Transcript Voltage Stability Simulation and Analysis

Reactive Power, Voltage
Control and Voltage Stability
Aspects of Wind Integration
to the Grid
V. Ajjarapu ([email protected] )
Iowa State University
1
Outline
• Basic Introduction
– Reactive power ; Voltage Stability ; PV curves
• FERC Order 661A
• Power Factor of +/- 95% at the point of interconnection ; Voltage
regulation capability ; Low Voltage Ride Through (LVRT) capability to
prevent tripping of wind turbines during voltage sag events
• Reactive Power Capability of DFIG
– Voltage security assessment and Penetrations levels
• Wind Variability on Voltage Stability
• Conclusions and Discussion
2
IEEE/CIGRE View on Stability 1
Power
System
Stability
Rotor
Angle
Stability
Small
Disturban
ce
Transient
Stability
Short Term
Frequency
Stability
Short
Term
Long
Term
Voltage
Stability
Large
Disturbance
Small
Disturbance
Start Term - Long Term
1. P. Kundur, J. Paserba, V. Ajjarapu , Andersson, G.; Bose, A.; Canizares, C.; Hatziargyriou, N.; Hill, D.; Stankovic, A.; Taylor,
C.; Van Cutsem, T.; Vittal, V “Definitions and Classification of Power System Stability “ IEEE/CIGRE Joint Task Force on
Stability Terms and Definitions , IEEE transactions on Power Systems, Volume 19, Issue 3, pp. 1387-1401 August 2004
3
Voltage Stability
• It refers to the ability of a power system
to maintain steady voltages at all buses
in the system after being subjected to a
disturbance.
• Instability may result in the form of a
progressive fall or rise of voltages of
some buses
4
Voltage Stability Cont…
• Possible outcomes of this instability :
– Loss of load in an area
– Tripping of lines and other elements
leading to cascading outages
• Loss of synchronism of some generators may
result from these outages or from operating
condition that violate field current limit
5
Voltage Stability Cont..
• Driving Force for Voltage instability (usually
loads):
– The power consumed by the loads is restored by
• Distribution Voltage regulators
• Tap-changing transformers
• Thermostats
– A run down situation causing voltage instability
occurs when the load dynamics attempt to restore
power consumption beyond the capability of the
transmission network and the connected
generation
6
Voltage Stability Cont..
• It involves : Small and Large
disturbance as well as Short Term and
Long Term time scales
– Short Term : Involves fast acting load
components
:
induction
motors,
Electronically controlled loads , HVDC
converters
• Short circuits near loads are important
7
Voltage Stability Cont..
– Long Term:
• Involves slow acting equipment:
– Tap changing transformers
– Thermostatically controlled loads
– Generator current limiters
• Instability is due to the loss of long-term
equilibrium
• In many cases static analysis can be used
• For timing of control Quasi-steady-state time
domain simulation is recommended
8
9
10
FERC Order 661A
• ZVRT
( Zero Voltage Ride Through)
– 2008 - present
• 3φ short of 0 V at POI for 0.15s
(9 cycles)
(Wind farms installed prior to Dec. 31, 2007
are allowed to trip off line in the case of a
fault depressing the voltage at the POI to
below 0.15 p.u., or 15 percent of nominal
voltage)
•
PF
• ± 0.95
(including dynamic voltage support, if
needed for safety and reliability)
11
Proposed WECC Low Voltage Ride-Through (LVRT)
requirements for all generators1
Most grid
codes now
require that
wind power
plants assist
the grid in
maintaining
or
regulating
the system
voltage
1. R. Zavadil, N. Miller, E. Mujadi, E. Cammand B. Kirby, “Queuing Up: Interconnecting Wind Generation into The Power
System” November/December 2007, IEEE Power and Energy Magazine
12
LVRT requirements of various National
Grid Codes2
DS: Distribution TS: Transmission
2. Florin Iov, Anca Daniela Hansen, Poul Sørensen, Nicolaos Antonio Cutululis ,”Mapping of grid faults and grid
codes” Risø-R-1617(EN), July 2007
13
Summary of ride-through capability for wind turbines2
2. Florin Iov, Anca Daniela Hansen, Poul Sørensen, Nicolaos Antonio Cutululis ,”Mapping of grid faults and grid
codes” Risø-R-1617(EN), July 2007
14
In general all generators which are coupled to the network
either with inverters or with synchronous generators are
capable of providing reactive power ( for Example Doubly Fed
Induction Generator)
In DFIG real
independently
Rotor Side Converter (RSC)
and
reactive
power
can
be
controlled
Grid side converter (GSC)
Grid
Source: http://www.windsimulators.co.uk/DFIG.htm
15
Voltage Controller
A voltage controller placed at the Point of Interconnect (POI) measures utility line voltage, compares it to
the desired level, and computes the amount of reactive power needed to bring the line voltage back to
the specified range .
• Monitors POI or
remote bus
• PI control adjusts
stator Qref signal from
Verror
• Qmx/n
– CC (capability curve)
– FERC
Qmax  Poutput tan(cos1 (0.95))
16
Grid Side Reactive
Power Boosting
By default the grid voltage is
controlled by the rotor-side
converter as long as this is not
blocked by the protection device
(i.e. crowbar), otherwise the grid
side converter takes over the
control of the voltage
MVAR
Impact of Grid Side Reactive Boosting
with (green) and without (red) Control
17
Capability curve of a 1.5 MW machine
Rated
electrical
power
Rated
generator
power
Rated stator
voltage
Rotor to stator
turns ratio
Machine
inertia
Rotor inertia
Inductance:
mutual, stator,
rotor
Resistance:
stator, rotor
Number
of
poles
Grid
frequency
Gearbox ratio
Nominal rotor
speed
Rotor radius
Maximum slip
range
1.5 MW
1.3 MW
575 V
3
30 kgm2
610000 kgm2
4.7351,
0.1107,
0.1193 p.u.
0.0059,
0.0066 p.u.
3
60 Hz
1:72
16.67 rpm
42 m
+/- 30%
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Converter Sizing
Ptot
[p.u.]
Qtot
[p.u.]
slip
[%]
Vrotor
[V]
1
0.05
0.80
25.26
244
2
0.25
0.72
11.50
3
0.50
0.63
4
0.75
5
6

