Reactive Power, Voltage Control and Voltage Stability Aspects of Wind Integration to the Grid V. Ajjarapu (vajjarap@iastate.edu ) Iowa State University

Outline Basic Introduction FERC Order 661A 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

IEEE/CIGRE View on Stability 1 Power System Stability Rotor Angle Stability Frequency Voltage Small Disturbance Transient Short Term Long Large Disturbance Small Disturbance 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

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

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

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

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

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

FERC Order 661A PF 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)

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

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

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

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

Voltage Controller Monitors POI or remote bus 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

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

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 Grid frequency 60 Hz Gearbox ratio 1:72 Nominal rotor speed 16.67 rpm Rotor radius 42 m Maximum slip range +/- 30%

Converter Sizing Ptot [p.u.] Qtot 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 0.33 458 348.6 Maximum converter capacity is 28% of machine rating

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

a) Impact of Capability Curve on System Losses

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

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% 20.6 18.5 66% 19.5 22.5 19.4 100% 18.1 13.5 Unstable Max system penetration possible is 20-25%

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

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

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

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

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

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) 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 POWER TRANSFER BUS VOLTAGE W2 W1 W3 WIND VARIABILITY The new voltage security assessment tool we propose here is called voltage secure region of operation or VSROP. It is similar to standard PV curves, except that we add a 3rd axis to it, i.e. the z axis on which we put the non-dispatchable wind generation. The surface incorporates different levels of wind generation by representing different PV curves at different wind generation levels to obtain a three dimensional region of voltage secure operation. Given the current amount of wind generation in the system, one would need to know for the next few hours, how much could the wind vary. ================================= A P-V surface for secure operation called the Voltage Secure Region of Operation (VSROp) is proposed. 3 axes: x axis: Existing power generation y axis: Per unit voltage z axis: Non-dispatchable wind generation 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. Step 1: Obtain Input Data The power flow data for the system under consideration The power flow data includes the committed generations and their bid curves, the load increase and generation increase directions. The generation increase scenario is provided for all other generations except wind. 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. Step 2: Optimal Power Flow in the base case Optimal power flow (OPF) is a well developed tool and standard procedure in power system planning and operation. The objective of this OPF is to minimize system costs while adhering to operation constraints such as line flow, generation, and bus voltage limitations. Step 3: Full Contingency based Margin Estimation For a fixed wind energy dispatch, plot the PV curves using powerflow for all (n-1) contingencies and store the one corresponding to the least power transfer margin is stored The series of PV curves on different planes corresponding to a particular wind penetration level will constitute a hyperspace which will represent the stable voltage operating zone The base case dispatch is then utilized to estimate the least available margin in the PV surface. Step 4: Margin Check and Remedial Action The margin obtained in Step 3 is verified to meet the power margin requirements. If the margin requirements are not met then remedial actions are taken to increase the margin and the modified load flow data is fed into step 1 and the entire process is iteratively repeated until the desired margin is obtained.

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.

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