Galina S. Antonova, ABB Inc., i-pcgrid Workshop - 2016 Combining subsynchronous oscillations detection and synchrophasor measurements to increase power system stability April 1, 2016 Slide 1
Sub synchronous oscillation (SSO) events 1970 s Mohave Generator plant turbine generator failure caused by near coincidence of the first torsional mode of oscillation of the turbine generator and the electrical resonance of the series capacitor and 500kV transmission network 1980 s Square Butte generators and a nearby HVDC terminal observed sub synchronous torsional interaction Recently, a related phenomena has been associated with modern wind turbine generator technologies: October 22, 2009 two wind generation systems and series compensated system of AEP-Texas in south Texas experienced SSOs. Turbine equipment and utility equipment were damaged during the event April 1, 2016 Slide 2
Sub synchronous oscillation (SSO) events captures 1250 MW nuclear unit caused by quick ramp down for HVDC link Peaks at 17,83 and 117 Hz caused by Swedish railway system April 1, 2016 Slide 3
Effects of SSO and sub synchronous resonance (SSR) Rotor damage due to excessive shaft torques High torque levels Steel has reached the yield point Resulting in shaft deformation can result in shaft misalignment Lateral bending could lead to shaft failure Low torque levels Where endurance limit has been exceeded Cyclic torque causes fatigue in the shaft Life expended is calculated using stress life April 1, 2016 Slide 4
Synchrophasors definition Im p/2 Im p/2 p Ae j0 Re p Ae -jp /2 Re -p/2 -p/2 Time Source Reference -p/2 0 p/2 p -p/2 0 p/2 p -p/2 0 p/2 Acos(wt) (a) f(t) = Acos(wt), q = 0 (b) f(t) = Acos(wt -p/2),q = -p/2 (-90 ) 0 p/2 p -p/2 0 p/2 p -p/2 0 p/2 Acos(wt p/2) New Second Pulse April 1, 2016 Slide 5
Synchrophasors measurement system GPS Satellite Time Synchronization Power System PMU PMU PMU PMU PMU Streaming synchrophasor data on the network to the PDC for archiving... ETHERNET PDC/Server PDC/Server APPLICATIONS... data display and real time control actions April 1, 2016 Slide 6
Combining SSO/SSR detection with synchrophasors Sub synchronous oscillation sources active power sources (e.g. turbine generators) reactive power sources (e.g. series capacitors) HVDC converters Power System Stabilizers (PSSs) Static Var Compensators (SVCs) PMUs are placed in strategic power system locations provide real-time monitoring of currents and voltages of the oscillation sources support detection of sub synchronous oscillations locally via specially designed and tuned filters. capture (in DFR) and stream SSO/SSR data to control center April 1, 2016 Slide 7
SSO/SSR applications examples Applications from simple exciter alarm to governor control FERC requires to calibrate exciters of generators (30+MW) every 5 years Continuous real-time monitoring provides early indications of torsional stress, and enables timely corrective actions April 1, 2016 Slide 8
SSO and SSR definitions Definitions are given by IEEE Sub synchronous Resonance WG of the System Dynamics Performance Subcommittee, June 1985. Sub synchronous Oscillations (SSO) are electromechanical interaction, either between a turbine-generator and passive system elements such as series capacitors, or between a turbinegenerator and active system elements such as HVDC transmission equipment controls, and static VAR system controls. Turbine-generator electromechanical interaction with series capacitors has historically been known as the phenomena of Sub synchronous Resonance (SSR). April 1, 2016 Slide 9
SSO and SSR categories Induction Generator Effect (IGE) An electrical phenomena that results from an electrical resonance between a series capacitor and a generator Torsional Interaction (TI) Occurs when the electrical system operation results in mechanical damping at the generator that is negative and sufficiently large to exceed the inherent mechanical damping of the shaft at a natural torsional frequency of the mechanical system Torque Amplification (TA) Shaft torsional stresses due to system disturbances which result in resonance between electrical and mechanical natural frequencies. Can occur because of a resonance with a series capacitor Can occur because of the control action of devices such as HVDC converters, SVC/s and STATCOM s Sub synchronous Control Interaction (SSCI) April 1, 2016 Slide 10 Interactions between a power electronic device (such as an HVDC link, SVC, wind turbine, etc.) and a series compensated system.
