上海交通大学：《电力系统自动化》课程教学资源（推荐论文）Excitation control of the synchronous generator

Richard C.Schaefer and Kiyong Kim Excitation Control of the Synchronous Generator n a world of change,new technologies replace bridge are similar to methods utilized for analog old ones ever-more quickly.In the early systems.Input signals for raise/lower or start/stop years,change was slow as the evolutionary can be controlled by a variety of methods.These in- process transitioned from electromechanical clude contact inputs,an RS485 serial communica- voltage regulators with motor-driven rheostats to tion port,or a PC using ASCII language connected high-gain rotating exciters.These exciters in- into an RS232 serial communication port.Flexi- cluded such systems as the Amplidyne,Regulex, bility in digital systems offers more efficient meth- and Rototrol,which improved performance to con- ods to accomplish automation,which lowers trol the generator output.In the past 30 years, installation cost. however,change has progressed from magnetic technology to analog control.Analog excitation Analog versus Digital Controller represented the multiple component module as- In the past,shaping the generator response using sembly with interlooping wire interconnections an analog excitation system was a matter of adjust- (see Fig.1). ing potentiometers or adding or deleting capaci- Today,we see another major technology change, tors and resistors in the control loops of the voltage a movement away from analog control to digital regulator stability circuit.Adjustments could be control.An excitation system is now reduced to a very time consuming because changes would often single integrated assembly that includes the auto- involve turning the excitation system on and off matic voltage regulator(AVR),field current regula- tor (manual control),excitation limiters,and even protection [3]-[6].Reliability is enhanced as multi- ple devices are implemented into the single compo- nent with multitasking microprocessors.Fig.2 highlights a classic digital excitation system inter- connected in a generator system. ●000 A single component forms the primary element of the excitation system.Inputs include the instru- 5== ment transformer for voltage sensing,current APppIp transformer signal from a line CT,and bus voltage AP transformer used for voltage matching.The output 信 of the controller is an analog signal designed to work directly into a separate firing circuit that generates pulses for a three-phase,fullwave recti- fier bridge to control the field(see Fig.2). In digital systems,the guidelines for selecting the power potential transformer and rectifier Ricbard C.Scbaefer and Kiyong Kim are with Basler Electric of Higbland,Illinois.Both Schaefer and Kim are Members of IEEE.This article appeared in its origi- Fig.I.A comparison of an analog-controlled nal form at the 2000 IEEE/IAS Pulp and Paper Con- excitation system (left)and an integrated,digitally ference. controlled excitation system (rigbt). 1077-2618/01/s10.00©20011EEE IEEE Industry Applications Mogazine March/April 2001

1077-2618/01/$10.00©2001 IEEE IEEE Industry Applications Magazine ■ March/April 2001 I n a world of change, new technologies replace old ones ever-more quickly. In the early years, change was slow as the evolutionary process transitioned from electromechanical voltage regulators with motor-driven rheostats to high-gain rotating exciters. These exciters in￾cluded such systems as the Amplidyne, Regulex, and Rototrol, which improved performance to con￾trol the generator output. In the past 30 years, however, change has progressed from magnetic technology to analog control. Analog excitation represented the multiple component module as￾sembly with interlooping wire interconnections (see Fig. 1). Today, we see another major technology change, a movement away from analog control to digital control. An excitation system is now reduced to a single integrated assembly that includes the auto￾matic voltage regulator (AVR), field current regula￾tor (manual control), excitation limiters, and even protection [3]-[6]. Reliability is enhanced as multi￾ple devices are implemented into the single compo￾nent with multitasking microprocessors. Fig. 2 highlights a classic digital excitation system inter￾connected in a generator system. A single component forms the primary element of the excitation system. Inputs include the instru￾ment transformer for voltage sensing, current transformer signal from a line CT, and bus voltage transformer used for voltage matching. The output of the controller is an analog signal designed to work directly into a separate firing circuit that generates pulses for a three-phase, fullwave recti￾fier bridge to control the field (see Fig. 2). In digital systems, the guidelines for selecting the power potential transformer and rectifier bridge are similar to methods utilized for analog systems. Input signals for raise/lower or start/stop can be controlled by a variety of methods. These in￾clude contact inputs, an RS485 serial communica￾tion port, or a PC using ASCII language connected into an RS232 serial communication port. Flexi￾bility in digital systems offers more efficient meth￾ods to accomplish automation, which lowers installation cost. Analog versus Digital Controller In the past, shaping the generator response using an analog excitation system was a matter of adjust￾ing potentiometers or adding or deleting capaci￾tors and resistors in the control loops of the voltage regulator stability circuit. Adjustments could be very time consuming because changes would often involve turning the excitation system on and off Richard C. Schaefer and Kiyong Kim Richard C. Schaefer and Kiyong Kim are with Basler Electric of Highland, Illinois. Both Schaefer and Kim are Members of IEEE. This article appeared in its origi￾nal form at the 2000 IEEE/IAS Pulp and Paper Con￾ference. ©Texture: 1998 Corbis Fig. 1. A comparison of an analog-controlled excitation system (left) and an integrated, digitally controlled excitation system (right). 37

