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V. Ajjarapu (vajjarap@iastate.edu ) Iowa State University

Reactive Power, Voltage Control and Voltage Stability Aspects of Wind Integration

to the Grid

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 Power System Stability

Rotor Angle Stability

Frequency Stability

Voltage Stability

Small Disturbance

Transient Stability

Short Term

Long Term

Large Disturbance

Small Disturbance

Short Term 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

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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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• 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)

FERC Order 661A

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Proposed WECC Low Voltage Ride-Through (LVRT) requirements for all generators1

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

Most grid codes now

require that wind power plants assist the grid in

maintaining or

regulating the system

voltage

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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

Source: http://www.windsimulators.co.uk/DFIG.htm

Grid side converter (GSC)Rotor Side Converter (RSC)

Grid

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Voltage Controller

• Monitors POI or remote bus

• PI control adjusts stator Qref signal from Verror

• Qmx/n

– CC (capability curve)– FERC

))95.0(tan(cos 1max

outputPQ

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 .

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Grid Side Reactive Power Boosting

Impact of Grid Side Reactive Boosting with (green) and without (red) Control

MVARBy 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

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Rated electrical power

1.5 MW

Rated generator power

1.3 MW

Rated stator voltage

575 V

Rotor to stator turns ratio

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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 1:72Nominal rotor speed

16.67 rpm

Rotor radius 42 mMaximum slip range

+/- 30%

Capability curve of a 1.5 MW machine

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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

Converter Sizing

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

Installed Wind CapacityPenetration Level

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 a) Impact of Capability Curve on System LossesSystem Losses

Installed Wind CapacityPenetration Level

Load

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b) Impact of Capability Curve on b) Impact of Capability Curve on Voltage Stability Margin Voltage Stability Margin

collapse actual MARGIN

actual actual

MW MW PVTMMW MW

TM Transfer Margin

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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

Power Transfer Margin at Different Penetration Levels (50 MVAr at 204 and 3008)

Base power transfer without wind is 13.5%

Max system penetration possible is 20-25%

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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

Security Assessment Methodology

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20% penetration at cut-in speed

Dynamic Performance Validation

3φ short Circuit at Bus 3001 , CCT 140 ms Operation Comparison

FERC +/- 0.95 CC

20% penetration at 15% output20% penetration at 100% output

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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

20% penetration at cut-in speed

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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

20% penetration at 15% output

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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/2007/W82.pdf

Voltage Secure Region of Operation (VSROp)

W1W2

W3

WINDGENERATION

0 POWERTRANSFER

BUSVOLTAGE

WINDVARIABILITY

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.

Methodology Flowchart

The assumed level of wind generation in the base case and wind variability that is to be studied.

The power flow data for the system under consideration.

The redispatch strategy for increase or decrease in wind generation.

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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.

Sample Test System

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

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