault. For high initial response 6
systems, the nominal response merely establishes the required ceiling voltage. The ceiling voltage and voltage response time are more meaningful parameters for such systems. 2.5.2 SMALL SIGNAL PERFORMANCEMEASURES: Small signal performance measures provide a means of evaluating the response of the closed loop excitation control systems to incremental changes in system conditions. Small signal performance may be expressed in terms of performance indices used in feedback control system theory:
Indices associated with time response and
Indices associated with frequency response.
The typical time response of a feedback control system to a step change in input is shown in Fig 2.3. The associated indices are rise time, ov er shoot and settling time.
Fig 2.3 Typical time response to step input
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A typical open loop frequency response characteristic of an excitation control system with the generator open circuited is shown in Fig 2.4
Fig 2.4 Open loop frequency response of an excitation control system with generator open circuited The performance indices associated with the open loop frequency response are the low frequency gain G, cross over frequency ωc, phase margin ϕm and gain margin Gm. Large values of G provide better steady state voltage regulation, and large cross over frequency ωc indicates faster response. Large values of phase margin ϕm and gain margin Gm provide a more stable excitation control loop. The typical closed loop frequency response of an excitation control system with generator open circuited is shown in Fig 2.5
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Fig 2.5 Closed loop frequency response of an excitation control system with generator open circuited The indices of interest associated with the closed loop frequency response are the bandwidth ωB and peak value M p. A high value of M p (>1.6) is indicative of an oscillatory system exhibiting large over shoot in its transient response. In general, a value of M p between 1.1 and 1.5 is considered as a good design practice. Large values of bandwidth indicate faster response. It approximately describes filtering or noise rejection characteristics of the system. The performance indices mentioned above are applicable to any feedback control system having a single major feedback loop, i.e., a single controlled output variable. Therefore they are applicable to an excitation control system with the synchronous machine on open circuit are feeding an isolated load. Stable operation of the excitation control system with the generator offline is ensured based on these performance indices a nd associated analytical techniques.
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On the other hand, synchronous machines connected to a power system form a complex multi loop, multi variable, and high order control system. For such a system, the performance indices mentioned above are not applicable. The state space approach using eigenvalue techniques is an effective method of assessing the performance of such complex systems. 2.6 CONCLUSION: Classification of excitation system and its performance measures was discussed in this chapter. Large signal and small signal performance measures are also defined along with their importance in design of excitation system. Modern excitation systems are designed to have high initial response such that they are able to respond to transient faults quickly and the system stability is maintained.
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CHAPTER - 3 BRUSHLESS EXCITATION SYSTEM 3.1 INTRODUCTION: As the name suggests in this excitation system there are no brushes for supply of power from exciter output to alternator field winding. The main components of brushless excitation system are:
Permanent Magnet Pilot Exciter (ELP 50/42 – 30/16)
Three phase Main Exciter (ELR 70/90 – 30/6 – 20N)
Rotating Diode Wheel
Multi Contactors
Automatic Voltage Regulator
Exciter Cooling Arrangement
Metering and Supervisory System
Exciter Bearings
Coupling to Alternator
Diode Wheels
Main Exciter Rotor
Sliprings
Pilot Exciter
Cooling Fan
Fig 3.1 Excitation System Rotor
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3.2 SCHEMATIC DIAGRAM OF EXCITATION SYSTEM:
Pilot Exciter Slip Rings Main Exciter AVR Quadrature Coil
Diode Wheel Set
Feedback to AVR
Multi Contactors Field Winding
Generator Stator
Fig 3.2 Line diagram of Excitation system
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3.3 PERMANENT MAGNET PILOT EXCITER: Pilot Exciter is a 16 pole revolving field unit. The rotor is salient pole type with 16 numbers of poles and its stator frame consists of laminated core with three ph ase winding. Each pole of rotor consists of 12 separate permanent magnets housed in a non-magnetic metallic enclosure. The output frequency of pilot exciter is 400Hz in rated speed condition.
Armature Winding
Permanent Magnets
Fig 3.3 Pilot Exciter Stator
Balance Weights
Fig 3.4 Pilot Exciter Rotor
Rated Voltage
220 Volts
Rated Current
195 Amps
Rated Speed
3000 RPM
No. of Phases & Frequency
3 & 400 Hz
Insulation Class & Type
Class F & Enamelled Glass Table 3.1 Ratings of Pilot exciter 13
3.3.1 TYPE OF PILOT EXCITER: E L P 50 / 42 – 30 / 16 E = Exciter
50 = Diameter of Rotor Body in cm
L = Air Cooling
42 = Length of Core in cm
P = 3 Phase Pilot Exciter
30/16 = 3000 rpm & No. of Poles
3.4 MAIN EXCITER: The three phase main exciter is a 6 pole revolving armature unit. The poles are arranged in stator frame with field and damper winding to reduce diode commutation reactance. The three phase winding is inserted in the slots of the laminated armature rotor. The winding conductors are transposed with in the core length and the end turns are secured with steel bands. The winding ends are run to a bus ring system to which the three phase leads to the rectifier wheels are connected. The output frequency of main exciter is 150Hz in rated speed condition.