many times to make modifications.Fig.3(a)is a Fig.3(b)is a block diagram of a PID block uti- block diagram of a typical analog lead-lag control- lized in the AVR control loop.The P term repre- ler utilized in the automatic voltage regulator con- sents the proportional gain,which affects the rate of trol loop.The feedback gain (KF)was adjusted by a voltage rise after a step change.The I term repre- potentiometer to achieve stable performance. sents the integral gain,which affects the generator An optimally tuned excitation system offers ben- voltage settling time after the initial voltage over- efits in overall operating performance during tran- shoot.Lastly,the D term represents the derivative sient conditions caused by system faults, gain,which affects the percent of overshoot allowed disturbances,or motor starting [5].During motor after the system disturbances.The derivative term is starting,a fast excitation system will minimize the used with those excitation systems that have a rotat- generator voltage dip and reduce the I'R heating ing exciter.For main field-excited systems,the D losses of the motor.After a fault,a fast excitation term is not required.Since the derivative term af- system will improve the transient stability by hold- fects the amount of generator overvoltage,the lower ing up the system and providing positive damping the voltage overshoot,the faster the voltage recovers to system oscillations.Additionally,a well-tuned to nominal.The combined effect of the PID terms excitation system will minimize the voltage over- will shape the response of the generator excitation shoot after a disturbance and avoid the nuisance system to reach the desired performance.In addi- tripping of generator protection relays. tion to PID,KG(loop gain)also provides an adjust- Today,the digital excitation system provides the able term to compensate for variations in system means to easily access the challenging parameters of input voltage to drive the power converting bridge. the analog system.The heart of most digital control- Variations in KG modify the PID terms by the same lers is the embedded microprocessors that perform proportions to vary the overall performance. various control functions for the excitation system. Today,a wide range of the PID gains can be eas- These control functions include the automatic volt- ily selected using PC menu screens as shown in Fig. age regulator,field current regulator (manual con- 6 or a front-panel display. trol)Var/power factor control,and a host of excitation The evaluation of system performance begins limiters to regulate and maintain the generator by performing voltage step responses to examine within safe operating limits of the machine. the behavior of the excitation system with the gen- Bndaes in Par alle Field Flashing Resistor and Contactor 0 Paralleling CT Gate Generato Drive 3-Phase Power PT [00000 Interface rom Alterrex PT 00000 Voltage 125 V dc 120V ac Power Input Sensing00000 Control Power Digital Excitation Controller LCD Screen Keypad Switches Analog AVR Mode ±10Vdc Manual Mode tpoint-R/L Se al Link Softstar 152出C0 Watchdog Start/Stop hetequengy Remote Console Contact Loss of Voltage Sensing Raise/Lowe to Cause Trip Auto/Manual (4)Field C to Redundant et Up Controlle Aux.Input Serial link Analoo (Modbus 10V protocoll Redundant Digital Excitation Controller Fig.2.A typical digital excitation control system. 38 IEEE Industry Applicafions Magazine March/April 2001

many times to make modifications. Fig. 3(a) is a block diagram of a typical analog lead-lag control￾ler utilized in the automatic voltage regulator con￾trol loop. The feedback gain (KF) was adjusted by a potentiometer to achieve stable performance. An optimally tuned excitation system offers ben￾efits in overall operating performance during tran￾sient conditions caused by system faults, disturbances, or motor starting [5]. During motor starting, a fast excitation system will minimize the generator voltage dip and reduce the I2 R heating losses of the motor. After a fault, a fast excitation system will improve the transient stability by hold￾ing up the system and providing positive damping to system oscillations. Additionally, a well-tuned excitation system will minimize the voltage over￾shoot after a disturbance and avoid the nuisance tripping of generator protection relays. Today, the digital excitation system provides the means to easily access the challenging parameters of the analog system. The heart of most digital control￾lers is the embedded microprocessors that perform various control functions for the excitation system. These control functions include the automatic volt￾age regulator, field current regulator (manual con￾trol) Var/power factor control, and a host of excitation limiters to regulate and maintain the generator within safe operating limits of the machine. Fig. 3(b) is a block diagram of a PID block uti￾lized in the AVR control loop. The P term repre￾sents the proportional gain, which affects the rate of voltage rise after a step change. The I term repre￾sents the integral gain, which affects the generator voltage settling time after the initial voltage over￾shoot. Lastly, the D term represents the derivative gain, which affects the percent of overshoot allowed after the system disturbances. The derivative term is used with those excitation systems that have a rotat￾ing exciter. For main field-excited systems, the D term is not required. Since the derivative term af￾fects the amount of generator overvoltage, the lower the voltage overshoot, the faster the voltage recovers to nominal. The combined effect of the PID terms will shape the response of the generator excitation system to reach the desired performance. In addi￾tion to PID, KG (loop gain) also provides an adjust￾able term to compensate for variations in system input voltage to drive the power converting bridge. Variations in KG modify the PID terms by the same proportions to vary the overall performance. Today, a wide range of the PID gains can be eas￾ily selected using PC menu screens as shown in Fig. 6 or a front-panel display. The evaluation of system performance begins by performing voltage step responses