Review of Series Compensation

Douglas Bowman, P.E.Research, Development, and Special StudiesDate: May 20-21, 2014

Contents1. Series Compensation2. Series Compensation Types3. Subsynchronous Interactions (SSI) - Terms 4. Fundamentals of SSI and Series Compensation5. Forms of SSI 6. SSI and Series Compensation 7. Tools for Assessment of SSI in Series Compensated Networks8. SSI Mitigation Measures9. SSI Protection Measures10. Protective Relay Considerations for Series Compensated Networks11. Protective Relay Solutions for Series Compensated Networks12. Project Planning for Implementation13. Design Studies14. Concluding Remarks

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Series Compensation

1. Increases power transfer capability

2. Improves transient performance

3. Improves reactive power balance

4. Improves Voltage Stability5. Improves power flow

balance on adjacent lines6. Deferral of major transmission investments7. Preservation of existing rights of way

3Benefits of Series Compensation

Series Compensation

Increases Power Transfer Capability4

Since transmission lines are mostly inductive, adding series capacitance decreases its total reactance

Reducing XL increases PR

Compensation Level K is defined as the percent of XLoffset by the series capacitorExample: For XL = 1 ohm, 30% compensation produces XL - XC = .7 ohm

Series Compensation

Improves Transient Performance Following Disturbances

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If A1 > A2, the generator will return to stability

Series compensation increases the system stability limits by reducing the system reactance between machines as this directly increases the synchronizing torque that can be interchanged between them

Series Compensation

Improves Reactive Power Balance and Self-Regulation

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Transmission Line Reactive Power Losses : Qlosses=I2Xline

Series Capacitor Reactive Power Output: Qoutput=I2Xcapacitor

As a transfer across the line increases, Qoutput partially offset Qlosses

Reactive Power Balance For A 300 Mile 500kV Line

Series Compensation

Improves Voltage Stability7

Increasing compensation levels K provides greater Qoutput capability

Maximum power transfer capability of the line is increased

Generator reactive power is made available for voltage control

Effect of Increasing Compensation Levels

Series Compensation Types• Continuous current rating

according to the line• Overvoltage protection

• Zinc Oxide Varistor (MOV)• Conducts when voltage level

across capacitor reaches protection level

• Fast Protective Device (FPD)• For example, an air gap

conducts when energy absorbed by MOV exceeds rated values.

• Bypass Breaker• Damping Reactor

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Fixed Series Compensation (FSC)

Series Compensation Types• Two Modules

• FSC as previously described• Capacitor with thyristor controlled, air cooled reactor to modulate

line impedance• FACTS Device

• Offers Dynamic Power Flow Control• Reactance can be modulated to effectively mitigate SSI

• Blocked Mode removes reactor from circuit• By-Passed Mode removes capacitor from circuit• Controlled Mode varies total reactance

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Thyristor Controlled Series Compensation(TCSC)

Series Capacitor

MOV

Thyristors

Reactor

Thyristor Controlled Series Compensation

Series Capacitor

Bypass Switch

MOV

FPD

Fixed Series Compensation

DampingReactor

Bypass Switch

FPD

DampingReactor

Subsynchronous Interactions (SSI) - Terms

• Subsynchronous Interaction – A general term describing the condition where two or more parts of the power system exchange energy at one or more frequencies below the fundamental frequency (60 hz).

• Subsynchronous Oscillation - An SSO is a condition where the electric network exchanges significant energy with a turbine generator at one or more of the natural frequencies of the combined system below the synchronous frequency of the system following a disturbance from equilibrium.

• SSI can lead to SSOs that must be damped before outage or damage to network equipment occurs

• Subsynchronous Resonance (SSR) – A type of SSI where the electric power system, most often a series compensated transmission line, exchanges energy with a turbogenerator at one or more natural frequencies below the fundamental 60hz frequency (three types of SSR)

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Fundamentals of SSI and Series Compensation

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• A power system’s natural electrical frequencies are a function of its inductance and capacitance.