How shaft of a turbo machine look like Properties of Turbo Machines Several masses on the shaft Long shaft / axle from 30m up to 100m Gen = generator Exc = exciter HP= high pressure turbine IP = intermediate pressure turbine LPA/LPB = low pressure turbines These masses can oscillate against each other April 1, 2016 Slide 11
Sub-synchronous phenomena Torsional oscillations Mass 1 HP Mass 2 IP Mass 3 LP1 Mass 4 LP2 Mass 5 Generator Mass 6 Exciter For sub-synchronous considerations, lumped masses for each turbine is typically sufficient If there are N masses, there will be N modes 1 non-oscillatory mode N-1 oscillatory modes Each mass participates differently in each mode Perturbations of the mechanical system will stimulate the modes Sudden change in input torque due to governor action Sudden change in electrical torque (fault or step-change of load on grid) The energy put into each oscillatory mode will exchange between kinetic energy (mass speed) and potential energy (shaft twist = spring) April 1, 2016 Slide 12
Sub-synchronous phenomena Torsional oscillations J 1 J 2 J J d 4 12 d 23 3 d 34 k 12 k 23 k 34 HP LPA LPB GEN M m1 M m2 M m3 K m1 D m1 K m2 D m2 K m3 D m3 Eigenvalue analysis of turbine-shaft allows the modes to be de-coupled and considered as N mass-spring systems Mode shapes are the eigenvectors normalized on the displacement of a given mass. Mass Mode 1 Mode 2 Mode 3 Mode 4 Mode 5 Mode 6 f (Hz) 0.0000 17.4390 26.1789 27.9803 36.0114 47.3719 HP 1.0000 1.0000 0.4261-0.0960 0.9326-0.7721 IP 1.0000 0.6889 0.1274-0.0191-0.3044 1.0000 LP1 1.0000 0.3170-0.1202 0.0374-0.6024-0.1880 LP2 1.0000-0.1264-0.0811 0.0046 1.0000 0.0603 GEN 1.0000-0.4821 0.0920-0.0372-0.4979-0.0141 EXC 1.0000-0.8074 1.0000 1.0000 0.6934 0.0072 April 1, 2016 Slide 13 These may be normalized on the largest value in the eigenvector or on the values associated with the generator. Nonoscillatory mode (stiff-shaft) Angular displacement of LP2 is largest. Generator displaces in opposite direction
Power system electro-mechanical model April 1, 2016 Slide 14
Frequency of induced I and V due to oscillations f f f SSR Rated Mechanical r or f f f 0 f o = the average synchronous frequency f er = the resonant frequency of the electrical system f r = the frequency of the rotor current as a result of f er er Thus the mechanical frequency is modulated on the fundamental power system frequency Component below the rated frequency is called sub-synchronous component April 1, 2016 Slide 15 Component above the rated frequency is called super-synchronous component
Current frequency spectrum April 1, 2016 Slide 16
SSO/SSR event spectrum April 1, 2016 Slide 17
SSO/SSR detection logic G VT U Filter #1 U SUP_2 Filter #2 U SUB_2 Filter #3 U SUP_3 Filter #4 U SUB_3 U SUP_2 U SUB_2 U SUP_3 U SUB_3 IDMT t a = f(u SUP-2 ) > SET_PICKUP IDMT t a = f(u SUP-3 ) > SET_PICKUP & & SSR RELAY OR TRIP CT I Filter #5 I SUB Filter #6 I SUP I SUB I SUP > SET_PICKUP > SET_PICKUP ALARM ALARM April 1, 2016 Slide 18
Multi-purpose filter functionality A 3-phase filter extracts frequency (2-500Hz) from the connected CTs and/or VTs Frequency to be extracted is settable Frequency of dominating oscillation can be determined Long filtering windows to achieve high frequency resolution Filter outputs Phasor (magnitude and the phase angle) at set frequency Exact frequency of the reported phasor April 1, 2016 Slide 19
Multi-purpose filter characteristics Precise phasor calculation (magnitude and phase angle) Different algorithm from the standard one-cycle Digital Fourier Filter (DFT) typically used by numerical IEDs Extremely good accuracy and excellent noise rejection Magnitude and phase angle can be estimated even if it has magnitude of one per mille (i.e. o ) Can be connected to measurement and protection functions Calculated and measured values captured in local disturbance recorder send to control center using communication, e.g. IEEE C37.118.2 synchrophasor data stream oo April 1, 2016 Slide 20
