Impact of excitation system on power system stability -1- 1. INTRODUCTION ABB Industrie AG ABB Impact of excitation system on power system stability THE POWER STATION HV SYSTEM STEP UP TRANSFORMER AC & DC AUXILIARY SYSTEMS AUX. TRANSF . GOVERNOR GENERATOR BREAKER SYNCHRONIZING CONTROL SYSTEMS PROTECTION 1 PT’s & CT’s EXCITATIO NSYSTEM SYNCHRONOUS GENERATOR TURBINE STAR POINT CUBICLE ABB Industrie AG CONTROL ROOM LV SWITCHGEAR HV- BREAKER 1 -2- EXCITATION TRANSFORMER ABB Impact of excitation system on power system stability If Synchronous Machine Ug -3- Grid Excitation System Controller The automatic voltage control system ABB Industrie AG ABB Impact of excitation system on power system stability ~ SM E 1 a 200 A Rotating exciter ABB Industrie AG -4- = SM ~ = 100 a 10000 A Static excitation ABB Impact of excitation system on power system stability -5- Basic Requirements • Excitation Current up to 10’000 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 ABB Industrie AG ABB Impact of excitation system on power system stability -6- Operational Application ABB Industrie AG ABB -7- Impact of excitation system on power system stability Main Components of an UNITROL 5000 System AVR ABB Industrie AG ABB Impact of excitation system on power system stability -8- System overview Measuring AVR 2 AVR 1 CONVERTER N Protection CONVERTER 2 CONVERTER 1 Monitoring AVR + FCR Converter Control Logic Control Field flashing Field suppression Fieldbreaker AVR = Autom. Voltage Reg. FCR = Field Current Reg. ABB Industrie AG ABB Impact of excitation system on power system stability -9- Main Functions of the AVR with Field Current Controller UG UG, IG, If UG, IG, If UG, IG AVR Setpoint -V/Hz limiter - Soft start - IQ Compensation - IP Compensation ABB Industrie AG + Σ Underexcit. limiter -Q=F(P,U,UG) -Stator current lead -Min. field current Overexcit. limiter -Max field current -Stator current lag Power System Stabilizer PSS Man setpoint If PID _ _ + Min. / max. value priority Measuring and A/D conversion UG, IG Σ Σ To pulse generation PI Σ ABB - 10 - Impact of excitation system on power system stability 2. IMPACT ON POWER SYSTEM STABILITY • Voltage Control Dynamics ( controller, power stage and power source) • Limiters • Power System Stabilizer ABB Industrie AG ABB - 11 - Impact of excitation system on power system stability 2.1 Voltage Control Dynamics ( controller, power stage and power source) ABB Industrie AG ABB - 12 - Impact of excitation system on power system stability Computer representation of the AVR from under-excitation Limiters UEL Generator voltage [p.u.] UT 1 1+sTR - HV Gate Σ + UT setpoint [p.u.] QT Generator Reactive Power [p.u.] PT Generator Active Power [p.u.] + 1 1+sTR 1 1+sTR ABB Industrie AG + Σ + KIR KIA from over-excitation Limiters OEL LV Gate + UP + KR UP + TB1 KR TC 1 Σ + UST Stabilizing signal from PSS 1+sTC2 1+sTC1 1+sTB2 1+sTB1 UP − TB1 KR TC 1 Parameter TR Ts KIR KIA KR Kc TB1 TB2 TC1 TC2 Up+ Up- UP − KR Up+∗ UT–Kc∗∗ If KR 1 Uf [p.u.] 1+sTs Up- ∗ UT Description Measuring filter time constant Gate control unit and converter time constant Reactive power compensation factor active power compensation factor Steady state gain Voltage drop to commutations and impedances Controller first lag time constant Controller second lag time constant Controller first lead time constant Controller second lead time constant AVR output positive ceiling value AVR output negative ceiling value Unit s s p.u. p.u. p.u. p.u. Range 0.020 0.004 -0.20...+0.20 -0.20...