Review of Series
Compensation
Douglas Bowman, P.E.
Research, Development, and
Special Studies
Date: May 20-21, 2014
Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Series Compensation
Series Compensation Types
Subsynchronous Interactions (SSI) - Terms
Fundamentals of SSI and Series Compensation
Forms of SSI
SSI and Series Compensation
Tools for Assessment of SSI in Series Compensated Networks
SSI Mitigation Measures
SSI Protection Measures
Protective Relay Considerations for Series Compensated Networks
Protective Relay Solutions for Series Compensated Networks
Project Planning for Implementation
Design Studies
Concluding Remarks
2
Series Compensation
1. Increases power transfer
capability
2. Improves transient
performance
3. Improves reactive power
balance
4. Improves Voltage Stability
5. Improves power flow
balance on adjacent lines
6. Deferral of major transmission investments
7. Preservation of existing rights of way
Benefits of Series Compensation
3
Series Compensation
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 capacitor
Example: For XL = 1 ohm, 30% compensation produces XL - XC = .7 ohm
Increases Power Transfer Capability
4
Series Compensation
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
Improves Transient Performance
Following Disturbances
5
Series Compensation
Transmission Line Reactive
Power Losses :
Qlosses=I2Xline
Reactive Power Balance For A 300 Mile 500kV Line
Series Capacitor Reactive Power
Output:
Qoutput=I2Xcapacitor
As a transfer across the line
increases, Qoutput partially offset
Qlosses
Improves Reactive Power Balance and
Self-Regulation
6
Series Compensation
Increasing compensation levels K
provides greater Qoutput capability
Effect of Increasing Compensation Levels
Maximum power transfer
capability of the line is increased
Generator reactive power is
made available for voltage
control
Improves Voltage Stability
7
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
Fixed Series Compensation (FSC)
8
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
Series Capacitor
Series Capacitor
MOV
Reactor
Damping
Reactor
Thyristors
MOV
FPD
Damping
Reactor
Bypass Switch
Fixed Series Compensation
FPD
Bypass Switch
Thyristor Controlled Series Compensation
•
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
Thyristor Controlled Series Compensation(TCSC)
9
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)
10
Fundamentals of SSI and Series Compensation
•
•
•
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
11
Forms of SSI
*
* Interaction with series compensation does not occur during SSTI
12
SSI and Series Compensation
•
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.
13
SSI and Series Compensation
•
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
14
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
•
Frequency Scan Screening
•
•
•
Eigenvalue Analysis
•
•
•
•
System model linearized, small pertubations examined
Identifies torsional mode damping characteristics
Used to study SSR-TI and SSCI problems
Damping Torque Analysis
•
•
•
•
Calculates apparent impedance from generator from 0 to 60 hz
Can identify potential IGE, SSR-TI, SSR-TA, and SSCI problems
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
16
SSI Mitigation Measures (SSI Prevention)
•
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)
17
SSI Mitigation Measures (SSI Prevention)
•
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
18
SSI Protection Measures (SSI Detected)
•
•
•
Series Capacitor Bypass
Newer relays developed for SSCI since 2009
Generator Relays
Relay
Signal Input Comments
Torsional 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. California
Edison patent
Terminal
voltage
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 Research
Ltd. patent
Generator
Terminal voltage
Micro-processor based relay developed in the 2011 timeframe.
ERLPhase
Power
Technologies
Generator
Terminal 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
Application
Innovation
Armature
current
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
19
Protective Relay Considerations for Series
Compensated Networks
•
•
•
•
•
•
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
20
Protective Relay Solutions for Series
Compensated Networks
•
Advanced Relays for Series Compensation
Application
•
•
•
•
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
•
•
Memory Polarization
Special Series Compensation Logic
Sequence Component Impedance for Directional
Discrimination
EMT simulation of various system conditions recommended for
the chosen protection scheme
See report for various case studies
21
Project Planning and Implementation
•
Location of Series Compensation – affects effectiveness,
voltage profile, protections settings, future configuration,
operation and maintenance
Mid-Line Installation
•
•
•
Modularity of Series Compensation for staged development
Design for Future Network Modifications
Operations and Maintenance Considerations
•
•
•
Line Ends Installation
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
22
Design Studies
•
•
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
23
Concluding Remarks
•
•
•
•
•
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.
24