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“Importance of Reactive Power Management, Voltage Stability and FACTS Applications in today’s Operating Environment” Sharma Kolluri Manager of Transmission Planning Entergy Services Inc Engineering Seminar Organized by IEEE Mississippi Section Jackson State University August 20, 2010.

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“Importance of Reactive Power Management, Voltage Stability and FACTS Applications in today’s Operating Environment”

Sharma Kolluri

Manager of Transmission Planning

Entergy Services Inc

Engineering Seminar

Organized by IEEE Mississippi Section

Jackson State University

August 20, 2010



  • Introduction

  • VAR Basics

  • Voltage Stability


  • Applications at Entergy

  • Summary


Voltage profile during aug 14 th blackout

Voltage Profile during Aug 14th Blackout

  • Voltages decay to almost 60% of normal voltage. This is probably the point that load started dropping off.

  • However, the recovery is too slow and generators are not able to maintain frequency during this condition.

  • Many generators trip, load shedding goes into effect, and then things just shut down due to a lack of generation.


A “Near” Fast Voltage Collapse in Phoenix in 1995

North American Electric Reliability Council, System Disturbances, Review of Selected 1995 Electric System Disturbances in North America, March 1996.

Recommendation 23


  • Strengthen Reactive Power and Control Practices in all NERC Regions

    “Reactive power problem was a significant factor in the August 14 outage, and they were also important elements in the several of the earlier outages”

    -Quote form the outage report

Reactive power

Reactive Power

Laws of reactive physics

Laws of Reactive Physics

  • System load is comprised of resistive current (such as lights, space heaters) and reactive current (induction motor reactance, etc.).

  • Total current IT has two components.

    • IR resistive current

    • IQ reactive current

    • IT is the vector sum of IR & IQ

    • IT = IR + jIQ




North American Electric Reliability Corporation

Laws of reactive physics1

Laws of Reactive Physics

  • Complex Power called Volt Amperes (“VA”) is comprised of resistive current IR and reactive current IQ times the voltage.

    • “VA” = VIT* = V (IR – jIQ) = P + jQ

  • Power Factor (“PF”) = Cosine of angle between P and “VA”

    • P = “VA” times “PF”

  • System Losses

    • Ploss = IT2 R (Watts)

    • Qloss = IT2 X (VARs)




North American Electric Reliability Corporation

Reactive physics var loss

Reactive Physics – VAR loss

  • Every component with reactance, X: VAR loss = IT2 X

  • Z is comprised of resistance R and reactance X

    • On 138kV lines, X = 2 to 5 times larger than R.

    • One 230kV lines, X = 5 to 10 times larger than R.

    • On 500kV lines, X = 25 times larger than R.

    • R decreases when conductor diameter increases. X increases as the required geometry of phase to phase spacing increases.

  • VAR loss

    • Increases in proportion to the square of the total current.

    • Is approximately 2 to 25 times larger than Watt loss.

North American Electric Reliability Corporation

Reactive power for voltage support

Reactive Power for Voltage Support

Reactive Loads

VARs flow from High voltage to Low voltage; import ofVARs indicate reactivepower deficit

Reactive power management compensation

Reactive Power Management/Compensation

What is Reactive Power Compensation?

  • Effectively balancing of capacitive and inductive components of a power system to provide sufficient voltage support.

    • Static and dynamic reactive power

  • Essential for reliable operation of power system

    • prevention of voltage collapse/blackout

      Benefits of Reactive Power Compensation:

  • Improves efficiency of power delivery/reduction of losses.

  • Improves utilization of transmission assets/transmission capacity.

  • Reduces congestion and increases power transfer capability.

  • Enhances grid reliability/security.

  • Transmission line real and reactive power losses vs line loading

    Transmission Line Real and Reactive Power Losses vs. Line Loading

    Source: B. Kirby and E. Hirst 1997, Ancillary-Service Details: Voltage Control,

    ORNL/CON-453, Oak Ridge National Laboratory, Oak Ridge, Tenn., December 1997.

    Static and dynamic var support

    Static and Dynamic VAR Support

    • Static Reactive Power Devices

      • Cannot quickly change the reactive power level as long as the voltage level remains constant.

      • Reactive power production level drops when the voltage level drops.

      • Examples include capacitors and inductors.

    • Dynamic Reactive Power Devices

      • Can quickly change the MVAR level independent of the voltage level.

      • Reactive power production level increases when the voltage level drops.

      • Examples include static VAR compensators (SVC), synchronous condensers, and generators.