+0.20 10...1000 Acc. Transf. s s s s p.u. p.u. TB1≥TB2 0<TB2≤TC2 0.01...10 0.01...2 Fixed Fixed ABB Impact of excitation system on power system stability IR - 13 - UR IDR UT If IT Uf IDT IDS IS US ABB Industrie AG Synchronous machine three-phase representation ABB Impact of excitation system on power system stability - 14 - D axis ra Id Ψd Stator Ud rdD Ψ dD IdD δ ω rf Uf Ψf Ψ Q1 If rQ1 IQ2 IQ1 Rotor Ψq Ψ Q2 Q axis rQ2 ra Iq Uq ABB Industrie AG d-q decomposition ABB Impact of excitation system on power system stability Torque TM TM = PM speed TE = US ⋅ Ep ⋅ sinδδ Xq + XT + XE - 15 - Motion equation: 0o 45o 180o δ dω TM − TE = 2 ⋅ H ⋅ dt The characteristic Dynamic Equation ABB Industrie AG ABB - 16 - Impact of excitation system on power system stability 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 421.1 NR= c UF ceiling UF nominal a o ABB Industrie AG ce-ao (ao)(oe) b d e=0.5s ABB Impact of excitation system on power system stability - 17 - Comparison of Excitation System Types DC-Exciter AC-Exciter stat. diodes AC-Exciter rot. diodes Stat. Exciter stat. thyristors yes yes yes 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 Mechanical - Conversion of mech. to electr. Energy ABB Industrie AG ABB Impact of excitation system on power system stability - 18 - Comparison of Exciter Characteristics DC-Exciter AC-Exciter stat. diodes AC-Exciter rot. diodes Stat. Exciter stat. thyristors Td’L+TE Td’L+TE Td’L+TE 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 Dynamic Performance - Major time constant of control circuit ABB Industrie AG ABB Impact of excitation system on power system stability - 19 - Ceiling limit ABB Industrie AG ABB Impact of excitation system on power system stability - 20 - 2.2 LIMITERS ABB Industrie AG ABB Impact of excitation system on power system stability - 21 - Setpoint building Automatic channel Follow up Automatic Ug AC -Setpoint -Soft start effective setpoint -V/Hz limiter -Q influence -P influence Power system stabilizer (PSS) + Transducer and filter Ug actual - Ucmax Σ HV Gate Under excitation limiters (UEL) Gain + LV Gate + + DC Gain Σ HF gain + P gain Σ Uc AVR Uc α Over excitation limiters (OEL) 1/TA Ucmin 1/TB ω Ucmax Gain IF Transducer and filter - DC Gain Σ + 1/TA Manual set point Follow up Manual Follow up Automatic ABB Industrie AG Uc FCR P gain Follow up control Uc AVR Ucmin 1/TB ω AVR FCR Uc FCR ABB Impact of excitation system on power system stability Theorectical stability limit Practical stability limit - 22 - Declared rated operation point of generating unit P [p.u.] Thermal limit of stator LEADING (underexcited) -1.0 p.u. U2 U2 Xq Xd Ex cit ati on po we r≈ IF Rate d ap pare nt po wer Sn Turbine ouput power limit ϕn Thermal limit of rotor LAGGING (overexcited) 1.0 Minimum Field current limit Q [p.u.] Safe operation range The Power Chart of a solid pole synchronous machine ABB Industrie AG ABB - 23 - Impact of excitation system on power system stability Synchronous machine operation Limits Max. field current limiter Min. field current limiter Stator current limiter Under excitation P,Q limiter P Theoretical stability limit -Q ABB Industrie AG 1/Xd +Q ABB Impact of excitation system on power system stability - 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. ABB Industrie AG ABB Impact of excitation system on power system stability VOLTAGE REGULATOR WITH LIMITERS ABB Industrie AG - 25 - ABB Impact of excitation system on power system stability - 26 - Supply ~ - + = Ifth setpoint + LOWER VAL. GATE Voltage setpoint ∫ I dt max 2 SM = - ∫ I dt 2 Ifmax COMP /# U =/~ Ifact + = ~ THE OPERATING PHILOSOPHY OF Ifmax LIMITER ABB Industrie AG ABB Impact of excitation system on power system stability - 27 - Field current x IFN Ceiling limit thermal limit setpoint switch time Time [s] ABB Industrie AG ABB Impact of excitation system on power system stability Machine frequency fT [p.u] 1 1+ sTR Machine terminal voltage UT [p.u] V / Hz Limiter x y=KVHZ.x+U_fn-KVHZ KVHZ y - UTMAX Σ U_fn Machine field current IF [p.u.] 1 1+ sTR x>0 + T_V_Hz Σ - Stator Current IT [p.u.] 1 1+ sTR UTMAX π IFth1 IFth2 0 