-1- Impact of excitation system on power system stability 1. INTRODUCTION ABB. ABB Industrie AG

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1 -1-1. INTRODUCTION Industrie AG

2 -2- HV SYSTEM THE POWER STATION CONTROL ROOM STEP UP TRANSFORMER LV SWITCHGEAR HV- BREAKER AC & DC AUXILIARY SYSTEMS 1 GOVERNOR GENERATOR BREAKER AUX. TRANSF. PROTECTION 1 CONTROL SYSTEMS SYNCHRONIZING PT s & CT s TURBINE SYNCHRONOUS GENERATOR EXCITATIO NSYSTEM STAR POINT CUBICLE EXCITATION TRANSFORMER Industrie AG

3 -3- If Synchronous Machine Ug Grid Excitation System Controller The automatic voltage control system Industrie AG

4 -4- ~ = SM E SM ~ = 1 a 200 A 100 a A Rotating exciter Static excitation Industrie AG

5 -5- Basic Requirements Excitation Current up to amps Input frequency range from 16 Hz to 400 Hz Adaptable to different redundancy requirements for controls and converters State of the art man machine interface Compatibility with most applied power plant control systems Remote diagnostics Comfortable commissioning tools Industrie AG

6 Operational Application -6- Industrie AG

7 Main Components of an UNITROL 5000 System -7- AVR Industrie AG

8 -8- System overview AVR 2 AVR 1 Measuring Protection Monitoring AVR FCR CONVERTER N CONVERTER 2 CONVERTER 1 Converter Control Logic Control Field flashing Field suppression Fieldbreaker Industrie AG AVR = Autom. Voltage Reg. FCR = Field Current Reg.

9 Main Functions of the AVR with Field Current Controller -9- Measuring and A/D conversion U G, I G U G U G, I G, I f U G, I G, I f U G, I G I f AVR Setpoint -V/Hz limiter - Soft start -I Q Compensation -I P Compensation Underexcit. limiter -Q=F(P,U,U G ) -Stator current lead -Min. field current Overexcit. limiter -Max field current -Stator current lag Power System Stabilizer PSS Man setpoint Min. / max. value priority PID PI To pulse generation Industrie AG

10 IMPACT ON POWER SYSTEM STABILITY Voltage Control Dynamics ( controller, power stage and power source) Limiters Power System Stabilizer Industrie AG

11 Voltage Control Dynamics ( controller, power stage and power source) Industrie AG

12 Computer representation of the AVR -12- from under-excitation Limiters UEL from over-excitation Limiters OEL UP TB1 KR TC1 UP KR Up UT Kc If Generator voltage [p.u.] UT setpoint [p.u.] Generator Reactive Power [p.u.] Generator Active Power [p.u.] QT PT UT 1-1sTR 1 1sTR 1 1sTR KIR KIA HV Gate LV Gate UST Stabilizing signal from PSS 1sTC2 1sTB2 UP TB1 KR TC1 UP KR 1sTC1 1sTB1 KR Uf [p.u.] Parameter Description Unit Range TR Measuring filter time constant s Ts Gate control unit and converter time constant s KIR Reactive power compensation factor p.u KIA active power compensation factor p.u KR Steady state gain p.u Kc Voltage drop to commutations and p.u. Acc. Transf. impedances TB1 Controller first lag time constant s TB1 TB2 TB2 Controller second lag time constant s 0<TB2 TC2 TC1 Controller first lead time constant s TC2 Controller second lead time constant s Up AVR output positive ceiling value p.u. Fixed Up- AVR output negative ceiling value p.u. Fixed 1 1sTs Up- UT Industrie AG

13 -13- IR UR IDR UT If Uf IT IDT IDS IS US Industrie AG Synchronous machine three-phase representation

14 -14- D axis ra Ψd Id Ud Stator ΨdD δ rdd IdD ω Uf If rf Ψf ΨQ1 ΨQ2 Ψq Q axis IQ1 rq1 IQ2 rq2 ra Rotor Iq Uq Industrie AG d-q decomposition