Current [ka] Current [ka] Filter Length parameter (blue data used for calculation) 0.4 Phase Current 1.0s 0.2 0-0.2-0.4 20 40 60 Cycles 0.4 Phase Current 0.2 0.5s 0-0.2-0.4 20 40 60 Cycles April 1, 2016 Slide 21
Filter configuration: Filter Length Configurable Filter Length Longer than 3 complete periods 1000 3 75ms 40 Longer than 5 complete periods for more accuracy 1000 5 125ms 40 April 1, 2016 Slide 22
Filter configuration: Pass Frequency Band Defaults per Filter Length are provided in the Table below Defaults are extendable by ½ of Frequency band width parameter Filter length = 0.2s Default band = 14Hz 5 Frequency band width = 5 => 14Hz 16. 5Hz 2 April 1, 2016 Slide 23
Multi-purpose filter operation A - Waveforms of the stator three-phase currents given in primary ka B - RMS value of the SSR current extracted by the filter in primary A April 1, 2016 Slide 24 C - Frequency of the extracted SSR current provided by the filter in Hz
Filter gives SSO/SSR phasors, what about Protection? Filter is connected further to standard 50/60Hz protection functions which then provide pickup and required time delays. The following protection functions can be used Over- or under-current protection function Over- or under-voltage protection function Over- or under-power protection function Over- or under-frequency protection function Multi-purpose protection function V, I, f measurement functions; value can be shown on builtin HMI or send to any control system via communication link April 1, 2016 Slide 25
Combined SSO/SSR and PMU application example April 1, 2016 Slide 26
Combined SSO/SSR and PMU application example April 1, 2016 Slide 27
Streaming SSO/SSR data in IEEE C37.118.2 frame IEEE C37.118.2 synchophasor data stream allows for binary and analog data streaming 2 data streams with 32 phasors each 28 binary and 24 analog signals each SSO/SSR data to be provided for each oscillation mode sub synchronous frequency phasor (magnitude and angle) output of over-voltage => sub synchronous magnitude exceeds limit super synchronous frequency phasor (magnitude and angle) output of over-voltage => super synchronous magnitude exceeds limit Same analog and binary data recorded locally in a disturbance record April 1, 2016 Slide 28
Conclusion PMUs placed in strategic power system locations provide real-time monitoring of I and V of the oscillation sources support detection of SSO/SSR locally via special filters Combining SSO/SSR detection and synchrophasors enables early indication of potential machine torsional stress initiation of mitigation techniques locally or from a central location Special filters provide extremely good accuracy and excellent noise rejection cover a broad range of frequencies support various applications April 1, 2016 Slide 29
References TERMS, DEFINITIONS AND SYMBOLS FOR SUBSYNCHRONOUS OSCILLATIONS, IEEE Subsynchronous Resonance Working Group of the System Dynamic Performance Subcommittee Power System Engineering Committee, IEEE Transactions on Power Apparatus and Systems, Vol. PAS- 104, No. 6, June 1985. L. Gross Sub-Synchronous Grid Conditions:New Event, New Problem, and New Solutions, WPRC 2010 conference, Spokane, WA, USA. R. Hedding, S. Roxenborg Subsynchronous Oscillation Detection Using Microprocessor Relays, 2015 WPRC conference, Spokane, WA USA. Z. Gajic, S. Roxenborg, T. Bengtsson, S. Lindahl, P. O.Lindström, H.Eriksson, M. Lindström Design Challenges for Numerical SSR Protection, CIGRE SC B5 Colloquium, September 2015, Nanjing, China. M. Larsson, G. Antonova, L-F Santos, P. Korba Monitoring and Control of Power System Oscillations using FACTS/HVDC and Wide-area Phasor Measurements, 2013 WPRC conference, Spokane, WA, USA. April 1, 2016 Slide 30
Questions? April 1, 2016 Slide 31
ABB Oy April 1, 2016 Slide 32