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Voltage Control with Distributed Generators to Enhance Voltage Stability
Presenter: Fangxing (Fran) Li, Ph.D.Assistant Professor, University of Tennessee
Advanced Electricity Infrastructure Workshop
Global Climate & Energy Project
STANFORD UNIVERSITY, Nov. 1~2, 2007
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Assistant Professor, University of Tennessee
Other contributors: Tom Rizy, John Kueck, and Yan X u (ORNL)
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Outline• Introduction to voltage collapse
– What is it and what causes the problem– Prevention of voltage collapse using reactive power
compensation
• Current research, development and demonstration (RD&D) effort at ORNL/UT
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demonstration (RD&D) effort at ORNL/UT – Using DG to provide dynamic reactive power– System and control models– Testing & simulation results– Future Plans
• Summary & Challenges
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Voltage Collapse
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Voltage Collapse: what and why?
• Most of recent blackouts are related to voltage collapse.• A severe voltage depression without the system’s ability
to recover.• The key cause is inadequate reactive power (VAr or Q)
supplies.• The following factors are usually involved:
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• The following factors are usually involved:– High impedance– High load current– Insufficient generation of Q– Line outage– Line congestion
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Voltage Collapse: Technical (1)Sending
EndReceiving
EndE V XIQ
RIP
loss
loss
2
2
=∆
=∆
• R
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Real power and reactive power
0. 5
1
1. 5
2
PS
6
- 1
- 0. 5
0
0. 5
Q
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Transmitting Reactive Power
Reactive power cannot be effectively
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transmitted across long distances or through power transformers due to high
I2X losses.
It is an uphill battle!
Source: Max Chau, Chairman, EEI Transmission Planning & Operations Working Group, 2004
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Reactive power should be addressed locally.
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Source: Max Chau, Chairman, EEI Transmission Planning & Operations Working Group, 2004
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RVθ∠LNZ
I φ∠LDZRR jQP +
E
Voltage Collapse: Technical (2)
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Voltage Collapse: Technical (3)
• A single-line system
P+jQ
EVR+jX
Nose Points
P-V Curves
Var compensation
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P+jQQc
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Voltage Collapse: Vulnerability (1)
• Power systems are operated under much more stress than in the past– Generation pattern changed under competitive
markets
– Continuous growth of consumption in heavy load
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– Continuous growth of consumption in heavy load areas
– Transmission expansion does not keep pace with Gen and Loads
– Future concerns
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Voltage Collapse: Vulnerability (2)
0.8
0.9
1
1.1
Tra
nsm
issi
on C
apac
ity (
1989
= 1
) NPCC
WECC
FRCC
MAPP
MAAC
SERC
U.S. Total 5
TR
AN
SM
ISS
ION
IN
VE
ST
ME
NT
(b
illio
n 1
99
9-$
/ye
ar)
Transmission investment has been declining for three decades
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0.6
0.7
0.8
1989 1994 1999 2004 2009
Tra
nsm
issi
on C
apac
ity (
1989
= 1
)
U.S. Total
MAIN
SPP
ECAR
ERCOT
98054
0
1
2
3
4
1975 1980 1985 1990 1995 2000
TR
AN
SM
ISS
ION
IN
VE
ST
ME
NT
(b
illio
n 1
99
9-$
/ye
ar)
-$117 million/year
Transmission capacity relative to load has been declining in everyNERC region since 1982
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Common Solutions
• Load shedding (only for urgent needs)
• Reactive power compensation• switched capacitor banks (ability drops quickly
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• switched capacitor banks (when needed most)
• power electronics such as SVC• Additional local generation like DG (dynamic
Var injection; responses very fast !)
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Voltage collapse prevention
• A single-line system
P+jQ
EVR+jX
Qc
Nose Points
P-V Curves
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Qc=V2/Xc for capacitors.
Ability drops quickly when V is low.
Blue: no compensation (Q c=0)
Green: capacitor compensation
Red: constant Q c injection
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Dynamic Performance: voltage control Vo
ltage
With fast response
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Volta
ge
Time
With slow response
Disturbance occurs
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DG to support voltage• DOE has goal of more penetration of DG
• 20% by 2020
• Modern DGs are interconnected to the grid with power electronic interface
• With the right controls, PE can provide flexible, dynamic reactive power (Var) to support voltage
• We are exploring the possibility of using DG’s dyna mic Var capability
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• We are exploring the possibility of using DG’s dyna mic Var capability to help improve voltage stability.
• PV, Wind, fuel cells, microturbines
• Rotating machine interface• Synchronous condenser• Reciprocating-engine synchronous generator
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Current RD&D (Research, Development, and
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Development, and Demonstration) at ORNL/UT
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• Objective– evaluate the impact and benefits of DG on voltage collapse
• Approach– develop the model of ORNL’s X-10 power distribution system to
perform voltage collapse studies– develop dynamic models of different DG sources– explore impact of DG placement location
Objective and Approach
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– explore impact of DG placement location– determine the degree of voltage collapse protection provided by
DG– identify general engineering guidelines for the design and
operation to use DG to prevent voltage collapse
• Facility– Distributed Energy Communications and Controls (DECC)
Laboratory at ORNL serves as the test-bed since it is actually interfaced to the ORNL Campus distribution system.