15 -15- Torque TM = PM speed TE = US Ep sinδ Xq XT XE TM Motion equation: 0 o 45 o 180 o δ TM TE H d ω = 2 dt Industrie AG The characteristic Dynamic Equation

16 -16- AVR - Voltage control dynamic System type (Static excitation, Brush-less, DC Exciter) Settings of control algorithm Response time Ceiling capabilities Excitation system nominal response IEEE c NR= ce-ao (ao)(oe) UF ceiling b a UF nominal d Industrie AG o e=0.5s

17 -17- Comparison of Excitation System Types Mechanical - Conversion of mech. to electr. Energy DC-Exciter yes AC-Exciter stat. diodes yes AC-Exciter rot. diodes yes Stat. Exciter stat. thyristors main machine - Size of exciter power and speed power and speed power and speed power - Sliprings yes yes no yes Electrical - Rectifier not required static external static rotary static - Direct measuring of field current possible possible not possible possible - Fast field suppression possible possible not possible possible Industrie AG

18 -18- Comparison of Exciter Characteristics Dynamic Performance -Major time constant of control circuit DC-Exciter Td LTE AC-Exciter stat. diodes Td LTE AC-Exciter rot. diodes Td LTE Stat. Exciter stat. thyristors Td L - Ceiling factor limited not limited limited not limited - Negative field current possible not possible not possible possible Reliability (MTBF) - Machine, Transformer good good good better -Converter good better better better Maintenance - Machine, Transf. at standstill at standstill at standstill at standstill -Converter at standstill during operation at standstill during operation Industrie AG

19 -19- Ceiling limit Industrie AG

20 LIMITERS Industrie AG

21 -21- Setpoint building Automatic channel Ug AC Follow up Automatic Transducer and filter -Setpoint -Soft start -V/Hz limiter -Q influence -P influence Ug actual effective setpoint Under excitation limiters (UEL) - Power system stabilizer (PSS) HV Gate LV Gate Gain DC Gain Ucmax HF gain P gain Uc AVR Uc α Over excitation limiters (OEL) 1/TA 1/TB ω Ucmin Ucmax Gain IF Transducer and filter - DC Gain P gain Uc FCR Manual set point Follow up Automatic Follow up Manual Follow up control Uc AVR Uc FCR Ucmin 1/TA 1/TB ω AVR FCR Industrie AG

22 Theorectical stability limit Practical stability limit P [p.u.] Declared rated operation point of generating unit -22- Turbine ouput power limit LEADING (underexcited) ϕ n Excitation power IF Rated apparent power Sn LAGGING (overexcited) Thermal limit of stator Thermal limit of rotor -1.0 p.u. U 2 U 2 Minimum Field 1.0 Q [p.u.] Xq Xd current limit Safe operation range Industrie AG The Power Chart of a solid pole synchronous machine

23 Synchronous machine operation Limits -23- Max. field current limiter Min. field current limiter Stator current limiter Under excitation P,Q limiter P Theoretical stability limit -Q 1/Xd Q Industrie AG

24 -24- Main duty of the limiters: Keep the synchronous machine operating within the safe and stable operation limits, avoiding the action of protection devices that may trip the unit. Industrie AG

25 -25- Industrie AG VOLTAGE REGULATOR WITH LIMITERS

26 -26- Voltage setpoint Supply ~ = - Ifth setpoint - LOWER VAL. GATE = / ~ = Ifact - 2 Idt Ifmax 2 Idtmax COMP / # U SM = ~ Industrie AG THE OPERATING PHILOSOPHY OF Ifmax LIMITER

27 -27- Ceiling limit Field current x IFN setpoint switch time thermal limit Time [s] Industrie AG