    Voltage stability

    Voltage Stability

    Common definitions

    Common Definitions

    Voltage stability- ability of a power system to maintain steady voltages at all the buses in the system after disturbance.

    Voltage collapse - A condition of a blackout or abnormally low voltages in significant part of the power system.

    Short term voltage stability - involves the dynamics of fast acting load components such as induction motors, electronically controlled loads, and HVDC converters.

    Long term voltage stability - involves slower acting equipments such as tap-changing transformer, thermostatically controlled loads, and generator limiters.

    What is voltage instability collapse

    What is Voltage Instability/Collapse?

    • A power system undergoes voltage collapse if post-disturbance voltages are below “acceptable limits”

      • voltage collapse may be due to voltage or angular instability

  • Main factor causing voltage instability is the inability of the power systems to “maintain a proper balance of reactive power and voltage control”

  • Voltage instability collapse

    Voltage Instability/Collapse

    • The driving force for voltage instability is usually the load

    • The possible outcome of voltage instability:

      • loss of loads

      • loss of integrity of the power system

    • Voltage stability timeframe:

      • transient voltage instability: 0 to 10 secs

      • long-term voltage stability: 1 – 10 mins

    Voltage stability causes and analysis

    Voltage stability causes and analysis

    Causes of voltage instability

    Increase in loading

    Generators, synchronous condensers, or SVCs reaching reactive power limits

    Tap-changing transformer action

    Load recovery dynamics

    Tripping of heavily loaded lines, generators

    Methods of voltage stability analysis

    Static analysis methods

    Algebraic equations, bulk system studies, power flow or continuation power flow methods

    Dynamic analysis methods

    Differential as well as algebraic equations, dynamic modeling of power system components required


    Generator Capability Curve

    Over-excitation Limit

    Lagging (Over-excited)

    0.8 pf line

    Stator Winding Heating Limit

    Normal Excitation (Q = 0, pF = 1)

    - Per unit MVAR (Q) +


    Turbine Limit

    Leading (Under-excited)

    Under-excitation Limit

    Stability Limit

    P v curve

    P-V Curve

    Q v curve

    Q-V Curve with Detailed Load Model

    Peak Load with Fixed Taps







    Base Case




















    Voltage (p.u.)

    Q-V Curve

    Key concerns

    Limit UVLS activation

    Minimize motor tripping

    Key Concerns

    Voltage (pu)

    Possible solutions for voltage instability

    Possible Solutions for Voltage Instability

    • Install/Operate Shunt Capacitor Banks

    • Add dynamic Shunt Compensation in the form of SVC/STATCOM to mitigate transient voltage dips

    • Add Series Compensation on transmission lines in the problem area

    • Implement UVLS Scheme

    • Construct transmission facilities


    Voltage Collapse

    Fault induced delayed voltage recovery fidvr

    Fault Induced Delayed Voltage Recovery (FIDVR)

    • FIDVR Definition

    • Load Models

    Fault induced delayed voltage recovery fidvr1

    Fault Induced Delayed Voltage Recovery (FIDVR)

    • What is it?

      • After a fault has cleared, the voltage stays at low levels (below 80%) for several seconds

    • Results in dropping load / generation or fast voltage collapse

    • 4 key factors drive FIDVR:

      • Fault Duration

      • Fault Location

      • High load level with high Induction motor load penetration

      • Unfavorable Generation Pattern

    Load characteristics

    Load characteristics

    The accuracy of analytical results depends on modeling of power system components, devices, and controls.

    Power system components - Generators, excitation systems, over/under excitation limiters, static VAr systems, mechanically switched capacitors, under load tap changing transformers, and loads among others.

    Loads are most difficult to model.

    Complex in behavior varying with time and location

    Consist of a large number of continuous and discrete controls and protection systems

    Dynamics of loads, especially, induction motors at low voltage levels should be properly modeled.

    Induction motor characteristics

    Induction motor characteristics

    Impact of fault on transmission grid

    Depressed voltages at distribution feeders and motor terminals

    Reduction of electrical torque by the square of the voltage resulting in slow down of motors

    The slow down depends on the mechanical torque characteristics and motor inertias

    With fault clearing

    Torque -per unit

    Square-law load torque

    Electric torque

    Constant load torque


    Speed – per unit

    Fig. 1 Induction motor characteristics

    • Partial voltage recovery

    • Slowed motors draw high reactive currents, depressing voltage magnitudes

    • Motor will reaccelerate to normal speed if, electrical torque>mechanical torque

    • else, the motors will rundown, stall, and trip

    • The problem is severe in the summer time with large proportion of air conditioner