Irotor
[A]
Vdc-link
[V]
Sconvert
[kVA]
352
440
258.5
108
449
195
146.2
1.33
8
425
14
10.2
0.49
-9.28
97
428
175
125.4
1.00
0.37
-25.14
254
468
460
357.9
1.00
0.33
-25.14
254
458
460
348.6
Maximum converter capacity is 28% of machine rating
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Impact of Capability Curve:
a) On System Loss b) On Voltage Stability Margin
A Sample Simulation Study
Penetration Level 
 Installed Wind Capacity
 Load
Various Wind Penetration Levels at
15, 20, 25 & 30% are simulated
At each penetration level, total wind
generation is simulated at 2, 15, 50 &
100% output
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a) Impact of Capability Curve on
System Losses
Percent Reduction in Losses
Percent Reduction in Losses
18
16
Penetration Level 
14
 Installed Wind Capacity
 Load
Penetration
Level
12
10
15%
20%
8
25%
6
30%
4
2
0
2%
15%
50%
100%
Plant Output
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b) Impact of Capability Curve on
Voltage Stability Margin
TM 
TM 
Transfer Margin
MWcollapse  MWactual
MWactual

PVMARGIN
MWactual
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Power Transfer Margin at Different
Penetration Levels (50 MVAr at 204 and 3008)
Base power transfer without wind is 13.5%
Penetration Level
Plant Output
0%
20%
25 %
30%
15.1
15.3
17.1
17.1
20.6
18.5
19.5
22.5
19.4
18.1
13.5
Unstable
33%
66%
100%
Max system penetration possible is 20-25%
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Security Assessment Methodology
 Develop peak load base case matrix:
 % Penetration of peak load (x)
 % Park output (y)
 Critical contingencies for case list
 n-1 outages
 Perform appropriate static analysis (PV)
 Identify weak buses