1 s x KIFmax Σ + EmaxF KCF KHF IFmax1 IFmax2 U_V/HZ Volt/Hz influence signal to AVR [p.u.] 0 - x2 x>0 Setpoint 2 for IF max limiter (option) Delay Max. field current limiter + y x>y LV Gate Stator current limiter + Σ - - x2 π KCS KHS x>0 EmaxS 0 1 s x KITind Σ + y To OEL limiter gate of AVR x>y ITmax To UEL limiter gate of AVR ITth - + Min. field current limiter + Σ KITcap Σ KIFmin Σ KPQ Generator reactive power QT [p.u.] Generator terminal voltage UT [p.u.] Generator active power PT [p.u.] ABB Industrie AG - 28 - HV Gate IFmin 1 P/Q limiter - 1+ sTR + x2 1 1+ sTR Look-up Table yn = f (xn) π ABB Impact of excitation system on power system stability - 29 - 2.3 Power system Stabilizer ABB Industrie AG ABB Impact of excitation system on power system stability Xq(s) Ep I XT - 30 - XE UG US XT+XE Infinite Bus Ep δEp-S = I. Xq Ep’ H =I.Xq’ UG X q(s ) = X q ⋅ = I. (XE+XT) 1 + sTq' 1 + sT q'' ⋅ 1 + sT q o ' 1 + sT q o '' US δ ABB Industrie AG Stationary and transient air gap voltages ABB - 31 - Impact of excitation system on power system stability Pe Ep ω UG US 90o ∆ω phasor rotation Transient behavior of ∆ω and ∆PE ABB Industrie AG ∆PE ABB - 32 - Impact of excitation system on power system stability A) H B) H1 taking Infinite Bus XT+XE H2 XT+XE TM − TE = 2 ⋅ H ⋅ dω dt For small oscillations keeping the driving torque constant the dynamic equation linearized can be written as: Equivalent Inertia H1 ⋅ H2 H1 + H2 d2 δ TM − TE = 2 ⋅ H ⋅ 2 dt 2 ⋅ H d2 δ D dδ ⋅ 2 + ⋅ + K1∆δ = 0 ωn dt ωn dt Inertia. Ang. Acc. ABB Industrie AG Heq = Damping torque Dynamic equation of synchronous machine+grid for small oscillations Synchronizing Torque ABB Impact of excitation system on power system stability - 33 - Oscillation frequency Ω phasor rotation Negative Damping (Unstable Region) ∆UG D/ws*∆ ∆ω ∆ω ∆δ Ω≅ K1 ⋅ ωn rad/s 2⋅H Positive Damping (Stable Region) Damping axis K1=synchronizing coefficient Torque operating point ∆Te=K1.∆ ∆δ K1 = slope = Resulting Torque Synchronizing Axis ABB Industrie AG 0o δo 90o Phasor diagram of machine+grid for small oscillations ( without AVR) ∆T Ep'.Us = ⋅ cos δo' ∆δ Xq'+ XE + XT δ ABB Impact of excitation system on power system stability - 34 - 2 ⋅ H d2δ D dδ ⋅ 2 + ⋅ + K1 ⋅ ∆δ + K2 ⋅ ∆Ep' = 0 ωn dt ωn dt Inertia.Ang. Acc. ∆UG Negative Damping (Unstable Region) K2.∆ ∆Ep’ Damping torque Synchron. Torque Add .Torque from exc. system phasor rotation D/ws*∆ ∆ω -∆ ∆UG ∆ω Positive Damping (Stable Region) Resulting torque component without excitation system Resulting torque component with excitation system ∆Te=K1.∆ ∆δ Synchronizing Axis ABB Industrie AG Phasor diagram of machine+grid+excitation system for small oscillations ABB Impact of excitation system on power system stability - 35 - FINAL CTRL ELEMENT ALTERNATIVE ~ AVR U PSS G M P ,Q Fig3: PSS in the excitation system ABB Industrie AG ABB - 36 - Impact of excitation system on power system stability 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 ABB Industrie AG ABB - 37 - Impact of excitation system on power system stability PSS2A acc. IEEE 421.5 1992 ∆PM 1 + s.2.H ∆PA 1 + s.2.H ω s.TW1 s.TW2 1 + s.TW1 1 + s.TW2 + Σ + (1 + s ⋅ T 8) (1 + s ⋅ T 9)M N s.TW3 s.TW4 Ks2 1 + s.TW3 1 + s.TW4 1 + s.T7 Σ - Ks1 VSTmax 1+s.T1 1+s.T3 1+s.T2 1+s.T4 VSTmin Ks3 PE + VSTmax VST VSTmin ∆PE 1 + s.2.H ABB Industrie AG ABB - 38 - Impact of excitation system on power system stability Adaptive PSS (APSS) – Alternative to PSS2A Driving Power Pa Uref + AVR + - Uf + + Synchronous Generator UG , IG GRID Measurement UG APSS 3rd Order System Model Estimator White noise ABB Industrie AG + + P Filter ∆P Regulator ABB Impact of excitation system on power system stability - 39 - Multi Band PSS (MBPSS) FB1(s) FB2(s) ∆ω FI1(s) FI2(s) FH1(s) FH2(s) ABB Industrie AG + Σ - Kb + + Σ - K + Σ - Kh i Σ Output Limit To AVR + ABB