ssr control of power systems

September 9, 2013 © Instrumentation Technology 1

SSR Control of Power Systems

Instrumentation Technology Inc.September 9, 2013

ERCOT

ByColin EJ Bowler


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 InteractionDamping 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


• 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

• 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

SSR Protection Options


SSR Relay Qualities

• Dependability– Always Trips when required

• Security– Never False Trips

• Undependable– Does NOT trip when required

• Insecure– Trips when NOT required (FALSE)


SSR Relay OptionsSensing Means Time Delay to Trip

Relay Type V/I Velocity T < 1 < 1 T < 10 T > 10

Points

SSO 3V/3I 0 I/U I/U S/D

SMF 0/0 1 or 2 I/U S/D S/D

SMF/DMF 3V/3I 2 S/D S/D S/D

Legend Meaning Description

I Insecure FALSE TripsU Undependable NO Trip when RequiredS Secure Never False TripsD Dependable Always Trips when required

SSOSubsynchronous Electrical Oscillations Time over Current

SMF Single Mode Frequency Time over VelocityDMF Digital Mode Frequency Time over Acceleration Developed for SCE Mohave High Speed Trip required3V 3 Phase voltage3I 3 Phase CurrentVelocity Rotor Velocity Measure at one or two tooth-wheel locations on Turbine-Generator

SSR Relay Options Security and Dependability


SSR Countermeasure Selection Example

SSRCounter measure 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

0 AC/SSO AC/SMF AC/SMF AC/DMF AC/DMF AC/DMF AC/DMF 1 AC/SSO AC/SMF AC/SMF AC/DMF AC/DMF AC/DMF AC/DMF 2 SSO SMF SMF AC/DMF AC/DMF AC/DMF AC/DMF 3 SSO SMF SMF DMF AC/DMF AC/DMF AC/DMF

LEGEND

AC Active SSR Counter MeasureSSO SSO ProtectionSMF SMF protectionDMF DMF ProtectionSEDC Supplementary Excitation Damping ControlTCSC Thyristor Controlled Series capacitorSRF 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 [email protected](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– 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

ssr control of power systems

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