Remedial Action Schemes: Practical Solutions for Power System Stability Problems Scott Manson, PE March, 2011

What Dictates Power System Stability? Frequency Response Characteristic Major Disturbances Volt/MVAr Margins Frequency/MW Margins Economics Undesired Oscillations

Governors/Turbines Simply Can’t Respond Instantly Red – Electrical Power

Typical Governor Controller WGOV1

Frequency Depressions Most turbines control packages trip off at ~ 57.5 Hz to protect themselves from damage Large, Expensive Motors trip for same reason Will Cascade into uncontrolled blackouts S S Power Out Power In – Df/dt = w J

frequency decay rate proportional to the magnitude of the power deficit

Frequency Response Characteristic Many different definitions and names throughout the world R, FRC, dF/dP, etc Some countries (not US) define generator FRC requirements Effects Dominated by: Load composition System Inertia Generator Tuning

Frequency Response Characteristic (FRC) Example for large offshore NGL plant Sudden increase of 0.3 pu load

Three common FRC Variants Point A - ‘Transient’ FRC = 50 (0.3)/ (50-48.7) = 11.5 Point B – Locked Rotor FRC = Extraction mode FRC = 50 (0.3)/ (50-48) = 7.5 Point C – ‘System Long Term FRC’ = ‘System Droop Characteristic’ = 50 (0.3)/ (50-49.4) = 25

What does FRC tell you about a Power System? A quantity of ‘stiffness’ Example: Long Term FRC 25*150 MW/50Hz = 75 MW/Hz 75 MW of load will reduce system frequency by 1 Hz Extraction Mode FRC = 22.5 MW/Hz Transient FRC = 34.5 MW/Hz

Solutions for a Poor FRC Governor tuning Add Inertia Limit electronic loads More Synchronous Machines BIG Battery Backed Statcom Load Shedding Generation Shedding/Runback

SEL Project to improve Power Quality Presidio, TX (By Controlling Some Big Batteries)

Power Corridor Transport Limits Out of Step (OOS) Behavior Lethal to machines and power systems Thermal limits must be obeyed to prevent conductor damage

Jim Bridger Power Plant – Long History of Severe Faults and OOS behavior

Power System Overview

SEL RAS Protection Required Prevent loss of stability caused by Transmission line loss Fault types Jim Bridger Plant output levels WECC requires Jim Bridger output reduced to 1,300 MW without RAS

Stability Studies Determine RAS Timing Requirements Total time from event to resulting action must not exceed 5 cycles 20 ms available for RAS, including input de-bounce and output contact

JB RAS Also protects against… Subsynchronous resonance (SSR) protection – capacitor bypass control Transmission corridor capacity scheduling limits

Dynamic Remedial Action for Idaho Power Co.

Idaho Power System Conundrum Maintain the stability, reliability, and security Operate system at maximum efficiency Prevent permanent damage to equipment Minimal Capital expenditures Maximize Revenue Serve increasing load base

RAS Was Lowest Cost Solution New transmission line: $100s of millions New transmission substation: $10s of millions This project: approximately $2 million

RAS Functional Requirements Protect lines against thermal damage Optimize power transfer across critical corridors Predict power flow scheduling limits dynamically Follow WECC requirements Track Changing power system topography 20 ms response requirement

RAS Actions Based on Combinations of Factors N events (64) J states (64) System states (1,000) Arming level calculation Action tables combinations (32) Crosspoint switch (32x32)

Gain Tables Allow Operations to Adjust RAS Performance for Any System Event 7 gain entries used in arming level equation 64 N events 32 actions 1,000 system states 4 seasons 8,192,000 possible gains per gain entry 57,344,000 total gains

RAS Gains Configured From HMI

Most Sophisticated RAS in the World exists in South Idaho 

Major Disturbances Put Power Systems at Risk Faults Critical Clearing Time to prevent OOS Fault Type Protection speed Fast breakers Load startup or trip (FRC problem) Generator trip (FRC problem)