to examine the behavior of the excitation system with the gen- 38 IEEE Industry Applications Magazine ■ March/April 2001 ▼ ▼ t ttt t t t Isolation Transducer Power Input 3-Phase Power PT from Alterrex Alterrex Generator Generator Field PT Paralleling CT Voltage Sensing Bridges in Parallel Watchdog Timer Contact to Cause Trip to Redundant Controller On/Off Status Analog Output ± 10 V dc (4) Field Configurable Output Contacts Interface Firing Circuit Chassis Analog Output ±10 V dc SCR Gate Drive Serial Link Communication to SCADA System (Modbus protocol) Start/Stop Remote Console Raise/Lower Auto/Manual } } } Field Flashing Resistor and Contactor Station Battery Control Power LCD Screen Keypad Switches Digital Excitation Controller Redundant Digital Excitation Controller Aux. Input 125 V dc 120 V ac Local Indicators: ·AVR Mode ·Manual Mode ·Droop ·Autotracking ·R/L Limit Indicator ·Under/Overexcitation Limit ·AC overvoltage ·AC undervoltage ·Underfrequency ·Null Control: Protection: ·Contactor Control ·Setpoint-R/L Serial Link ·Mode Transfer-Serial Link ·Preposition-52b Contact ·Field Overvoltage ·Field Overcurrent ·Gen. Undervoltage ·Gen. Overvoltage ·Loss of Voltage Sensing Functions: Communications - RS485 ·Voltage Regulation ·Generator Softstart ·Volts/Hertz ·Reactive Droop ·Voltage Matching ·Var/P.F. Control ·Field Current Regulator ·Control ·Calibration ·Monitoring ·Exciter Input/Output Status ·Set Up t ttt t t t Fig. 2. A typical digital excitation control system

Generator Generator No Load VRMAX No Load Saturation Curve KA(1+STc) Air Gap Line Generator Full Load s(1+sTB) VRMIN 100% Saturation △VT Gain Curve △VF SKE (1+STF1)(1+STF2) VT=Generator Rated Voltage VF=Generator Field Voltage Field Voltage 100% VRMAX KGKA Fig.4.Generator saturation curve illustrating 1+STA VR generator gain. VRMIN 1+STD Vac Fig.3.Simplified block diagrams of automatic voltage regulators.(a)Analog AVR with lead-lag Generator Voltage 325 V ac min/div controller.(b)Digital AVR witb PID coutroller. 1 s/div erator.It is performed with the breakers open Excite Field because the generator is in the least stable condi- V dc tion,i.e.,the least saturation and the highest gain Exciter Field Voltage 1s min/div (see Fig.4).Similar tests can be performed with the 25 V/div generator breaker closed,but with caution.Here, the voltage step change needs to be very small to Fig.5.Voltage step responses performed on a bydro avoid large changes in generator vars. turbine generator with analog voltage regulation. Comparing Analog to Digital Fig.5 and Table I represent step responses per- Control System Performance formed with the system described above.The origi- nal analog excitation system used a lead/lag stability Performance comparison and tuning techniques network and a three-phase,single-quadrant, are demonstrated by replacing the analog excita- halfwave-controlled semiconductor-controlled rec- tion system controlling the field of a rotating ex- tifier(SCR)bridge for field control [7].The excita- citer with a digital system. tion system was designed to provide 5 p.u.field Analog system:The analog voltage regulator was forcing to speed the response of the generator. installed on a hydroturbine generator in the late IEEE 421.2 [8]provides the guidelines for per- 1980s for a machine rated for 40 MVA,13,800 V formance analysis after a voltage step change.Here, ac,at 0.85 pf.It has a 14-s main field time constant a criterion is established for expected ranges of and a 2-s exciter field time constant. voltage overshoot,voltage rise times,and settling One of the factors affecting machine perfor- time.Depending upon the type of excitation pro- mance is the generator field time constant.As the vided and the type of control used,performance ex- machine time constant increases,the speed of sys- pectations will vary from acceptable to excellent. tem response slows due to the inherent lag caused For these tests,a voltage step change was intro- by the field inductance that will resist change.Ad- duced into the voltage regulator setpoint causing ditionally,since many systems have rotating excit- the generator voltage to move from 12.2 to 13.5 ers,the speed of response is further attenuated kVac.During the test,generator voltage overshoot because of the additional phase lag introduced by reached 303 Vac before recovering to nominal.The the second field (see Fig.2). generator voltage reached the peak value in 3.5 s, Table I.Initial Performance Starting Generator V ac Final Generator Voltage Generator Voltage Rise Time to Reach Peak Voltage Recovery Time Overshoot from Final Settling Value 12.2kW 135kW 303 3.55 755 Exciter field forcng Field response 160yk Optimum EEE Industry Applications Mogazine March/April 2001 39

erator. It is performed with the breakers open because the generator is in the least stable condi￾tion, i.e., the least saturation and the highest gain (see Fig. 4). Similar tests can be performed with the generator breaker closed, but with caution. Here, the voltage step change needs to be very small to avoid large changes in generator vars. Comparing Analog to Digital Control System Performance Performance comparison and tuning techniques are demonstrated by replacing the analog excita￾tion system controlling the field of a rotating ex￾citer with a digital system. Analog system: The analog voltage regulator was installed on a hydroturbine generator in the late 1980s for a machine rated for 40 MVA, 13,800 V ac, at 0.85 pf. It has a 14-s main field time constant and a 2-s exciter field time constant. One of the factors affecting machine perfor￾mance is the generator field time constant. As the machine time constant increases, the speed of sys￾tem response slows due to the inherent lag caused by the field inductance that will resist change. Ad￾ditionally, since many systems have rotating excit￾ers, the speed of response is further attenuated because of the additional phase lag introduced by the second field (see Fig. 2). Fig. 5 and Table I represent step responses per￾formed with the system described above. The origi￾nal analog excitation system used a