    • motors

    Air conditioner motor characteristics

    Air conditioner motor characteristics


    Main portion (80-87%) consumed by compressor motor

    Electromagnetic contactor drop out between (43-56%) of the nominal voltage and reclose above drop out voltage

    Stalling at (50-73%) of the nominal voltage

    Thermal overload protection act if motors stall for 5-20 seconds

    The operation time of thermal over load (TOL) protection relay is inversely proportional to the applied voltage at the terminal

    Air conditioner should be modeled to analyze the short term voltage stability problem

    Quite important for utilities in the Western interconnection

    Load modeling

    Load modeling

    Old models – Loads are represented as lumped load at distribution feeder

    Does not consider the electrical distance between the transmission bus and the end load components

    The diversity in composition and dynamic behavior of various electrical loads is not modeled


    WECC interim model

    20% of the load as generic induction motor load

    80% constant current P and constant impedance Q

    Transmission Bus


    Distribution Bus

    Distribution Capacitor

    Lumped Load (ZIP load)

    Fig. 2 Traditional load model

    Composite load modeling

    Composite load modeling

    Representation of distribution equivalent

    Feeder reactance

    Substation transformer reactance

    Parameters of various load components

    Discharge lighting

    Electronic Loads

    Constant Impedance loads

    Motor loads

    Distribution Capacitor

    Transmission Bus

    Bus 1


    Distribution Bus

    Bus 2

    Substation Capacitor

    Feeder Equivalent

    Distribution Feeder

    Bus 3

    Dynamic Loads

    (Small motor, Large motor, trip motor loads)

    Static Loads

    (Constant impedance, constant current, constant impedance loads)

    Distribution Capacitor

    Fig. 3 Composite load model structure




    What is FACTS?

    • Alternating Current Transmission Systems Incorporating Power Electronic Based and Other Static Controllers to Enhance Controllability and Increase Power Transfer Capability.

    • power semi-conductor based inverters

    • information and control technologies

    Major facts controllers

    Major FACTS Controllers

    • Static VAR Compensator (SVC)

    • Static Reactive Compensator (STATCOM)

    • Static Series Synchr. Compensator (SSSC)

    • Unified Power Flow Controller (UPFC)

    • Back-To-Back DC Link (BTB)

    Facts applications


    RTO System





    Power Generation

    Voltage Control

    Power System Stability

    Power Flow Control

    System Reliability

    Inter-area Control

    Inter-tie Reliability


    Import Capability







    Power Quality


    Transmission Capacity


    Power System




    FACTS Applications

    Static var compensator svc

    Static VAr compensator (SVC)

    Variable reactive power source

    Can generate as well as absorb reactive power

    Maximum and minimum limits on reactive power output depends on limiting values of capacitive and inductive susceptances.

    Droop characteristic




    Firing angle control



    Fig. 4 Schematic diagram of an SVC

    Static compensator statcom

    Static compensator (STATCOM)

    Voltage source converter device

    Alternating voltage source behind a coupling reactance

    Can be operated at its full output current even at very low voltages

    Depending upon manufacturer's design, STATCOMs may have increased transient rating both in inductive as well as capacitive mode of operation

    System bus






    DC-AC switching converter



    Fig. 5 Schematic diagram of STATCOM

    Technology applications at entergy

    Technology Applications at Entergy

    Technology applications at entergy to address reactive power issues

    Technology Applications at Entergy to Address Reactive Power Issues

    • Large Shunt Capacitor Banks

    • UVLS

    • Series Compensation

    • SVC

    • Coordinated Capacitor Bank Control

    • DVAR

    • AVR


    Determining Reactive Power Requirements in the Southern Part of the Entergy System for Improving Voltage Security – A Case Study

    Sharma Kolluri

    Sujit Mandal

    Entergy Services Inc

    New Orleans, LA

    Panel on Optimal Allocation of Static and Dynamic VARS for Secure Voltage Control

    2006 Power Systems Conference and Exposition

    Atlanta, Georgia

    October 31, 2006

    Areas of voltage stability concern

    Areas of Voltage Stability Concern

    North Arkansas


    West of the Atchafalaya Basin


    Southeast Louisiana

    Western Region

    Amite South/DSG

    Study objective

    Study Objective

    • Identify Voltage Stability Problems in the DSG area

    • Determine the proper mix of reactive power support to address voltage stability problem

    • Determine size and location of static and dynamic devices.