Asian Electrical Operating Company (National Grid) Generator Trip at Chevron Refinery Cause Massive Financial and Environment Problems Generation Station No. 1 Production Plant No. 1 Load ~ 120MW No. 2 & Prod. Plt 2 Load ~ 40MW Generation Station No. 3 & Prod. Plt. No. 3 Load ~ 60MW Fig. 1 – Simplified One-Line Asian Oil Production Complex Asian Electrical Operating Company (National Grid) 4 x 32 MW ea 3 x 34.5 MW ea. 2 x 105 MW ea. Potential for power system collapse

Preloaded and Ready to Go Generation Tripping Remediated by sub-cycle load shedding Techniques Invented at SEL f Trigger Inputs Crosspoint Switch Preloaded and Ready to Go t CB Opens X Trip G2 N5 N4 N3 N2 N1 Trip G1 Output Remediation Contingency Trip G3 Trip G4 Bypass C1 Bypass C2 Tripping Outputs

Generation Tripping Problem Requires a sub-cycle Load Shedding Scheme

Three main techniques for Load Shedding Contingency-based (aka ‘FLS’) Tie line Bus Tie Generator Asset Overloads U/F based Traditional technique in relays (lots of problems) Enhanced SEL technique, generally a backup to contingency-based system U/V based

Contingency Based Load Shed Systems for Chevron Plant Sub cycle response time prevent frequency sag Advises operator of every possible future action Expandable to thousands of sheddable loads with modern protocols Tight integration to existing protective relays

Contingency Based Load Shed system for Chevron Must have live knowledge of machine IRMs, Spinning Reserves, Power output initiating event is the sudden loss (circuit breaker trip) of a generator, bus coupler breaker, or tie breaker. perform all of their calculations prior to any contingency event System topology tracking

Typical Volt/VAR Stability problems Typical problems Fault induced long term suppressed voltage conditions Large Motor Starting Risk Plant blackouts Typical Solutions Dynamic control of exciters on large synchronous motors FACTS devices Misc power quality improvement electronics

Low Cost Solution: Controlling Exciters on 15 MVA SM on a 700 MW GOSP preserves VAR margins

How to contain a Voltage Collapse? Increase generation – reduce demand, match supply and demand Increase reactive power support Reduce power flow on heavily loaded lines (use Flexible AC Transmission Systems) Reduce OLTC at distribution level, to reduce loads and avoid blackouts (Brownout)

Frequency/MW Margins Problem1: Long Term Problem. Caused by Insufficient Reserve Margins (RM) of generation. Solution: Add more generators. Problem2: Short Term Problem. Caused by insufficient Incremental Reserve Margin (IRM) of generators. Solution1: RAS load/generation shedding Solution2: Machines with larger IRM

Typical Steam Turbine IRM characteristic Output (%) 100 % 0 % Time (Seconds) 500

Typical IRM values Steam Turbines: 20-50% Combustion Turbines Single Shaft Industrials: 5-10% Aero Derivatives: 10 – 50% Hydro Turbines: 1 - 25%

Economics Affecting Stability Danger: Fewer, larger generators Less expensive, more efficient More risk upon losing one generator Economic Dispatch Contradicting Stability Optimization NIMBY: Local Thermal/ Remote Hydro plants MW transactions across critical corridors put plants or system islands at risk

Solution: Active Load Balancing and Tie flow control for Optimal Stability Economic Dispatch (Low Risk Scenarios) Tie line flows (MW) per contracted schedule Distributes MW between units per Heat Rate Tie-line closed (High Risk Scenarios): Control intertie MW to a user defined low value Distributes MW between units, equal % criterion Tie-line open (Islanded Operation – high risk) Control system frequency to a user defined set-point Distributes MW between units , equal % criterion

Common PowerMAX Screen: AGC/VCS Interface

Common PowerMAX Screen: ICS Interface

Unwanted Oscillations Explain Spectrum of a power system Sub Synchronous Resonance (SSR) First detected in 1970’s during commissioning of high speed/gain exciters Mechanical/Electrical Mode Interaction Shaft oscillation modes Heavily Series compensated lines Improperly Reactive Compensation in Exciters

Power System Stabilizers Provide Damping based on two possible input types: Frequency (Hz)/Speed (rpm) – US Power (MW) - Europe

Any Questions?