Excitation System

EXCITATION The injection of dc in the field winding to produce magnetic field is called excitation

PURPOSE OF EXCITATION • The purpose of excitation system is to monitors line voltage and current constantly and produce proper excitation voltage necessary to maintain terminal voltage constant under all conditions of generator operation (no load, full load etc). • At no load, the excitation system should only supply that much amount of volts necessary to maintain the terminal voltage of generator constant. when a sudden load is applied to a generator, it’s terminal voltage decreases slightly, therefore an efficient excitation system senses the voltage dip and increase the excitation volts immediately and thus maintains the terminal voltage.

TWO TYPES OF CONTROLS ASSOCIATED WITH SYNCHRONOUS GENERATOR 1. AVR/ Excitation system Control (Voltage, Power factor and MVAr are Controlled by AVR)

2. Governor/Turbine Control (Speed, Frequency and MW output is controlled by Governor)

EXCITATION SYSTEM 1. Excitation system requirements 2. Elements of excitation systems 3. Dynamic performance measures  Large signal performance measures  Small signal performance measures

Basic functions of excitation system • The basic function of an excitation system is to provide direct current to the synchronous machine field winding. • This is also performs control and protective functions essential to the satisfactory performance of power system. 1. Control Functions 2. Protective Functions

Excitation system has a strong interface with the generator protection, generator control and power system stability as indicated below

Control functions 1. Control of voltage and reactive power flow 2. Enhancement of system stability (30-60 sec)

Protective functions  Ensure that the capability limits of synchronous machine excitation system and other equipment are not exceeded.  Operating of the system within the capability limits of all equipments belongs to excitation system and associated equipment.  Extra capability for short time, long time and transient stability of the system

When exceed the outside of the capability curve protection system comes into picture then generator life going to be not affected. I. EXCITATION SYSTEM REQUIREMENTS: 1. Generator considerations:  Main terminal voltage as output varies  Must be able to respond to transient disturbances with field forcing (power system consideration)

Field forcing: Supplying of more field voltage (or) current to the field winding for short duration during power system disturbance is nothing but a high ceiling voltage. 2. Power system considerations:  Effective control of voltage and enhancement of system stability  Rapid response to improve transient stability  Modulation of field current to enhance small signal stability (PSS)  The system is never in steady state conditions it oscillates state as load varies on the system

II. 1. 2. 3.

ELEMENTS OF AN EXCITATION SYSTEM: Exciter Regulator Terminal voltage transducer and load compensator 4. Power system stabilizer 5. Limiters and protective circuits

Total Excitation System

1. Excitation supplies field current. 2. Output of exciter is control by providing regulator. 3. If senses the output of Generator voltage it may be 11 kv or 16 kv depends on machine, step downs and rectified to DC signal and sends to regulator. Exciter output sensing signal is closed loop control for stability for excitation, otherwise exciter is unstable. 4. Auxiliary controller it will operate only in the dynamic conditions not in the steady state. Senses deviation in the power output or frequency (speed) or combination of these two.

III. DYNAMIC PERFORMANCE MEASURES:  Large signal performance measures  Small signal performance measures Small signal: When small disturbance, basically look into the stability of the system. Large signal: 3-phase faults, Transmission line trips

• Excitation system ceiling voltage: The maximum direct voltage that the excitation system is able to supply from its terminals under specified conditions. It is the indicative of the field forcing capability of the excitation system, higher ceiling voltage tend to improve transient stability. • Excitation system ceiling current: (1.6 times of rated current): The maximum direct current that the excitation system is able to supply from its terminals for specific time for prolonged disturbances. This value depends on thermal duty of the excitation system.

• Excitation system voltage time response: The excitation output voltage expressed as a function of time under specified conditions • Excitation system voltage response time: The time in seconds for excitation voltage to attain 95% of the difference between the ceiling voltage and rated load field voltage under specified conditions

Excitation system voltage response time

• High initial response excitation system: An excitation system having a voltage response time of 0.1 sec (or) less. • Excitation system nominal response: The rate of increase of excitation system output voltage determined from the excitation system voltage response curve, divided by the rated field voltage

Excitation system nominal response

• Nominal reponse = cd/(ao)(oe) Where oe = 0.5 sec ao = rated field voltage

• The basis of considering a nominal time span of 0.5 sec in the above definition is that, following a severe disturbance the rotor angle swing normally peaks between 0.4 sec to 0.75 sec. The excitation system must act within this time period to be effective in enhancing the transient stability. For high initial response excitation system, the nominal response merely establishes the required ceiling voltage.