    Downstream of gypsy area critical facilities

    Ninemile Units

    1 - 50 MW

    2 - 60 MW

    3 - 128 MW

    4 - 740 MW

    5 - 750 MW

    Michoud Units

    1 - 65 MW

    2 - 240 MW

    3 - 515 MW

    115 kV

    115 kV

    - 230 kV

    230 kV

    Downstream of Gypsy Area - Critical Facilities

    Little Gypsy-South Norco 230kV line

    Waterford-Ninemile 230kV line


    DSG Issues

    • Area load growth

      • 1.6% projected for 2003 - 2013

      • Weather normalized to 100º F

      • Projected peak load – 3800 MW

    • Area power factor - Low

      • 94% at peak load

    • Worst double contingency

      • Loss of the Waterford to Ninemile 230 kV transmission line and one of the 230 kV generating units at Ninemile or Michoud



    New Orleans area voltage profile on June 2, 2003

    (with 2 major generators offline)

    • Area Problems

      • Thermal overloads of underlying 115 kV and 230 kV transmissionsystem

      • Depressed voltages throughout New Orleans metro area potentially leading to voltage collapse and load shedding

    Various steps used for determining reactive power requirements

    Various Steps Used for Determining Reactive Power Requirements

    • Step 1 – Problem identification

    • Step 2 – Determining total reactive power requirements

    • Step 3 – Sizing and locating dynamic devices

    • Step 4 – Sizing and locating static shunt devices

    • Step 5 – Verification of reactive power requirements

    Tools techniques used

    Tools & Techniques Used

    • Various tools and techniques used for analysis purposes

      • PV analysis using PowerWorld

      • Transient stability using PSS/E Dynamics

      • Mid-term stability using PSS/E Dynamics

      • PSS/E Optimal Power Flow

    • Detailed Models used

      • Motor models and appropriate ZIP model for dynamic analysis

      • Tap-changing distribution transformers, overexcitation limiters, self-restoring loads modeled in mid-term stability study

    Criteria requirements

    Improve post-fault voltage

    Minimize motor tripping


    Voltage (pu)

    Steady state analysis results

    Steady State AnalysisResults

    Pv curve ninemile unit 4 out of service trip ninemile unit 5 and waterford ninemile 230 kv line


    PV CurveNinemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line

    Dynamic analysis

    Dynamic Analysis


    Stability Simulation Ninemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line

    Process for determining reactive power requirements

    Process for Determining Reactive Power Requirements

    • Approx 700 MVAr of reactive power shortage identified in the DSG

      • How much static and how much dynamic?

    • Criteria for determining static and dynamic requirements

      • Voltage at critical buses should recover to 1 pu in several seconds

      • Voltage at critical buses should recover to 0.9 pu within 1.5 - 2 seconds

      • Voltage should not dip below 0.7 pu for more than 20 cycles

      • Generator reactive power output should be below Qmax

    • Factors considered in sizing static/dynamic devices

      • Short circuit levels, size & location of the stations, number and existing size of cap banks, back-to-back switching, etc

    Svc size and location

    Optimal size and location

    SVC Size and Location

    • Sites considered

      • Ninemile 230 kV

      • Gretna 115 kV

      • Paterson 115 kV

    • Size

      • 300 MVAR

      • 500 MVAR

    Steps to locate static shunt devices

    Steps to locate Static Shunt Devices

    • Static shunt requirements – 400 MVAR approximately

    • Options available to locate the static shunt devices on the transmission or distribution systems

    • OPF Program used to come up with size and location of shunt devices

    Opf application

    OPF Application

    • PSS/E OPF Program used

    • Objective Function – Minimize adjustable shunts

    • OPF simulated for critical contingencies

    List of shunt capacitor banks banks recommended

    List of Shunt Capacitor Banks Banks Recommended


    Simulation Results with the Capacitors and SVCNinemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line


    SVC PerformanceNinemile Unit 4 out-of-serviceTrip Ninemile Unit 5 and Waterford – Ninemile 230 kV line



    • Process for determining static and dynamic reactive power requirements discussed

    • OPF program utilized for sizing/locating static shunt capacitor banks

    • Results verified using mid-term stability simulations

    • Study recommendation – 400 MVAR of static shunt devices and 300 MVAR of dynamic shunt compensation

    Ninemile svc configuration

    = 75 MVAr

    = 75 MVAr

    = 150 MVAr

    Ninemile SVC Configuration

    External device control single line diagram of svc and msc

    External Device ControlSingle line diagram of SVC and MSC


    SVC Ninemile

    Svc ninemile

    SVC Ninemile

    Porter 0 300mvar svc

    Porter 0/+300Mvar SVC

    SVC Topology: 2 x 75MVAr TSC & 1 x 150MVAr TSC

    Porter static var compensator svc

    Porter Static Var Compensator (SVC)

    Maintains system voltage by continuously varying VAR output to meet system demands Controls capacitor banks on the transmission system to match reactive output to the load requirements.

