SSR Control of Power Systems By Colin EJ Bowler Instrumentation Technology Inc. September 9, 2013 ERCOT

Electromagnetic Resonance Concerns Causes SSR from series capacitor application Electrical self excitation Torsional interaction Transient resonance Device dependent torsional interaction Control design ramification for: HVDC, SVC, UPFC Excitation systems Super synchronous resonance Torsional modes too close to f0 and 2*f0 frequency Strong coupling to generator torque Negative sequence torque transients Unbalanced transmission faults Breaker high-speed reclosing

SSR Torsional Interaction Damping v.s. Series Compensation Level An unstable electric & mechanical response Limited by magnetic saturation or plasticity A broad band phenomena 0->60 Hz Occurs over a wide range of series compensation Mitigated by damping or filtering

SSR Transient Response Mostly a post fault phenomena – torsional interaction driven by post fault resynchronization Mechanical response integrates the electrical response at resonance Even when stable, event will produce severe shaft fatigue

Application RULES for SSR Must be Base Case Stable Probability of SSR should be low N-2 or better at onset Must NOT Operate near Resonance Control Shaft Torque Keep same as equivalent uncompensated system SSR Relay Only Protection Only when probability of SSR is very low Backing up calculated expectation of stability Backing up switching countermeasures Load Dependent Capacitor switching SSR Relay only for backing up primary SSR control

Quantification of SSR Level of Torsional Destabilization Excess over positive damping Turbine Positive Damping 0.05 to 0.1 rad/sec Negative Damping due to SSR 500 kV – 3 Rad/sec 345 kV - 1 Rad/sec Level of Shaft Torque Excess over High Cycle Fatigue Torque 1 to 3 Times on uncompensated system Do not exceed levels for uncompensated system

SSR Protection Options Instantaneous electro-magnetic torque Instantaneous three phase voltages Instantaneous three phase line current Subtract stator iR drops from voltage Integrate to measure magnetic flux Multiply flux by current Instantaneous velocity response Each end of TG set Basic Elements of Torsional Protection & Monitoring SSO – Subsynchronous Time over-current SMF – Subsynchronous Time over-velocity DMF – Subsynchronous Time over-acceleration Monitoring internal stress and fatigue

SSR Relay Qualities Dependability Security Undependable Insecure Always Trips when required Security Never False Trips Undependable Does NOT trip when required Insecure Trips when NOT required (FALSE)

SSR Relay Options Security and Dependability Sensing Means Time Delay to Trip Relay Type V/I Velocity T < 1 < 1 T < 10 T > 10 Points SSO 3V/3I I/U S/D SMF 0/0 1 or 2 SMF/DMF 2 Legend Meaning Description I Insecure FALSE Trips U Undependable NO Trip when Required S Secure Never False Trips D Dependable Always Trips when required Subsynchronous Electrical Oscillations Time over Current Single Mode Frequency Time over Velocity DMF Digital Mode Frequency Time over Acceleration Developed for SCE Mohave High Speed Trip required 3V 3 Phase voltage 3I 3 Phase Current Rotor Velocity Measure at one or two tooth-wheel locations on Turbine-Generator

SSR Countermeasure Selection Example SEDC SEDC/TCSC/SRF   Onset Instability level 0.05 0.1 0.2 0.4 0.8 1.6 3.2 Radians/Sec Contingency Doubling Time 13.86 6.93 3.46 1.73 0.87 0.43 0.22 Seconds AC/SSO AC/SMF AC/DMF 1 2 SSO SMF 3 DMF LEGEND AC Active SSR Counter Measure SSO Protection SMF protection DMF Protection Supplementary Excitation Damping Control TCSC Thyristor Controlled Series capacitor SRF SSR Power Filter

Instrumentation Technology Experience Original Developer of SMF Protection in US Developed DMF Observer Protection Commissioned by SCE First use on Mohave Plant Installed, Operating, Ordered - 24 systems 16 gas turbines 4 Nuclear 4 Fossil

The End THANK YOU Colin EJ Bowler President IT Inc. cbowler@inst-tech.com (919)-656-5853 m (919)-380-1039 p

DMF Protection for Combined Cycle Group SMF & DMF Protection Operator Interface Velocity Target vs Mode Acceleration Targets vs Mode Trip Target Shaft Torque Display

Post Event Data Evaluation Enhanced replacement for generator oscillograph 120 Samples per cycle 32 Channels Analog 24 channels digital input 24 channels digital output GPS Time synchronized Available with phasor measurement sub-system COMTRADE file generation Arbitrary long events Nonvolatile storage Windows human interface Independent from protection system Easy to understand & use

SSR Protection Traditional methods - SMF Observer based methods – DMF Filter modal response from velocity Inverse time over-velocity Cannot mitigate resonance due to filter delay Observer based methods – DMF Calculate response where inaccessible to direct measurement Measure modal torque without filter delay Inverse time over-velocity and acceleration Can trip before max response is reached Extremely Dependable & Secure A class of both protection and mitigation for SSR Valuable for super synchronous response observation Eliminates false trip and high associated cost