1. Reactive Power, Voltage Control and Voltage Stability Aspects of Wind Integration to the Grid
V. Ajjarapu )
Iowa State University

2. 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

3. 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 August 2004

4. 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

5. 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

6. 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

7. 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

8. 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

11. 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)

12. 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

13. 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

14. 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

15. 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:

16. 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

17. 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

18. 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
kgm2
Inductance: mutual, stator, rotor
4.7351, , p.u.
Resistance: stator, rotor
0.0059, 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%

19. 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

20. 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

21. a) Impact of Capability Curve on System Losses

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

23. 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%

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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 :

30. 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.

31. 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.

32. 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.

33. Results: Comparison of Redispatch Strategies at Location 1

34. Results: Comparison of Redispatch Strategies at Location 2

35. 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.

36. VSROP for Large System

37. 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.

38. 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