    Porter svc

    Porter SVC

    Series capacitor dayton bulk 230kv station

    Series Capacitor – Dayton Bulk 230kV Station

    The Capacitor offsets reactance in the line, making it appear to the system to be half of its actual length. Power flows are redirected over this larger line, unloading parallel lines and increasing transfer capability.

    Dsmes unit

    DSMES Unit

    Stores Energy in a superconducting coil

    Automatically releases energy to the system when needed to ride through voltage dips caused by faults. This unit improves power quality and reduces customer loss of production.

    Industry issues

    Industry Issues

    • Coordination of reactive power between regions

    • No clearly defined requirements for reactive power reserves

    • Proper tools for optimizing reactive power requirements

    • Incentive to reduce losses



    • The increasing need to operate the transmission system at its maximum safe transfer limit has become a primary concern at most utilities

    • Reactive power supply or VAR management is an important ingredient in maintaining healthy power system voltages and facilitating power transfers

    • Inadequate reactive power supply was a major factor in most of the recent blackouts



    Under voltage load shed logic western region

    Under Voltage Load Shed Logic - Western Region

    T&D Planning

    April 2010


    Western Region – Overview

    ≤ 230 kV Tie Lines


    Load Center

    Load projection

    Load Projection

    • 2010 peak: 1770 MW

    • 2012 peak: 1852 MW


    Sample PV Curve ResultLewis Creek Unit 1 & China-Porter 230kV Out - 2010

    2010 summer pv curve analysis

    2010 Summer PV Curve Analysis

    Approved Construction Plan Projects included:

    *Relocate Caney Creek 138kV

    Dynamic analysis results

    Dynamic Analysis Results

    Results 2010 case without load shed

    Results: 2010 case without load shed

    Case 3 Voltages (pu): Goslin: 0.810; Conroe: 0.855; Cleveland: 0.909; Jacinto: 0.924; Dayton: 0.944; Huntsville: 0.944

    Case 4 Voltages (pu): Goslin: 0.757; Conroe: 0.800; Dayton: 0.913; Huntsville: 0.928; Cleveland: 0.928; Rivtrin: 0.941

    2010 summer conditions dynamics analysis

    2010 Summer Conditions - Dynamics Analysis

    • Lewis Creek Unit 1 outaged in the base case

    • 50% induction motor load is modeled

    • Result: Shed Load Block 1 (183 MW)


    • Observations for 2010 Summer Peak Conditions

    • Existing load shed logic in Western Region OK for 2010 Summer conditions

    • Voltage at some critical buses drop below 0.7 pu for more than 20 cycles – Potential of motor load tripping

    • Conclusions for 2010 Summer

    • Reducing load shed blocks to 180 + 70 MW in Western Region has no negative impact

    Results 2010 case with load shed block 1

    Results: 2010 case with load shed (Block 1)

    Case 3 Voltages (pu): Goslin: 0.872; Conroe: 0.902; Cleveland: 0.934; Jacinto: 0.948; Dayton: 0.966; Huntsville: 0.968

    Case 4 Voltages (pu): Goslin: 0.827; Conroe: 0.855; Dayton: 0.939; Cleveland: 0.951; Huntsville: 0.954; Jacinto: 0.964

    Conclusions and recommendations

    Conclusions and Recommendations

    • Retain the exiting UVLS logic

    • Change the load blocks

      • Block one: 180 MW

      • Block two: 70 MW (existing size 111 MW)


    Proposed Load Shed Logic

    Voltage @ 4/8 buses <0.90 pu

    Armed all time

    Drop load

    One or more Lewis Creek units in-service?

    OEL at Lewis Creek units

    Voltage @ 4/8 buses

    < 0.92 pu

    Time Delay 3 seconds

    Load Blocks:

    Block 1: 175 MW

    Alden: 50 MW

    Metro: 35 MW

    Oakridge:30 MW

    Goslin: 60 MW

    Block 2: 75 MW

    In the vicinity of Block 1

    Monitored Buses:

    Metro 138kV

    Goslin 138kV

    Alden 138kV

    Oakridge 138kV

    Huntsville 138kV

    Rivtrin 138 kV

    Poco 138 kV

    Conroe 138 kV

    Reset the Process for next LVSH block

    Load Blocks:

    Block 1: 175 MW

    Block 2: 75 MW

    The above conditions need to be met for 3 scans to trigger load shedding

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