• Small signal performance measures: Small signal performance measures provide a means of evaluating the response of the closed loop excitation control system to incremental changes in system conditions. Small signal performance may be expressed in terms of performance indices used in feed back control system theory.  Indices associated with time response  Indices associated with frequency response Indices associated with time response 1. Rise time 2. Over shoot 3. Settling time

Which characterize the performance of the small signal response of excitation system

Indices associated with frequency response: Low frequency gain Cross over frequency Phase and gain margins For tuning of AVR to reduce steady state error phase and gain margins must be low For any particular systems to have good characteristic the features are the Gain margin ≥ 6 db Phase margin ≥ 400 Over shoot = 5-15%

CONTROL AND PROTECTIVE FUCNTIONS • Which are provided in the excitation system 1. AC & DC Regulators 2. Load Compensation 3. Under Excitation Limiter 4. Over Excitation Limiter 5. Field shorting circuits

Control and protective functions are required to achieve the desired performance of the generator and power system 1. Excitation control 2. Excitation limiting 3. Excitation protection

• AC & DC Regulators: The basic function of the AC regulator is to maintain stator voltage(terminal voltage). The dc DC regulator hold constant generator field voltage and is commonly referred to as manual control. • In order to have stable performance of the exciter/AVR the simplest circuit is used is derivative feed back and his derivative feedback is achieved through transfer function.

Load Compensation

• Load compensator is to give the voltage signal to voltage regulator as per load. • If load compensation is not required it can be set zero and voltage regulator regulates by the direct signal of terminal voltage. • With Rc & Xc positive, the voltage drop across the compensator is added to the terminal voltage. The compensator regulates the voltage at a point with in the generator and then provides voltage drop. This will be applied when two generators are in parallel for sharing of reactive power.

• With Rc & Xc negative, the compensator regulates voltage at a point beyond the machine terminals. This type of compensation is used to compensate the voltage drop across the step up transformer. This will be applied when two generators are paralleled with their individual generator transformers.

• Under Exciter Limiter (O/E): When the excitation of synchronous generator comes below certain value this operation comes into picture. The under excitation limiter is intended to prevent reduction of generator excitation to a level where the small signal stability limit or the stator core end region heating limit is exceeded. This limiter is also referred to by other names such as under excitation-reactive ampere limiter and minimum excitation limiter.

Active power Vs Reactive power

• Over Excitation limiter (O/E): The purpose of the over excitation limiter is to protect the system from over heating due to prolonged field over current. This limiter is also referred to as the maximum excitation limiter. O/E limiting function typically detects the high filed current condition and after a time delay, acts through the ac regulator to ramp down the excitation to the present value. If it is unsuccessful it trips the ac regulator, and transfers control to the dc regulator and repositions the set point to a value corresponding to rated value.

• Volts / Hz Limiter (V/f): This function is used to protect the generator, generator transformer and unit auxiliary transformer from damage due to excessive magnetic flux resulting from low frequency and over voltage. If excessive magnetic flux is sustained for long time serious over heating and may result in damage to the unit transformers and to the generator core. • Flux produced in the core is function of the terminal voltage.

• Field Shorting Circuits: In excitation system we can not make field current reverse because rectifiers are provided. But under certain situations it is desirable to allow the current to flow in the reverse direction, particularly during pole slipping and short circuit conditions where the stator carries the dc component. whenever fault occurs at the terminal of the synchronous generator. The voltages which are produced in the field winding of the synchronous generator try to drive the current in reverse direction. In case we do not have the provision for reversing then the voltage produced will be very high which may damage the field winding of the generator.

• When the voltage is higher than the certain set value thryrister gets ‘ON’ and allow to flow in reverse direction. When fault occurs voltage raises then it allow to flow in reverse direction and flow through FDR, other wise it damages the life of generator.