Damper Winding
Field Winding
Fig 3.5 Main Exciter Stator
Fig 3.6 Main Exciter Rotor
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Rated Voltage
413 Volts
Rated Current
5192 Amps
Rated Field Current
110 Amps
No. of Phases & Frequency
3 & 150 Hz
Insulation Class & Type for armature
Class F & Epoxy Glass Lapped
Table 3.2 Ratings of Main exciter 3.4.1 TYPE OF MAIN EXCITER: E L R 70/90 – 30/6 – 20N E = Exciter
70/90 = Rotor Diameter / Core Length in cm
L = Air Cooling
30/6 = 3000 rpm / Six Poles
R = Main Exciter with Rotating Rectifier
20N = 20 Diodes connected in Parallel
3.5 DIODE WHEELS: The main components of the rectifier wheels are the silicon diodes which are arranged in the rectifier wheels in a three phase bridge circuit. Two diodes are connected in parallel and mounted in each aluminum alloy heat sink. Associated with each heat sink there is a fuse in series with diodes which serves to switch off the two diodes if one diode fails (i.e. Loss of Reverse Blocking Capability). For suppression of the momentary voltage peaks arising from commutation, each wheel is provided with six RC networks consisting of a capacitor and a damping resistor each, which are combined in a single resin encapsulated unit. The insulated and shrunken rectifier wheels serve as DC buses for negative and positive side of Rectifier Bridge. The two wheels are identical in mechanical design and differ only in forward direction of diodes (i.e. Positive group and Negative group). The direct current from the rectifier wheels is fed to DC leads arranged in the center bore of the shaft via radial bolts. The three phase AC power is supplied to diode rectifier from the main exciter by the conductors taken axially along the surface of the shaft.
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Fuses
Power Diode
Hot air outlet holes
Diode Wheel Negative
Diode Wheel Positive
Fig 3.7 Diode Wheels The three phase bridge circuit consists of six bridge arms (Three arms for each diode wheel) Number of parallel paths for each bridge arm is 10 and each parallel path consists of two diodes in parallel. Number of fuses and heat sinks for each diode wheel is 30.
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Diode Rating Rated average forward current
690 Amps
Maximum repetitive peak inverse voltage
2600 Volts
Maximum junction temperature
90
Fuse Rating Voltage Rating
800 Volts
Current Rating
800 Amps
Fuse failure indicator
Stroboscope Table 3.3 Ratings of Fuse and Diode
Fig 3.8 Schematic Diagram of Rectifier Wheels 1) A.C Lead
4) Diode
7) Tension Bolt
2) Fuse
5) Diode Wheel (+ ve)
8) Terminal Bolt
3) Heat Sink
6) Hot air Outlet
9) Diode Wheel (- ve)
10) DC Lead
* Average forward current is the average value of current flowing through the diode in forward bias condition of diode in rectifier. ** Repetitive Peak inverse voltage is the Maximum voltage a diode can withstand in the reverse biased direction before breakdown repetitively.
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3.6 MULTI CONTACTORS: Mechanical coupling of generator and exciter shaft assemblies results in simultaneous coupling of D.C leads in the central shaft bore through the Multicontanct electrical system consisting of plug in bolts and sockets. The DC leads are connected to the diodes via radial bolts. This contact system is also designed to compensate for length variations of the leads due to thermal expansion. The DC supply is connected to main field winding through the radial bolts connection at alternator end.
Diode Wheel +ve
Tension Bolt
Diode Wheel -ve
Fig 3.9 Diode wheels and Tension Bolt
Fig 3.10 Multicontact Pins 18
Coupling to Alternator
3.7 EXCITER BEARING: Exciter rotor is mounted on generator bearing and another journal bearing located between main exciter and cooling fan. The bearing temperatures are measured with thermocouples located in the bearing lower halves. The actual measuring point is located at the babbitt / sleeve interface. Generator rotor and exciter rotors are manufactured with high precision and carefully balanced. However some unavoidable residual balance will result in vibrations during operation, which are transmitted to stator frame and foundation via the bearings. To permit a reliable assessment of the running condition, vibration pickups are located at the bearings. Measurement and recording of temperature and vibration are performed in conjunction with the turbine supervision. The overall turbine protection is tripped when the maximum permissible temperature or vibration is exceeded.