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DECC Lab is Interfaced with the ORNL Distribution System
Circuit #4Other Circuit
3000 Substation C
ircuit #2 Other Circuit
ORNL Substation
161kV/13.8kV
13.8kV/2.4kV
TVA TransmissionSystem
Other Substations
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2.4kV/480V750kVATransformer
2.4kV/480V750kVATransformer
2.4kV/480V300kVATransformer
Loads
300kVar SCC
ircuit #4Other Circuit
Loads
Loads
Circuit #2 Other Circuit
Loads
Panel PP2
150A/88kVar Inverter
30kW Capstone Micro-turbine
Panel PP3Panel PP1
Induction Motor
DECC Lab
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DECC Lab LayoutPanel PP2
Transformer 4-4 Load Banks
Panel PP3 Transformer 2-3
Disconnect
Bldg. 3114
1000A Service
600A Service
door
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Inverter Test Area
Synchronous Condenser Test Area
PP2
PP3
Bay Area
N
door
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DG with Power Electronics Interface
• DG with an inverter (compensator) interface is connected in parallel at PCC (point of common coupling).
• PCC voltage is regulated by the inverter.
Load
ic vdc
vtis il
ic
vt il Cf
Ls Rsvs
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• By generating or consuming reactive power, the inverter regulates the PCC voltage.
Controller
switchingsignals
Lc
ic
vcvdcDE
Compensator
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Synchronous Condenser (SC)
• SC only generates or consumes reactive power.
• System voltage is controlled by increasing or
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increasing or decreasing the SC current.
• SC current is controlled by the SC excitation voltage.
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Control Diagrams for each DG TypeInverter• Inverter reference voltage vc*
is always in phase with vt, so that the inverter only provides reactive power.
• The magnitude of vc* is controlled according to the error of PCC voltage.
Inverter control diagram
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error of PCC voltage.Synchronous Condenser (SC)• 85 V** is the base excitation
voltage for the SC used in the system.
• Excitation voltage is controlled according to the error of the system voltage.
SC control diagram
**The level at which the SC (an overexcited synchronous motor) neither absorbs nor generates reactive power.
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Experimental Results (RL Load)
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Local Voltage Regulation on May 1, 2007
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Synchronous Motor Start
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Regulation with balanced sags Aug. 28, 2007
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Regulation with unbalanced sags 08/30/2007
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Motor Start Current (Ph a) Inverter Current
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Multiple DGs on the Same Circuit
• SC and inverter are installed in a system at ORNL.– Currently they are on
different circuit for testing
– Simulated on the
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– Simulated on the same circuit
• They control the same voltage.
• Inverter response time is faster than SC, therefore an integrated control is required.
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Integrated Control DiagramInverter• Faster response, but is
current is limited.• Responsible for the fast
changing voltage ripples.SC• Picks up what the
inverter is unable to.
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inverter is unable to.Integrated System• The overall power rating
of the combined Inverter/SC system is increased.
• Faster response for the integrated system.
• No competing between the inverter & SC.
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Simulation Results
Regulated voltage Inverter current SC current
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• Voltage is controlled at the desired level.• Inverter current is limited to 100A peak (70Arms).• SC and inverter share the reactive power required for
the voltage regulation.• The total power rating of the voltage regulation
devices is improved (greater than the inverter and SC individually)
Regulated voltage Inverter current SC current
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Micro-Turbine Available for FY08• 100kW Micro-turbine• Donated by SCE• Plan to wire up in FY08• Capable of limited voltage
& power factor correction– ±2% voltage regulation
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• Plan to connect to circuit #4– Panel PP2 has an available
350A CB
Elliott TA-100 Micro-Turbine
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New DG Technologies to be Added
• Photovoltaic (PV) Array– assess opportunity for
non-active power compensation
– 50kW PV Array to be added by end of the year
– Storage could be added
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– Storage could be added later to provide output when it is cloudy/rainy/night
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Summary• The key cause of voltage collapse is inadequate
reactive power supplies.
• DG, equipped with Var-capable power electronic device is an effective approach to provide dynamic reactive power.
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reactive power.
• ORNL/UT is carrying out research work on dispatching DG for reactive power compensation to better regulate voltage and enhance system voltage stability.
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Challenges
• Many utilities are still reluctant to DG interconnection because they are worried that DG may mess up their network and make them lose control of the grid. – IEEE Standard 1547 is restrictive in DG interconnection– Technical issues (e.g. protection) related to
interconnection need to be addressed.
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interconnection need to be addressed.
• Cost of dynamic Var from DG power electronic interface– Has been improved and still needs further improvement
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Questions and Answers
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Questions and Answers