Post on 19-Mar-2016
description
transcript
1
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
2
3
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
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
9
10
11
• 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
12
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
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 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
16
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 .
17
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
18
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 1:72Nominal rotor speed
16.67 rpm
Rotor radius 42 mMaximum slip range
+/- 30%
Capability curve of a 1.5 MW machine
19
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
20
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
21
a) Impact of Capability Curve on a) Impact of Capability Curve on System LossesSystem Losses
Installed Wind CapacityPenetration Level
Load
22
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
23
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%
24
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
25
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
26
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
27
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
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 : 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.
32
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
38