lead/lag stability network and a three-phase, single-quadrant, halfwave-controlled semiconductor- controlled rec￾tifier (SCR) bridge for field control [7]. The excita￾tion system was designed to provide 5 p.u. field forcing to speed the response of the generator. IEEE 421.2 [8] provides the guidelines for per￾formance analysis after a voltage step change. Here, a criterion is established for expected ranges of voltage overshoot, voltage rise times, and settling time. Depending upon the type of excitation pro￾vided and the type of control used, performance ex￾pectations will vary from acceptable to excellent. For these tests, a voltage step change was intro￾duced into the voltage regulator setpoint causing the generator voltage to move from 12.2 to 13.5 kV ac. During the test, generator voltage overshoot reached 303 V ac before recovering to nominal. The generator voltage reached the peak value in 3.5 s, IEEE Industry Applications Magazine ■ March/April 2001 39 Generator No Load Air Gap Line Generator No Load Saturation Curve Generator Full Load Saturation Curve VT = Generator Rated Voltage VF = Generator Field Voltage 0 100% 100% Field Voltage Generator Terminal Voltage = Gain ∆VT ∆VF Fig. 4. Generator saturation curve illustrating generator gain. Exciter Field Voltage Generator Voltage 325 V ac min/div 1 s/div 1 s min/div 25 V/div V ac Exciter Field V dc Fig. 5. Voltage step responses performed on a hydro turbine generator with analog voltage regulation. Table I. Initial Performance Starting Generator V ac Final Generator Voltage Generator Voltage Overshoot from Final Settling Value Rise Time to Reach Peak Voltage Recovery Time 12.2 kV 13.5 kV 303 3.5 s 7.5 s Exciter field forcing Field response 160 V dc Optimum VC VC VREF VREF VS VS + + + VRMAX VRMAX VRMIN VRMIN VR VR K (1+sT ) A C s(1+sT ) B sKF (1+sT )(1+sT ) F1 F2 K1 s KP sKD 1+sTD K KG A 1+sTA + + + + + − Fig. 3. Simplified block diagrams of automatic voltage regulators. (a) Analog AVR with lead-lag controller. (b) Digital AVR with PID controller

with total recovery in 7.5 s.Note the generator Digital system:After testing was completed us- voltage swing prior to the generator voltage set- ing the analog system,the digital controller was tling to the steady state value.During the initial installed.The new excitation system included a tuning of the analog excitation system,different two-quadrant power SCR bridge that could pro- phase lead and lag compensation was implemented vide both positive and negative field forcing,com- to determine best settings for performance.This bined with a digital controller to hasten flux was accomplished by adding external capacitors to changes in the field winding and quicken system optimize unit performance. voltage recovery. With the use of digital control,the ingredients DEL for fast,but stable performance can be readily ac- commodated by adjusting the controller gains.To set the control gains,a graphical user interface, such as the one shown in Fig.6,provides the user with an accessible method to modify the PID gains as well as loop gain(KG)settings.The loop gain compensates for variations in system configuration that are associated with the power input voltage used to drive the SCRs in the field circuit.Varia- Fig.6.Typical grapbical user interface allows tions in the magnitude of KG loop gain affect the digital excitation controller setup. overall response. To analyze the effect of the PID gains on genera- tor performance,a number of tests were executed. ac The voltage step changes were performed open cir- cuited in 5%voltage steps to increase and decrease AVR Mode 15 s/drv generator voltage.Fig.7 illustrates initial perfor- Generator Terminal Voltage 189 V/div mance settings for the PID controllers prior to op- ■■■■■■■■ Vdc timization.Initial readings indicate performance Main 十十十十十 with voltage overshoot reaching 168 V ac from a Field 13.2-kV ac base value.The generator voltage takes Generator Field Voltage 15s/di 100 V/div 4.5 s to reach the peak voltage and more than 10 s to recover back to nominal after the step change. Vdc Notice that the field voltage offers very little Excite Field field forcing during the disturbance and the field voltage is very slow to decay back to the Exciter Field Voltage 1.5 s/div 30 V/div steady-state value (see Table ID). In the next test,the PID gains were modified to Fig.7.Initial performance settings for PID controllers improve transient performance (see Fig.8). Here,the derivative gain (KD)was increased from 100 to 120,the proportional gain(Kp)from 80 to 200,and the integral gain from 20 to30.The Vac voltage overshoot now reduces to 100 V ac as the increased derivative gain anticipates voltage recov- ery and improves damping.Note that the time it AVR Mode 1.5 s/div Generator Terminal Voltage 189 V/div takes to reach the voltage maximum is down to 2s, and the total recovery time is now only 4.5 s.As the gains were increased,the voltage regulator now V dc reaches 110 V dc,and the field voltage becomes Main Field under damp momentarily and forces the field a negative 15 V dc (see Table IID). Generator Field Voltage 1.5 s/div Fig.9 further demonstrates the performance 100 V/div variation,as PID gain settings are further modified to improved system performance.An increase of V dc the proportional gain from 200 to 300 reduces the Excite time to reach the maximum voltage peak to 1.5 s. Field At the same time,increasing the derivative gain from 120 to 150 decreases the voltage overshoot to Exciter Field Voltage 1.5 s/div a level of 48 V ac.Notice how field behavior pro- 30 V/div duces the cause and effect that improves perfor- Fig.8.Increased PID gains. mance,i.e.,higher field forcing,faster response. 0 IEEE Industry Applicafions Magazine March/April 2001