Voltage criteria limit


0.90 – 1.05 V p.u.
Max load is 5% below collapse point for cat. B (n-1)
 Add shunt compensation

Transfer Margin Limit
 Repeat for all % output (y) and % penetration (x)
levels
 Perform dynamic analysis
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Dynamic Performance Validation
3φ short Circuit at Bus 3001 , CCT 140 ms
 Operation Comparison
 FERC +/- 0.95
 CC
20% penetration at cut-in speed
20% penetration at 15% output
20% penetration at 100% output
25
20% penetration at cut-in speed


Cut-in (4 m/s)
Q limits




153 voltage
RPF control



CC (0.72,-0.92)
RPF (0.0, 0.0)
unable to recover post
fault
PEC crowbar protection
does not activate
 reactive injections
during fault.
Extended reactive
capability stabilizes
system
26
20% penetration at 15% output

Q limits



CC (0.70, -0.90)
RPF (0.08, -0.08)
CC control provides enhanced
post fault voltage response
 Reduced V overshoot /
ripple
 Increased reactive
consumption at plant 3005
27
20% penetration at 100% output

Q limits



CC (0.36, -0.69)
RPF (0.34, -0.34)
Near identical reactive
injections

voltage recovery at
bus 153
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Voltage Stability Assessment Incorporating
Wind Variability
Electricity generated from wind power can be highly
variable with several different timescales –
 hourly, daily, and seasonal periods
Increased regulation costs and operating reserves.
 Wind variations in the small time frame (~seconds) is very
small (~0.1%) for a large wind park. [1]
Static tools can be used to assess impact of wind variation
[1] Design and operation of power systems with large amounts of wind power , Report available Online :
http://www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf
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Voltage Secure Region of Operation
(VSROp)
For each PV curve the amount of wind generation is kept
constant and the load and generation is increased
according to a set loading and generation increase scenario
BUS
VOLTAGE
WIND
VARIABILITY
0
POWER
TRANSFER
W2
W1
W3
WIND
GENERATION
Redispatch strategy for increase or decrease
in wind generation.
Methodology Flowchart
The power flow data
for the system under
consideration.
The assumed level of wind
generation in the base case and wind
variability that is to be studied.
The redispatch strategy for
increase or decrease in
wind generation.
Sample Test System
 Two locations are chosen for adding
wind generation.
 Each wind unit is of size 800 MW.
 Two redispatch strategies are chosen
 Gen 101 and Gen 3011 [ remote
to load] (RED)
 Gen 206 and Gen 211 [ close to
load] (GREEN)
 Base case wind output is 560 MW.
Any change in wind power is
compensated by redispatch units
 Determine – minimum margin and
most restrictive contingency.
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Results:
Comparison of Redispatch Strategies at Location 1
Results:
Comparison of Redispatch Strategies at Location 2
Large System Implementation
• 5600 buses with 11 areas constitute the Study area
with 2 wind rich regions.
• Total base case load is 63,600 MW with 6500 MW
coming from Wind.
• With a given set of 50 critical contingencies the
minimum power transfer margin possible is 300 MW
• 3000 MW of wind is varied between 0 to 3000.
• To compensate for reduced wind additional units are
brought online to compensate for the loss of wind.
VSROP for Large System
Observations
 A larger power transfer margin available over the entire range
of variability with Capability Curve
 Leads to higher penetration levels
 This tool helps determine the wind level at which minimum
power transfer margin is obtained.
 This power level need not be at minimum wind or maximum
wind.
 The tool also provides the most restrictive contingency at
each wind level.
Conclusions
• As levels of wind penetration continue to increase the
responsibility of wind units to adequately substitute
conventional machines becomes a critical issue
– Recent advancement in wind turbine generator technology provides
control of reactive power even when the turbine is not turning. This
can provide continuous voltage regulation. A performance benefit ,
not possible with the conventional machines
– Wind generators can become distributed reactive sources.
Coordination of this reactive power is a challenging task
• The FERC order 661-A, gives general guidelines for
interconnecting wind parks, but for specific parks employing
DFIG units the restriction on power factor can be lifted
38