28 -28- Machine frequency ft [p.u] Machine terminal voltage UT [p.u] Machine field current IF [p.u.] 1 1 str 1 1 str V / Hz Limiter x KVHZ y=kvhz.xu_fn-kvhz U_fn Max. field current limiter - x 2 KCF KHF x>0 y - x>0 T_V_Hz π IFmax1 IFmax2 0 EmaxF 1 s Delay UTMAX x y x>y - 0 UTMAX KIFmax U_V/HZ Volt/Hz influence signal to AVR [p.u.] IFth1 Setpoint 2 for IF max limiter (option) Stator Current IT [p.u.] 1 1 str IFth2 Stator current limiter - x 2 KCS KHS x>0 π 0 EmaxS 1 s x y x>y - KITind LV Gate To OEL limiter gate of AVR ITth ITmax To UEL limiter gate of AVR - KITcap Min. field current limiter - KIFmin HV Gate IFmin Industrie AG Generator reactive power QT [p.u.] Generator terminal voltage UT [p.u.] Generator active power PT [p.u.] 1 1 str 1 1 str P/Q limiter Look-up Table y n = f (x n ) x 2 π - KPQ

29 Power system Stabilizer Industrie AG

30 -30- Xq(s) I XT XE Ep UG US XTXE Infinite Bus Ep δ Ep-S = I. Xq Ep H =I.Xq UG Xq( s) = Xq 1 1 stq' stqo' 1 1 stq'' stqo' ' = I. (XEXT) US δ Industrie AG Stationary and transient air gap voltages

31 -31- Pe ω E p U G U S 90 o ω phasor rotation Transient behavior of ω and PE PE Industrie AG

32 -32- A) H XTXE Infinite Bus B) H1 H2 Equivalent Inertia Heq XTXE = H1 H2 H1 H2 TM TE H d ω = 2 taking TM TE H d 2 δ = 2 dt 2 dt For small oscillations keeping the driving torque constant the dynamic equation linearized can be written as: 2 H ωn 2 d δ D dδ K1 δ = 0 2 dt ωn dt Inertia. Ang. Acc. Damping torque Synchronizing Torque Industrie AG Dynamic equation of synchronous machinegrid for small oscillations

33 -33- Oscillation frequency Ω phasor rotation Ω K1 ωn 2 H rad/s Negative Damping (Unstable Region) UG δ D/ws* ω ω Positive Damping (Stable Region) Damping axis Torque K1=synchronizing coefficient operating point Te=K1. δ Resulting Torque T K1 = slope = = δ Ep'. Us cos δo' Xq' XE XT Synchronizing Axis 0 o δo 90 o δ Industrie AG Phasor diagram of machinegrid for small oscillations ( without AVR)

34 -34-2 H ωn 2 d δ D dδ K1 δ K2 Ep' = 0 2 dt ωn dt Inertia.Ang. Acc. Damping torque Synchron. Torque Add.Torque from exc. system Negative Damping (Unstable Region) K2. Ep UG phasor rotation D/ws* ω ω - UG Positive Damping (Stable Region) Resulting torque component without excitation system Te=K1. δ Resulting torque component with excitation system Synchronizing Axis Industrie AG Phasor diagram of machinegridexcitation system for small oscillations

35 -35- FINAL CTRL ELEMENT ALTERNATIVE ~ AVR PSS U G M P,Q Industrie AG Fig3: PSS in the excitation system

36 -36- Main Target of PSS: It provides an additional torque component in order to get: 1) A positive resulting torque component on damping axis, even for the highest possible rotor oscillation frequency. 2) A positive torque component on synchronizing axis for partial compensation of generator terminal voltage variations even for the highest possible oscillation frequency Industrie AG

37 -37- PSS2A acc. IEEE PM 1 s.2.h PA 1 s.2.h VSTmax VSTmax ω s.tw1 1 s.tw1 s.tw2 1 s.tw2 ( 1 s T8) ( 1 s T9) M N - Ks1 1s.T1 1s.T2 1s.T3 1s.T4 VST Ks3 VSTmin VSTmin PE s.tw3 1 s.tw3 s.tw4 1 s.tw4 Ks2 1 s.t7 PE 1 s.2.h Industrie AG

38 Adaptive PSS (APSS) Alternative to PSS2A -38- Driving Power P a U ref AVR - U f Synchronous Generator U G, I G Measurement GRID U G P APSS 3rd Order System Model Filter Estimator P White noise Regulator Industrie AG

39 -39- Multi Band PSS (MBPSS) FB 1 (s) FB 2 (s) - K b ω FI 1 (s) FI 2 (s) - K i Output Limit To AVR FH 1 (s) FH 2 (s) - K h Industrie AG