Fig 3.11 Bearing temperature measurement 3.8 CONCLUSION: In this chapter the main parts of excitation system are explained along with the ratings related to 600MW BHEL unit. In next chapters the cooling arrangement of brushless excitation system components will be discussed. 19
CHAPTER - 4 EXCITER COOLING SYSTEM 4.1 INTRODUCTION: In any electrical system heat will be dissipated due to the its copper losses and other losses. There should be proper cooling arrangement to remove heat from system to avoid its damage. Air is the cooling media for excitation system. The complete exciter is housed in an enclosure through which the cooling air circulates and cooled in two cooler sections arranged alongside the exciter. The main exciter receives the air from the fan which draws the cold air through the pilot exciter. The air enters the main exciter from both ends and is passed into ducts below the rotor body and discharged through radial slots in the rotor core to the lower compartment. The rectifier wheels draw the cold air in at both ends and expel the hot air to the compartment beneath the base plate. The warm air is cooled in cooler sections and then returns to the main enclosure 4.2 REPLACEMENT OF AIR INSIDE EXCITER ENCLOSURE: While the generator is running, the air leaving the exciter enclosure via the bearing vapor exhaust system and the leakage air outlet in the foundation provides for a pull through system. The volume of extracted from the cooling air circuit is replaced via the filters located at the top of the enclosure. When the generator is at rest the air drier of the exciter unit discharges dry air inside the exciter enclosure. The air leaves the exciter enclosure via the leakage air filter and the leakage air outlet at the shafts as well as via the bearing vapor exhaust system (if the system is in service). The air 3
volume inside the exciter enclosure requires an air change rate of 125m /hr. 4.3 EMERGENCY COOLING: In the event of cooler failure three flaps are provided on the exciter for emergency cooling. One flap is provided on the main housing for admitting cold air. Two flaps are provided below the coolers for discharging the hot air through the openings in the base frame. These flaps open
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o
when cold air temperature in exciter housing raises above 48 C and an open circuit cooling is maintained in the exciter.
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2
8
3
1
4
5
6
Fig 4.1 Exciter Cooling Air Circuit during Operation 1. Exciter Coolers
5. Exciter Cooling Fan
2. Rotating Rectifier
6. Pilot Exciter
3. Main Exciter
7. Air leakage air at Shaft ends
4. Exciter Bearing
8. Air inlet through Filters
4.4 EXCITER AIR DRYING SYSTEM: A dryer (dehumidifier) and an anti-condensation heating system are provided to avoid the formation of moisture condensate inside the exciter with the alternator at rest or on turning gear. The dehumidification takes place in a slowly rotating dryer wheel (approximately 7 revolutions per hour). The dryer wheel consists of a magnesium silica alloy containing crystalline lithium chloride (hygroscopic material) as adsorbent material. The inlet side of dryer wheel is subdivided th
th
so that 1/4 is available for regeneration and 3/4 for adsorption section. When air passes through the adsorption section of dryer wheel, the moisture is removed by adsorbent material by the result of partial pressure drop existing between the air and adsorbent 21
material. In regeneration section of the dryer wheel, the accumulated moisture is removed from the dryer wheel by the heated regenerated air. The tubular ducts on the inlet side of dryer wheel are dimensioned so that a laminar flow with low pressure loss is obtained even at higher air velocity. An Anti-condensation heating system to support the dryer is installed in the exciter base frame. The heaters are rated and arranged so that the temperature of air in the exciter interior is within limits. There are 5 heaters and the on/off of these heaters is based on the output of Duplex RTD (MKC80CT012) at exciter fan inlet.
Fig 4.2 Schematic Diagram of Dryer
1. 2. 3. 4. 5. 6. 7.