with total recovery in 7.5 s. Note the generator voltage swing prior to the generator voltage set￾tling to the steady state value. During the initial tuning of the analog excitation system, different phase lead and lag compensation was implemented to determine best settings for performance. This was accomplished by adding external capacitors to optimize unit performance. Digital system: After testing was completed us￾ing the analog system, the digital controller was installed. The new excitation system included a two-quadrant power SCR bridge that could pro￾vide both positive and negative field forcing, com￾bined with a digital controller to hasten flux changes in the field winding and quicken system voltage recovery. With the use of digital control, the ingredients for fast, but stable performance can be readily ac￾commodated by adjusting the controller gains. To set the control gains, a graphical user interface, such as the one shown in Fig. 6, provides the user with an accessible method to modify the PID gains as well as loop gain (KG) settings. The loop gain compensates for variations in system configuration that are associated with the power input voltage used to drive the SCRs in the field circuit. Varia￾tions in the magnitude of KG loop gain affect the overall response. To analyze the effect of the PID gains on genera￾tor performance, a number of tests were executed. The voltage step changes were performed open cir￾cuited in 5% voltage steps to increase and decrease generator voltage. Fig. 7 illustrates initial perfor￾mance settings for the PID controllers prior to op￾timization. Initial readings indicate performance with voltage overshoot reaching 168 V ac from a 13.2-kV ac base value. The generator voltage takes 4.5 s to reach the peak voltage and more than 10 s to recover back to nominal after the step change. Notice that the field voltage offers very little field forcing during the disturbance and the field voltage is very slow to decay back to the steady-state value (see Table II). In the next test, the PID gains were modified to improve transient performance (see Fig. 8). Here, the derivative gain (KD) was increased from 100 to 120, the proportional gain (KP) from 80 to 200, and the integral gain from 20 to 30. The voltage overshoot now reduces to 100 V ac as the increased derivative gain anticipates voltage recov￾ery and improves damping. Note that the time it takes to reach the voltage maximum is down to 2 s, and the total recovery time is now only 4.5 s. As the gains were increased, the voltage regulator now reaches 110 V dc, and the field voltage becomes under damp momentarily and forces the field a negative 15 V dc (see Table III). Fig. 9 further demonstrates the performance variation, as PID gain settings are further modified to improved system performance. An increase of the proportional gain from 200 to 300 reduces the time to reach the maximum voltage peak to 1.5 s. At the same time, increasing the derivative gain from 120 to 150 decreases the voltage overshoot to a level of 48 V ac. Notice how field behavior pro￾duces the cause and effect that improves perfor￾mance, i.e., higher field forcing, faster response. 40 IEEE Industry Applications Magazine ■ March/April 2001 V ac 1.5 s/div 189 V/div 1.5 s/div 100 V/div 1.5 s/div 30 V/div V dc Main Field V dc Exciter Field AVR Mode Exciter Field Voltage Generator Terminal Voltage Generator Field Voltage Fig. 7. Initial performance settings for PID controllers. V ac 1.5 s/div 189 V/div 1.5 s/div 100 V/div 1.5 s/div 30 V/div V dc Main Field V dc Exciter Field AVR Mode Exciter Field Voltage Generator Terminal Voltage Generator Field Voltage Fig. 8. Increased PID gains. Fig. 6. Typical graphical user interface allows digital excitation controller setup

Table IV shows that the field forcing now shift exhibited illustrates a very sluggish respond- reaches 140 V dc positive and becomes tempo- ing system as noted in Fig.4. rarily underdamp,forcing the field a-25 V dc be- The range of 0.1 to 0.3 Hz represents the criti- fore recovering to nominal.The total response cal frequency of the generator.The critical fre- improves to 3.2 s. quency of the generator is derived from the 90 Fig.10 highlights further improvements as phase shift produced from the main field and the gains settings determine the final performance of 90 phase lag caused by the exciter field.The two the generator.Increasing the derivative gain from 90 phase shifts add directly to provide 180 phase 150 to 200 causes the generator voltage overshoot to be nearly zero,and the voltage recovery,since the voltage overshoot is nearly eliminated.Re- sponse time becomes 1.5 s(see Table V). Step response tests can be performed using the Vac graphical interface tools to enhance system perfor- mance and minimize commissioning time.The above study clearly demonstrates the performance AVR Mode tuning benefits of the digital system over the ana- Generator Terminal Voltage 189 Vdr log system. Frequency Response Boosts Performance Analysis A more complex method of determining generator Main voltage stability and unit performance is by per- Field forming a frequency response of the excitation sys- tem with the generator over a range of frequencies. Generator Field Voltage 15 sdI This is known as a "closed-loop"analysis.The 100 V/div closed-loop frequency response provides informa- tion on the behavior of the excitation/generator sys- tem over a frequency range that represents the Vdc potential oscillating region of the generator system. Exciter Field The test is performed by injecting a small sine wave at various frequencies (0.1 to 3.0 Hz)into the summing point of the excitation controller and mea- Exciter Field Voltage 1.5 s/div suring the response of the generator terminal voltage. 