• When new capacitance is added, new natural electrical frequencies result and the system natural frequency approaches the fundamental frequency fo

• A generator’s shaft may also have multiple natural frequencies of oscillation• Four natural frequencies

or torsional modes for the system shown

Forms of SSI

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*

* Interaction with series compensation does not occur during SSTI

SSI and Series Compensation

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• SSR – TI (Torsional Interaction)• When a small disturbance occurs, simultaneous excitation of all

natural frequencies (modes) of oscillation occurs in both the electrical system and the generator

• If the electrical and mechanical natural frequencies are close to one another, sustained or growing rotor oscillations can occur resulting in possible torsional fatigue damage to the turbine generator shaft. This is classic SSR-TI.

• SSR – TA (Torsional Amplification)• When a large disturbance occurs, the subsynchronous transient

current frequency may be close to the generator natural torsional frequency

• Can lead to prolonged generator shaft oscillations with high amplitude causing increased stress and accelerated loss of life.

SSI and Series Compensation

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• IGE (Induction Generator Effect)• Purely electrical resonance effect• Combined generator and electric power system results in a negative

effective rotor resistance at a natural frequency below 60 hz• If the negative rotor resistance is greater than the apparent stator

plus network resistance, self –excited, subsynchronous current and electromagnetic torque can result

• SSCI – (Control Interaction) ERCOT 2009 Event• Event between wind generators and series compensated

transmission line• 2 pu overvoltage damaged rotor side protection circuits• Wind farm became radially connected through series capacitor• 1.5 seconds before capacitor was bypassed• Resonance between Capacitor and Wind Turbine Converter/Control• Only Type 3 and Type 4 Turbines Can Be Affected• See report for ERCOT’s SSI study process for new wind generation

Mohave SSR-TI Incident (1970)• Mohave generator: 1,580 MW coal-fired in NV.

• Gradually growing vibration that eventually fractured a shaft section.

• First investigations incorrectly determined cause. After 2nd failure in 1971 cause was identified as Subsynchronous Resonance.

• An electrical resonance at 30.5 Hz excited a mechanical resonance at 30.1 Hz.

• Problem was solved by reducing compensation and installing a torsional relay.

D. Baker, G. Boukarim, “Subsynchronous Resonance Studies and Mitigation Methods for Series Capacitor Applications,” IEEE 2005.D. Walker, D. Hodges, “Results of Subsynchronous Resonance Test At Mohave,” IEEE 1975.

Tools for Assessment of SSI in Series Compensated Networks

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• Frequency Scan Screening• Calculates apparent impedance from generator from 0 to 60

hz• Can identify potential IGE, SSR-TI, SSR-TA, and SSCI problems

• Eigenvalue Analysis• System model linearized, small pertubations examined• Identifies torsional mode damping characteristics• Used to study SSR-TI and SSCI problems

• Damping Torque Analysis• EMT type software used for analysis• Examines electrical torque response to small change in

generator speed to determine damping characteristic• Practical for evaluating SSR-TI

• Time Domain Analysis• EMT type software used for analysis• Most useful in studying SSR-TA problems

SSI Mitigation Measures (SSI Prevention)

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• Network Based Preventative Measures to Reduce a Known Risk of SSI• Operational Procedure

• Alter the network configuration or generation dispatch• Bypass the Capacitor or reduce its compensation level

• Passive Filter Damping for series resonance network condition• Shunt or Series• Shunt and Series

• FACTS Active Shunt Filter Damping• STATCOM or SVC

• FACTS Active Series Filter Damping• Thyristor Controlled

Series Compensation (TCSC)• Unified Power Flow Controller (UPFC)

SSI Mitigation Measures (SSI Prevention)

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• Generator Based Preventative Measures to Reduce a Known Risk of SSI• Passive Filter Damping• Active Filter Damping (FACTS devices such as TCR or STATCOM)• Supplemental Excitation Control Damping• Wind Turbine Control Damping

