Reactive Power, Voltage Control and Voltage Stability Aspects of Wind Integration to the Grid

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• Reactive Power, Voltage Control and Voltage
  Stability Aspects of Wind Integration to the Grid

V. Ajjarapu (vajjarap_at_iastate.edu )
Iowa State University
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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

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IEEE/CIGRE View on Stability 1
Start Term - Long Term
Short 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
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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

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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

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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
  

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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

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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

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FERC Order 661A

• ZVRT
• ( Zero Voltage Ride Through)
• 2008 - present
• 3f 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)

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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
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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
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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
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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 and reactive power can be controlled
independently
Grid side converter (GSC)
Rotor Side Converter (RSC)
Grid
Source http//www.windsimulators.co.uk/DFIG.htm
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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

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Grid Side Reactive Power Boosting
MVAR
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
Impact of Grid Side Reactive Boosting with
(green) and without (red) Control
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Capability curve of a 1.5 MW machine
Rated electrical power 1.5 MW
Rated generator power 1.3 MW
Rated stator voltage 575 V
Rotor to stator turns ratio 3
Machine inertia 30 kgm2
Rotor inertia 610000 kgm2
Inductance mutual, stator, rotor 4.7351, 0.1107, 0.1193 p.u.
Resistance stator, rotor 0.0059, 0.0066 p.u.
Number of poles 3
Grid frequency 60 Hz
Gearbox ratio 172
Nominal rotor speed 16.67 rpm
Rotor radius 42 m
Maximum slip range /- 30
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Converter Sizing
Ptot p.u. Qtot p.u. slip Vrotor V Irotor A Vdc-link V Sconvert kVA
1 0.05 0.80 25.26 244 352 440 258.5
2 0.25 0.72 11.50 108 449 195 146.2
3 0.50 0.63 1.33 8 425 14 10.2
4 0.75 0.49 -9.28 97 428 175 125.4
5 1.00 0.37 -25.14 254 468 460 357.9
6 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
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
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b) Impact of Capability Curve on Voltage
Stability Margin
Transfer Margin
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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 20 25 30
0 15.1 15.3 17.1
33 17.1 20.6 18.5
66 19.5 22.5 19.4
100 18.1 13.5 Unstable
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

• 3f 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
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20 penetration at cut-in speed

• Cut-in (4 m/s)
• Q limits
• CC (0.72,-0.92)
• RPF (0.0, 0.0)
• 153 voltage
• RPF control
• unable to recover post fault
• PEC crowbar protection does not activate
• reactive injections during fault.
• Extended reactive capability stabilizes system

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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

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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/2
007/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
Redispatch strategy for increase or decrease in
wind generation.
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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.
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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
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Results Comparison of Redispatch
Strategies at Location 2
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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.

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VSROP for Large System
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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.

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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