Regeneration air Outlet Dryer Wheel Heater Ventilator Filter Shutoff Valve Dry air Outlet
4.5 CONCLUSION: In this chapter the cooling system of brushless excitation system is described along with the flow path of cooling air in the enclosure. The working of air drying system is also discussed. 22
CHAPTER - 5 EXCITER SUPERVISORY SYSTEM
5.1 INTRODUCTION: The most essential measuring and supervisory devices in the excitation system are: 1. Temperature monitoring system 2. Excitation current measuring device 3. Fuse monitoring system 4. Ground fault detection system 5.2 TEMPERATURE MONITORING SYSTEM: The exciter is provided with devices for monitoring the temperatures of the cold air after the exciter cooler and the hot air leaving the rectifier wheels and hot air leaving the rectifier wheels and main exciter. There are1 total 6 RTD‟s for exciter air temperature measurement and 2 RTDs and 2 Dial type thermometers for cooling water te mperature measurement. They are:
RTD CODE
RTD POSITION
TEMPERATURE MEASUREMENT
MKC82 CT001
Hot air inlet to Coolers
Hot air from Diode wheel and Main Exciter
MKC82 CT002
Hot air inlet to Coolers
Hot air from Diode wheel and Main Exciter
MKC82 CT003
Hot air inlet to Coolers
Hot air from Diode wheel and Main Exciter
MKC84 CT002
Hot air inlet to Coolers
Hot air from Rotating Diode wheel
MKC80 CT014
At Exciter Fan inlet
Cold air inlet to Main exciter
MKC80 CT012
At Exciter Fan inlet
For switching on/off heaters
PGB42 CT001
Cooling water outlet
For Hot cooling water at cooler 1 outlet
PGB42 CT002
Cooling water outlet
For Hot cooling water at cooler 2 outlet
Table 5.1 RTD‟s in Excitation system 23
5.3 EXCITATION CURRENT MEASURING DEVICE:
Main Exciter Damper Winding
Quadrature Coils
Main Exciter Poles
Fig 5.1 Quadrature coil in Main exciter The exciter armature winding is placed on the shaft, so it is not possible for direct measurement of exciter outlet current. So, the exciter current is measured indirectly through a coil (quadrature coil) arranged between two poles of the main exciter (i.e. along the quadrature axis). The voltage induced in this coil is proportional to the exciter current thus enabling a determination of the excitation current. There 2 quadrature coils which are placed in the bottom half of the main exciter stator. 5.4 FUSE MONITORING SYSTEM: During operation of exciter set the fuses on the rotating rectifier wheel are monitored with the help of stroboscope. On each of the two wheels A and B, separate flash tubes are provided. A common control unit is provided to control these flashing tubes. The control unit is mounted on the exciter enclosure whereas the tubes are permanently installed in the rectifier wheel enclosure.
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The capacitor and high voltage transformer required to produce the firing pulses for the flash tubes are located on a printed circuit board which is accommodated in the handle of flash lamp. To synchronize the sequence of flashes with the generator rotation the system frequency is utilized to activate the flashes, so that a slow motion observation of the fuse becomes possible. 0
The observation period for one full revolution of the rectifier wheel (360 ) is approximately 25 seconds. After approximately 2 minutes the stroboscope is automatically switched off. If this period should not be sufficient for fuse checking switching on the stroboscope for another two minutes without delay can be repeated for any desired number of times.
Observation point of Fuse Bolt to connect to the Diode wheel
Diode Wheel Enclosure
Fig 5.2 Rectifier wheel Fuse
Flash tube to control unit
Stroboscope
Fig 5.3 Fuse monitoring system
5.5 GROUND FAULT DETECTION SYSTEM: Any Single ground fault occurring on field winding or exciter circuit is not a big problem for machine. But more than one earth fault will leads to magnetic unbalances with very high currents flowing through the faulted part, resulting in its destruction within a very short time. Two sliprings are installed on the shaft between main exciter and bearing. One is connected to the star
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point of the three phase winding of main exciter and the other to the frame. These sliprings permit ground fault detection. If the field ground fault detection system detects a ground fault, an alarm is activated at value of st
resistance to earth (R E) falls below 80kΩ (1 stage). If the insulation resistance between exciter field circuit and ground either suddenly or slowly drops to R E < 5kΩ the generator electrical nd
protection is tripped (2 stage).
1. Measuring Slipring 2. Measuring Brush 3. Mounting Plate 4. Brush carrier segment 5. Plug in brush holder 6. Measuring rod 7. Measuring Leads
Fig 5.4 Brushes for Ground fault detection
The location of various measuring points on the excitation system is pointed in Fig 5.5 along with the KKR codes of measuring instruments.