30 V/div Various methods are used to analyze system perfor- Fig.9.Performance variation with modified P+D settings. mance such as Root Locus,Nyquist,and Bode.In this study,Bode plots are utilized to compare phase versus frequency and gain versus frequency to evaluate sys- tem performance.The Bode plots show phase shift and relative gain of the overall generator system.For Vac good performance,the gain should remain essentially flat at the low frequencies,then fall off as the signal frequency increases.It is important for the gain to re- AVR Mode 1.5 s/div main high over a wide bandwidth for greater contri- Generator Terminal Voltage 189 V/di bution to positive damping.The peak value of gain just before it rolls off is an indicator of the voltage overshoot during voltage step changes.Typically a Vdc Main 2-4dB rise as referenced in IEEE 421.2"Performance Field Guidelines to Testing and Evaluating Generator Per- formance"is favored for good system response.Peak Generator Field Voltage 1.5 s/div values higher than 4 dB would imply potentially un- 100 V/div stable systems [9]. Figs.12 and 13 represent the closed-loop fre- quency response of the twoexcitation systems under V dc study.They are used to demonstrate the difference Exciter Field in performance based upon the optimum settings established during the voltage step changes.Notice in Fig.12,the maximum gain is zero at 0.1 Hz and Exciter Field Voltage 15 s/div 30 V/div quickly falls as the frequency is increased to 1.0 Hz. The phase shift follows similarly as it falls from-40 Fig.10.Further improvements with gain settings increasing at 0.1 Hz to-160 at 1.0 Hz.The gain and phase derivative (KD) IEEE Industry Applications Mogazine March/April 2001

Table IV shows that the field forcing now reaches 140 V dc positive and becomes tempo￾rarily underdamp, forcing the field a -25 V dc be￾fore recovering to nominal. The total response improves to 3.2 s. Fig. 10 highlights further improvements as gains settings determine the final performance of the generator. Increasing the derivative gain from 150 to 200 causes the generator voltage overshoot to be nearly zero, and the voltage recovery, since the voltage overshoot is nearly eliminated. Re￾sponse time becomes 1.5 s (see Table V). Step response tests can be performed using the graphical interface tools to enhance system perfor￾mance and minimize commissioning time. The above study clearly demonstrates the performance tuning benefits of the digital system over the ana￾log system. Frequency Response Boosts Performance Analysis A more complex method of determining generator voltage stability and unit performance is by per￾forming a frequency response of the excitation sys￾tem with the generator over a range of frequencies. This is known as a “closed-loop” analysis. The closed-loop frequency response provides informa￾tion on the behavior of the excitation/generator sys￾tem over a frequency range that represents the potential oscillating region of the generator system. The test is performed by injecting a small sine wave at various frequencies (0.1 to 3.0 Hz) into the summing point of the excitation controller and mea￾suring the response of the generator terminal voltage. Various methods are used to analyze system perfor￾mance such as Root Locus, Nyquist, and Bode. In this study, Bode plots are utilized to compare phase versus frequency and gain versus frequency to evaluate sys￾tem performance. The Bode plots show phase shift and relative gain of the overall generator system. For good performance, the gain should remain essentially flat at the low frequencies, then fall off as the signal frequency increases. It is important for the gain to re￾main high over a wide bandwidth for greater contri￾bution to positive damping. The peak value of gain just before it rolls off is an indicator of the voltage overshoot during voltage step changes. Typically a 2-4 dB rise as referenced in IEEE 421.2 “Performance Guidelines to Testing and Evaluating Generator Per￾formance” is favored for good system response. Peak values higher than 4 dB would imply potentially un￾stable systems [9]. Figs. 12 and 13 represent the closed-loop fre￾quency response of the two excitation systems under study. They are used to demonstrate the difference in performance based upon the optimum settings established during the voltage step changes. Notice in Fig. 12, the maximum gain is zero at 0.1 Hz and quickly falls as the frequency is increased to 1.0 Hz. The phase shift follows similarly as it falls from -40 at 0.1 Hz to -160° at 1.0 Hz. The gain and phase shift exhibited illustrates a very sluggish respond￾ing system as noted in Fig. 4. The range of 0.1 to 0.3 Hz represents the criti￾cal frequency of the generator. The critical fre￾quency of the generator is derived from the 90° phase shift produced from the main field and the 90° phase lag caused by the exciter field. The two 90° phase shifts add directly to provide 180° phase IEEE Industry Applications Magazine ■ March/April 2001 41 V ac 1.5 s/div 189 V/div 1.5 s/div 100 V/div 1.5 s/div 30 V/div V dc Main Field V dc Exciter Field AVR Mode Generator Terminal Voltage Exciter Field Voltage Generator Field Voltage Fig. 9. Performance variation with modified P+D settings. V ac 1.5 s/div 189 V/div 1.5 s/div 100 V/div 1.5 s/div 30 V/div V dc Main Field V dc Exciter Field AVR Mode Generator Terminal Voltage Exciter Field Voltage Generator Field Voltage Fig. 10. Further improvements with gain settings increasing derivative (KD)

lag.The actual critical frequency depends upon the ity network minimizes the lagging phase shift to machine time constant;the larger the machine provide a stable and quickly responding system. time constant,the lower the critical frequency.For Fig.13 illustrates the Bode plot of the digital this system,the range of 150 to 180 phase lag in excitation system utilizing optimized PID gains the range of 0.1 to 0.3 Hz represents the potential noted in Fig.10 and Table V.Notice how the gain oscillating area of the generator,which when com- remains relatively flat from.05 to 0.2 Hz.then bined with the overall loop gain of the generator, peaks (3 dB above nominal)and rolls off as the sig- exciter,and voltage regulator can result in instabil- nal frequency increases.The slight gain rise at 0.2 ity if the gain is equal to one.The excitation stabil- Hz provides desirable positive damping and com- Table ll.Initial Performance Settings for PID Controllers Starting Generator V ac Final Generator Voltage Generator Voltage Rise Time to Reach Peak Voltage Recovery Time Overshoot from Final Settling Value 12,552 13,180 168 4.5s 145 KG-loop gain KD-derivative gain KP-proportiondlgoin KI-integral gain 10 100 80 20 Exciti fiorg Field response 90V dc Very sluggish Table lll.Results of Increased PID Gains