• Type 3 and 4 turbines use VSC as basis for control• Newer controls since 2009 mitigate SSI

SSI Protection Measures (SSI Detected)

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• Series Capacitor Bypass• Newer relays developed for SSCI since 2009• Generator Relays

Relay Signal Input CommentsTorsional Motion (Stress) Relay

Shaft Speed Developed and applied in the late 1970s. Speed is processed by band-pass filters to calculate conditions at particular sub-synchronous frequencies of interest. Torsional Stress Relays (TSR) have been applied at several generator units and are still available. Newer torsional motion relays are micro-processor based. Appears to be the most widely applied measure to protect genertors from the potential of SSI due to proximity of HVDC or series compensated lines.

S. CaliforniaEdison patent

Terminalvoltage

Micro-processor relay that uses exclusive time domain analysis on wave parameters of successive half cycles. More research is recommended as to the application of this 1986 patent, performance information, and current status.

ABB ResearchLtd. patent

GeneratorTerminal voltage

Micro-processor based relay developed in the 2011 timeframe.

ERLPhasePowerTechnologies

GeneratorTerminal voltage and currents

Micro-processor based relay is used to perform frequency spectrum analysis on the inputs to compare sub-synchronous frequency components with fundamental component.

Relay ApplicationInnovation

Armaturecurrent

Micro-processor based relay. Developed in late 2009 and applied in 2010 by AEPSC at two locations as backup generator protection.

Summary of Generator Based SSI Relays

Protective Relay Considerations for Series Compensated Networks

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• Voltage and current inversion due to nearby fault• Measured Impedance of Distance Protection when

series compensation switched in and out• Subsynchronous Transient Signal Impacts on

apparent impedance• Adjacent Line Impacts• Unbalanced Line and Mutual Impedance Impacts• Automatic Reclosing for Series Compensated

Transmission Lines• Series Capacitor Switching• Three Phase Automatic Reclosing • Single Phase Automatic Reclosing• Spurious Bypass Operation

Protective Relay Solutions for Series Compensated Networks

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• Advanced Relays for Series Compensation Application• Memory Polarization• Special Series Compensation Logic• Sequence Component Impedance for Directional

Discrimination• Protection Schemes

• Line Current Differential Protection• Directional Comparison Protection

• Permissive Overreach Scheme• Underreaching Direct Trip and Direct Transfer Trip Scheme

• Protection Design and Performance Verification• EMT simulation of various system conditions recommended for

the chosen protection scheme• See report for various case studies

Project Planning and Implementation

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• Location of Series Compensation – affects effectiveness, voltage profile, protections settings, future configuration, operation and maintenance Mid-Line Installation Line Ends Installation

• Modularity of Series Compensation for staged development• Design for Future Network Modifications• Operations and Maintenance Considerations

• FSC - majority of equipment used is already likely found in the system• TCSC – redundant power electronic modules allows replacement of faulty

modules• Operations and Reliability

• Remote control functionality

Design Studies

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• Steady State and Short Circuit analysis• Transient Stability Analysis

• Harmonics and Subsynchronous Frequency Scans to identify possible resonance issues

• Short-Term Transient Voltage and Switching Studies (EMTP type) to determine• Maximum energy on varistors• Maximum transient voltage and current on capacitors• TRV on circuit breakers• Required size of MOV and damping circuit components

• Small Signal Analysis to determine impact of series capacitor on current modes of oscillation

Concluding Remarks

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• Series Compensation used worldwide since 1950s• Series Compensation is a tried and true technology that

continues to grow in popularity as an effective means of resolving a number of network issues

• The risk of SSI is relatively low; however, the consequences of an SSI event can be significant. The risk and consequences must factor into series compensation design including controls and protection.

• The SSI phenomenon is well understood and effective mitigations measures are available

• Series Compensation should be included in the planners’ toolbox and considered as an available option.