• When the voltage across the circuit is below set value then Varistor(non-linear resistance) offers infinite resistance. When voltage goes beyond certain value it offers low resistance to flow the field current in reverse direction. • These two are the special protections of the field winding

TYPES OF EXCITATION SYSTEMS 1. DC Excitation systems 2. AC Excitation Systems a) Stationary rectifier systems b) Rotating rectifier systems

3. Static Excitation systems a) Potential source controlled rectifier systems b) Compound source rectifier systems c) Compound controlled rectifier excitation system

• Excitation power required for a synchronous generator is of the order of 2-3.5 KW per MW or MVA rating of the machine. DC Excitation system: It is oldest of all the excitation systems and main limitation of DC excitation system is the amount of power can be delivered and the commutation is the main problem. In 1960s DC is replacled by AC excitation systems.

DC EXCITATION SYSTEM

AC EXCITATION SYSTEM

With brushes and Un controlled rectifier (stationary Diodes)

AC EXCITATION SYSTEM

With brushes and controlled rectifier (stationary Controlled rectifier Diodes)

AC EXCITATION SYSTEM

Brush less excitation system

STATIC EXCITATION SYSTEM

•Advantages no rotating part of excitation system •Response time of static excitation system is very low

COMPOUND-SOURCE RECTIFIER EXCITATION SYSTEM

OPERATING MODES OF GENERATOR 1. Single generator is operating in isolation and supplying stand alone load (without supply from the grid) – Terminal voltage varies with excitation current (Vt ∞ If ) – Power factor of generator stator current is equal to load current power factor – Reactive power supplied by the generator depends on the reactive power demand by the load and load power factor.

2. Two or more generators operating in parallel and supplying stand alone load (without supply from the grid). The terminal voltage depends on the operating conditions of parallel machines and the load conditions.

3. Generator connected to Infinite bus ( grid or several generators operating in parallel) – Terminal voltage is constant and equal to grid voltage – Terminal voltage does not vary with excitation current – Power factor of generator stator current and reactive power Q shared by the generator varies with the excitation current.

• Under steady state conditions, the terminal voltage of generator connected to infinite bus bar is constant and is determined by the prevailing grid voltage and not by the generator field current. The power factor of armature current is decisively influenced by the excitation current.

V/f OPERATING RANGE DIAGRAM OF A SYSNCHRONOUS GENERATOR FOR STEADY STATE OPERATION OF EXCITATION SYSTEM

• The synchronous machines are designed for continuous rated output, at rated power factor, at rated voltage with tolerance 5% and rated frequency with tolerance 2%. The combination of operating voltage and operating frequency is of importance with reference to overheating of excitation winding and over fluxing of power transformers of the generator unit. High V/f is harmful for unit transformer and auxiliary transformer as the core gets heated due to over fluxing. V/f limiter in excitation system prevents V/f above permissible limit.

• Vt & f operating range for steady state operation

CAPABILITY CURVE OF GENERATOR • The capability of the generator terms of Active Power (P) and Reactive Power (Q) is usually represented in the form of the Generator Capability Cures.

Unit 2 & 3 Gen. Capability Curve

Armature heating

Armature core end heating

Field heating

TERMS RELATED TO EXCITATION SYSTEM • Synchronous machine regulator: The regulator that compares the output variables of a synchronous machine to the input of the exciter through a feed back and feed forward control elements for controlling the synchronous machine output variables. • Rated field current: Direct current in the field winding of the synchronous machine operating at rated voltage, current power factor.

• Rated field voltage: Direct voltage required across the terminals of the filed winding of the synchronous machine under rated continuous load conditions with its field winding at specified temperature. • Excitation system nominal response: Rate of increase of excitation system output voltage divided by rated field voltage. Rate of increase of excitation system output voltage is determined from excitation system nominal response curve.

• Exciter voltage response time. Time in seconds for exciter voltage to reach 95% of the (difference between ceiling voltage and rated load field voltage) under specified conations. • Excitation system ceiling voltage: The maximum DC voltage which the excitation system can supply to the generator field winding for a specified short time. • Excitation system ceiling current: The maximum DC current which the excitation system can supply to the generator field winding for a specified short time.

• Field forcing: The control function that rapidly forces (changes) the field current in the synchronous machine in positive or negative direction. • Under excitation limiter: Prevents the voltage regulator from lowering field current below specified limit. • Over excitation Limiter: Prevents the voltage regulator from raising field current above specified limit.

• Volts per hertz limiter: Acts through voltage regulator to limits V/f ratio within specified limits and takes corrective action to make V/f normal. • De-excitation : Removal of excitation (field current) of main exciter or pilot exciter. For example by opening field circuit and discharging the field by means of field discharge circuit breaker and discharge resistor.