Starting Generator V ac Final Generator Voltage Generator Voltage Rise tTme to Reach Peak Voltage Recovery Time Overshoot from Final Settling Valve 12483 13,107 120 2.0s 455 KG-loop gain KD-derivative gain KPproprg KI-integral gain 10 120 200 Exciter fiedfong11V Exciter field response under damp -15 V dc 110Vde Under damp-15 V dc Table IV.Results of Further Modification to PID Gains Starting Generator V ac Final Generator Voltage Generator Voltage Rise Time to Reach Peak Voltage Recovery Time Overshoot from Final Settling Valve 12,552 13,180 48 1.5s 3.25 KG-loop goin KD-derivative gain KP-propgin KI-integral gain 10 150 300 30 Exciter fiedg Exciter field response 140 V dc Under damp-25Vd Table V.Results of Further Improvements to Gain Settings Starting Venerator V ac Final Generator Voltage Generator Voltage Rise Time to Reach Peak Voltage Recovery Time Overshoot from Final Settling Valve 12,544.8 13,171 5Vac 15s 1.55 KG-loop gain KD-derivative gain KPron KI-integral gain 10 200 300 30 Exiter fiedfrng Exciter field response 150 V de -10V de 42 IEEE Industry Applicafions Magazine March/April 2001

lag. The actual critical frequency depends upon the machine time constant; the larger the machine time constant, the lower the critical frequency. For this system, the range of 150 to 180° phase lag in the range of 0.1 to 0.3 Hz represents the potential oscillating area of the generator, which when com￾bined with the overall loop gain of the generator, exciter, and voltage regulator can result in instabil￾ity if the gain is equal to one. The excitation stabil￾ity network minimizes the lagging phase shift to provide a stable and quickly responding system. Fig. 13 illustrates the Bode plot of the digital excitation system utilizing optimized PID gains noted in Fig. 10 and Table V. Notice how the gain remains relatively flat from .05 to 0.2 Hz, then peaks (3 dB above nominal) and rolls off as the sig￾nal frequency increases. The slight gain rise at 0.2 Hz provides desirable positive damping and com- 42 IEEE Industry Applications Magazine ■ March/April 2001 Table III. Results of Increased PID Gains Starting Generator V ac Final Generator Voltage Generator Voltage Overshoot from Final Settling Value Rise tTme to Reach Peak Voltage Recovery Time 12,483 13,107 120 2.0 s 4.5 s KG – loop gain KD – derivative gain KP – proportional gain KI – integral gain 10 120 200 30 Exciter field forcing 110 V dc Exciter field response under damp -15 V dc 110 V dc Under damp -15 V dc Table IV. Results of Further Modification to PID Gains Starting Generator V ac Final Generator Voltage Generator Voltage Overshoot from Final Settling Value Rise Time to Reach Peak Voltage Recovery Time 12,552 13,180 48 1.5 s 3.2 s KG – loop gain KD – derivative gain KP – proportional gain KI – integral gain 10 150 300 30 Exciter field forcing Exciter field response 140 V dc Under damp -25 V dc Table V. Results of Further Improvements to Gain Settings Starting Venerator V ac Final Generator Voltage Generator Voltage Overshoot from Final Settling Value Rise Time to Reach Peak Voltage Recovery Time 12,544.8 13,171 5 V ac 1.5 s 1.5 s KG – loop gain KD – derivative gain KP – proportional gain KI – integral gain 10 200 300 30 Exciter field forcing Exciter field response 150 V dc -10 V dc Table II. Initial Performance Settings for PID Controllers Starting Generator V ac Final Generator Voltage Generator Voltage Overshoot from Final Settling Value Rise Time to Reach Peak Voltage Recovery Time 12,552 13,180 168 4.5 s 14 s KG – loop gain KD – derivative gain KP – proportional gain KI – integral gain 10 100 80 20 Excitation field forcing Field response 90 V dc Very sluggish

pliments the performance observed during the [7]R.C.Schaefer,"Steam turbine generator excitation system voltage step responses in Fig.10 as being optimal. modernization,"presented at the IEEE Pulp and Paper Technical Conference,Rome,GA,1995 [8]IEEE Guide for Identification,Testing,and Evaluation of the Tools for Easier Programming Dynamic Performance of Excitation Control Systems,IEEE Std To simply the process of custom tuning the excita- 421.2.1990. tion,tools are available in the excitation system to [9]R.C.Schaefer,"Voltage regulator influence on generator provide starting gains for the PID settings needed stability,"presented at Waterpower,1991. [10]R.C.Schaefer,"Voltage versus var/power factor regulation for commissioning.The often-provided graphical on hydro generators,"presented at the IEEE Power System user interface screens are used to help select the Relaying Conference,1993. PID numbers for the excitation controller stabil- [11]IEEE Guide for Specification for Excitation Systems,IEEE Std ity.Here,the time constant of the generator fields 421.4,1990. is typed into a user screen and the PID numbers are automatically calculated.Step responses are then 0 performed to verify system performance. Exciter Field -20 Although this article focuses primarily on PID -40 settings of the voltage regulator controller,other -60 -80 control loops such as excitation limiters,var/PF controllers [10],and field-current regulators uti- Generator Field -100 -120 lize similar gain-setting techniques for control sys- Exciter+ Generator Field -140 tem optimization.As before,step responses are -160 again used to verify that the settings in the control- TrIn TT rTTI -180 lers will deliver stable operation when limits are 004.01 .1 1.0 10 reached or other modes enabled. (Hz) Machine/Regulator Oscillating Frequency Condusion Fig.11.Pbase sbift of the exciter field,the generator field,and It has become increasingly important to have an the sum of the two. optimally tuned excitation system to provide the highest degree of reliability possible for the sys- Gain(dB) Phase(deg) tem.The use of digital controllers helps dramati- Γ0 cally improve the transient stability of the system -20 with tools to easily achieve ideal performance [11]. 