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Fig 5.5 Exciter Measuring Points 27
CHAPTER 6 AUTOMATIC VOLTAGE REGULATOR 6.1 INTRODUCTION: Excitation system performance is controlled by excitation control system which consists of automatic voltage regulator (AVR). AVR will control the excitation system output such that it will supply sufficient field current to alternator rotor to have constant terminal voltage for alternator. The AVR will generate control signal by considering other generator and exciter parameters along with alternator terminal voltage such that the operation of alternator is within its capability limits. Alternator terminal voltage and output current are taken as feedback to AVR with help of instrument transformers for continuous monitoring and controlling. 6.2 TYPES OF AVR: Based on the type of control circuits designed AVR is of two types. They a re, i) Analog type AVR ii) Digital type AVR Now days most of the AVRs are of digital type due to their advantages over analog type in terms of response time, configuration and control etc. In JPL 600MW unit the AVR used is of Digital AVR type. 6.3 ADVANTAGES OF DIGITAL AVR:
Lesser number of electronic cards
Better configuration and control
Fast reaction to network disturbances
Self-monitoring and fault diagnosis features
User friendly software can be used for setting parameters and measuring variables
Troubleshooting easier
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6.4 COMPONENTS OF AVR: Automatic voltage regulator consists of, a) Automatic generator voltage regulator – Auto b) Exciter current regulator – Manual c) Gate control unit – Auto/ Manual channel d) Thyristor sets – Auto/ Manual channel e) De excitation Equipment f) Follow up control unit g) Open loop system for exchange of signals between regulator and control room The basic block diagram of AVR system is as shown in Fig 6.1
Fig 6.1 Block diagram of AVR
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6.5 LIMIT CONTROLLERS: With increase in size of generating units, the requirements to be met by excitation systems are also increased. Generators running in parallel with the power network even under extreme conditions must remain in synchronism with the maximum load limit on it being not exceeded. Optimum utilization of the generator can be ensured only if the basic AVR is influenced by additional limiting signals along with generator terminal voltage. Various types of limit controllers are as discussed below. a) Rotor current limiter: The field current is measured on the AC input side of rotating rectifier and it is converted into proportional DC voltages. The signal is compared with an reference value, amplified and with necessary time lapse fed to the voltage regulator input. Rotor current limiter avoids thermal overloading of rotor winding. The ceiling excitation is limited to a predetermined limit and is allowed to flow for a time which is dependent upon the rate of rise of field current before being limited to the thermal limit vlue.
Fig 6.2 Rotor current limiter block diagram 30
b) Stator current limiter: The stator current limiter has to influence the AVR differently depending on whether the machine is over excited or under excited. The excitation current is to be suitably reduced to limit the inductive stator current and is to be increased to limit the capacitive current.
Fig 6.3 Stator current limiter block diagram The generator stator current is converted into polarized DC signal positive or negative, depending upon whether the machine is over excited or under excited. This voltage forms the actual value for the controllers which process each of bipolar signals independently. One of these controllers compare the capacitive stator current against its reference and acts directly on the regulator via a decoupling diode to increase excitation. The action of second controller which limits the inductive stator current is delayed by means of an integrator before it influences the control input of the AVR so as to reduce the excitation. The time lag offered is acceptable as far as stator overheating is concerned. The integrator time constant is set one order less than the stator thermal time constant.
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c) Rotor angle limiter: The load angle is the electrical angle between the voltage vector of the system and the vector of the machine voltage. In the event of a short circuit in the system the generators may accelerate owing to the abrupt partial removal of the electrical load. Due to this the rotor angle increases and the angle can become so large relative to the system vector that the machine may fall out of step. The rotor angle limiter limits the load angle of the machine to an acceptable present value and provides a more definite protection in preventing the machine from falling out of step.
Fig 6.4 Rotor angle limiter block diagram d) Minimum excitation limiter: Minimum excitation limiter measures the reactive power and compares it with the set reactive power in the leading side. It gives positive output to increase the voltage regulator reference. This limiter assures to feed the generator the minimum excitation that is necessary to keep the synchronization defined by the capability curve. e) Power system stabilizing unit: In large power system network sudden change in load steps all kinds of oscillatory responses. These oscillations are essentially exchange of power or energy between rotors of machines. The power system stabilizing unit is used for the suppression of rotor oscillations of the machines through the additional influence of excitation. The power as well as acceleration signals needed for the stabilization are derived from active power delivered by the alternator. Both the signals will be correspondingly amplified and summed up to influence
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the excitation of the synchronous machine through AVR in a manner as to suppress the rotor oscillations. 6.6 CAPABILITY DIAGAM OF GENERATOR: Capability diagram of generator gives the safe operating regimes and limitations etc. This is of great help to the operating engineers to ensure operations of the machines accordingly. Their information particularly for limiting zones of operations are useful in setting the various limiters of AVR. MW values are marked on Y- axis and MVAR values on X-axis on per unit basis rated MVA. The typical capability diagram of a generator will be as shown below.
Fig 6.5 Generator capability diagram
Safety factor a 12.5 percent (1.125 p.u) power margin to increase in power demand with no corresponding increase in excitation gives Practical stability limit line. From the point „A‟ the dotted line „AS‟ denotes the theoretical stability line. The diagram “FBED” is the „Capability Diagram‟ of the machine (i.e. the safe operating region of the machine).
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