4 -40 Tuning of the overall system can be accomplished -60 inashort period oftime,even if the datasupplied is -10 -80 incorrect.The changing of parameters can be -100 quickly accomplished.This is not the case with the -16 -18 ,-120 analog-type regulator used here.Certain startup -20 -140 functions can be easily accomplished,such as step -160 changes and monitoring of operating levels,with 26 -Gain(dB)--Phase (deg) -180 the internal functions of the digital program.New -30 -200 technology provides a medium to achieve a higher 0.01 0.1 1 10 standard of quality control in generation. Frequency(Hz) Fig.12.Bode plot of off-line frequency response with analog References voltage regulator. [1]K.A.Riddle,"Renovation of apaper mill steam driven tur- bine-generator,"presented at IEEE Pulp and Paper Techni- cal Conference,Rome,GA,1995. Gain(dB) Phase(deg) 10r 100 [2]A.Godhwani,M.J.Basler,and T.W.Eberly,"Commis- sioning and operational experience with a modern digital excitation system,"IEEE Trans.Energy Comversion,vol.13, Pp.183-187,June1998. [3]A.Godhwani and M.J.Basler,"Design,test,and simulation 0 results of a var/power factor controller implemented in a modern digital excitation system,"presented at the IEEE Power Engineering Society Summer Meeting 1998,San 50 Diego,CA. [4]A.Godhwani and M.J.Basler,"An under and over excita- 10 100 tion limiter implementation in a digital excitation system for synchronous generator,"presented at IEEE/PES Winter -Gain(dB)--Phase(deg) Meeting,New York,NY,1999. -15 -150 [5]R.C.Schaefer,"Application of static excitation systems for 0.01 0.1 1 10 rotating exciter replacement,"presented at IEEE Pulp and Frequency(Hz) Paper Technical Conference,Cincinatti,OH,1997. [6]A.Godhwani and M.J.Basler,"Supplemental control in a Fig.13.Bode plot of off-line frequency response witb digital modern digital excitation system,"submitted for publication. voltage regulator. EEE Industry Applications Mogazine March/April 2001

pliments the performance observed during the voltage step responses in Fig. 10 as being optimal. Tools for Easier Programming To simply the process of custom tuning the excita￾tion, tools are available in the excitation system to provide starting gains for the PID settings needed for commissioning. The often-provided graphical user interface screens are used to help select the PID numbers for the excitation controller stabil￾ity. Here, the time constant of the generator fields is typed into a user screen and the PID numbers are automatically calculated. Step responses are then performed to verify system performance. Although this article focuses primarily on PID settings of the voltage regulator controller, other control loops such as excitation limiters, var/PF controllers [10], and field-current regulators uti￾lize similar gain-setting techniques for control sys￾tem optimization. As before, step responses are again used to verify that the settings in the control￾lers will deliver stable operation when limits are reached or other modes enabled. Conclusion It has become increasingly important to have an optimally tuned excitation system to provide the highest degree of reliability possible for the sys￾tem. The use of digital controllers helps dramati￾cally improve the transient stability of the system with tools to easily achieve ideal performance [11]. Tuning of the overall system can be accomplished in a short period of time, even if the data supplied is incorrect. The changing of parameters can be quickly accomplished. This is not the case with the analog-type regulator used here. Certain startup functions can be easily accomplished, such as step changes and monitoring of operating levels, with the internal functions of the digital program. New technology provides a medium to achieve a higher standard of quality control in generation. References [1] K.A. Riddle, “Renovation of a paper mill steam driven tur￾bine-generator,” presented at IEEE Pulp and Paper Techni￾cal Conference, Rome, GA, 1995. [2] A. Godhwani, M.J. Basler, and T.W. Eberly, “Commis￾sioning and operational experience with a modern digital excitation system,” IEEE Trans. Energy Conversion, vol. 13, pp. 183-187, June 1998. [3] A. Godhwani and M.J. Basler, “Design, test, and simulation results of a var/power factor controller implemented in a modern digital excitation system,” presented at the IEEE Power Engineering Society Summer Meeting 1998, San Diego, CA. [4] A. Godhwani and M.J. Basler, “An under and over excita￾tion limiter implementation in a digital excitation system for synchronous generator,” presented at IEEE/PES Winter Meeting, New York, NY, 1999. [5] R.C. Schaefer, “Application of static excitation systems for rotating exciter replacement,” presented at IEEE Pulp and Paper Technical Conference, Cincinatti, OH, 1997. [6] A. Godhwani and M.J. Basler, “Supplemental control in a modern digital excitation system,” submitted for publication. [7] R.C. Schaefer, “Steam turbine generator excitation system modernization,” presented at the IEEE Pulp and Paper Technical Conference, Rome, GA, 1995. [8] IEEE Guide for Identification, Testing, and Evaluation of the Dynamic Performance of Excitation Control Systems, IEEE Std 421.2, 1990. [9] R.C. Schaefer, “Voltage regulator influence on generator stability,” presented at Waterpower, 1991. [10] R.C. Schaefer, “Voltage versus var/power factor regulation on hydro generators,” presented at the IEEE Power System Relaying Conference, 1993. [11] IEEE Guide for Specification for Excitation Systems, IEEE Std 421.4, 1990. IEEE Industry Applications Magazine ■ March/April 2001 43 0 0 −20 −40 −60 −80 −100 −120 −140 −160 −180 .01 .1 1.0 10. (Hz) Machine/Regulator Oscillating Frequency Phase Shift In Degrees .004 Exciter Field Generator Field Exciter + Generator Field Fig. 11. Phase shift of the exciter field, the generator field, and the sum of the two. Gain (dB) Phase (deg) 2 0 −2 −10 −12 −14 −16 −18 −20 −22 −24 −26 −28 −30 −6 −4 −8 0 −20 −40 −60 −80 −100 −120 −140 −160 −180 −200 0.01 0.1 1 10 Gain (dB) Phase (deg) Frequency (Hz) Fig. 12. Bode plot of off-line frequency response with analog voltage regulator. Gain (dB) Phase (deg) 100 50 0 −50 −100 −15 −150 −10 −5 0 5 10 Gain (dB) Phase (deg) 0.01 0.1 1 10 Frequency (Hz) Fig. 13. Bode plot of off-line frequency response with digital voltage regulator