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Connecting Wind Power Plants to Weak Grids
Lessons learned from the analysis, design and connection of wind power plants
to weak electricity grids
Wind Industry Forum, 26 March 2015
Antonio Martinez | Manager, BoP Engineering APAC | Vestas Wind Systems A/S
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Connecting Wind Power Plants to Weak Grids, Vestas Wind Systems
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Agenda
1. Characteristics of a weak grid.
2. Weak grid challenges.
3. Power system study.
4. Wind Power Plant solutions.
5. Questions?
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Characteristics of a weak grid
Weak grid definition
 Short Circuit Ratio (SCR) < 3 and Xgrid/Rgrid ratio < 5;
 The SCR indicates the amount of power (Swpp) that can be accepted by the power
system without affecting the power quality (V, f, harmonics, flicker) at the PoC.
 Low grid inertia constant (H).
Where,
SCR = Smin/Swpp;
Smin = Minimum fault level at the WPP MV bus without the WPP [MVA];
Swpp = WPP rating [MW].
Rgrid
Grid Impedance
Wind Power Plant
(WPP)
WPP MV Bus
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Xgrid
Point of Connection
(PoC)
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Characteristics of a weak grid
Weak grid definition
 Both the fault level at the point of connection (PoC) and WPP MW rating determines if
the WPP connection will experience the power quality issues of a weak grid.
SCR vs Swpp
9.00
8.00
7.00
SCR
6.00
5.00
Smin=200 MVA
4.00
Smin=300 MVA
3.00
Smin=400 MVA
2.00
Weak Grid Boundary
1.00
0.00
0
50
100
150
200
Swpp (MW)
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250
Characteristics of a weak grid
Weak grid connections
 Large WPPs located in remote locations far from generation/load centers, and
interconnected to the power system using long transmission lines.
 GW of weak grid projects are expected from the global wind power market, including
Australia.
 Examples in Australia:
WPP
Musselroe
Collgar
Silverton (stage1)
Swpp (MW)
168
250
300
SCR
1.74*
2.65
1.24
*at Derby;
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Characteristics of a weak grid
Weak grid connections
SCR = 1.24
SCR = 1.74
Musselroe WPP
Silverton WPP
100km+
Transmission Line
to Norwood
250km+
Transmission Line
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Weak grid challenges
Weak grids present technical challenges to WPP connections.
Steady State Issues
 Voltage Stability if affected by both P and Q injected into the grid. PV and QV analysis
can be applied to determine the stability limits (critical V, max P, Q margins);
 WPP active power rating limited according to the PV stability limit and/or the Surge
Impedance Loading of the long radial transmission line;
 Grid continuous operating voltage range limits the reactive power capability of the WPP.
This becomes an issue with Q capability requirements from grid codes;
 Voltage change, overshoot, etc. limit the P and Q ramp rates. This becomes an issue
with P control and Q control requirements from grid codes;
 N-1 (element put of service) power system amplifies the weak grid issues by lowering
further the SCR.
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Weak grid challenges
WPP MW rating limitation
PV Curves
1.05
1
0.95
Note:
X=0.6 represents weaker grid
X=0.3 represents stronger grid
VS (pu)
0.9
0.85
X=0.6 pf=0.95; X/R= 10
X/R↓→Pmax↓
0.8
X=0.3; pf=0.95; X/R= 10
X=0.6 pf=0.95; X/R= 2
0.75
SCR↓→Pmax↓
X=0.3; pf=0.95; X/R= 2
0.7
0.65
Pmax = 0.6pu
Pmax = 1.2pu
0.6
0
0.5
1
1.5
P (pu)
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Weak grid challenges
Poor voltage regulation due to large dV for small dQ
 On the weaker grid 20% change in Q changes the grid voltage by 20%;
 On the stronger grid 20% change in Q changes the grid voltage by 7%.
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QV Curves
0.8
Note:
X=0.6 represents weaker grid
X=0.3 represents stronger grid
0.6
Slope~1 for
weak grid
0.4
Q (pu)
0.2
X=0.6; P=0.5; X/R= 10
0
-0.2
0.5
0.6
0.7
0.8
0.9
-0.6
-0.8
1
1.1
Slope~2.85 for
stronger grid
-0.4
X/R↓→Qmargin↓
1.2
X=0.3; P=0.5; X/R= 2
X=0.6; P=0.5; X/R= 2
Stronger grid has reactive power margin
Weaker grid has NO reactive power margin
-1
9
X=0.3; P=0.5; X/R= 10
Vs (pu)
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Weak grid challenges
Reduced Reactive Power Capability
200
Typical/Stronger Grid – Grid doesn’t
affect WPP reactive power capability
150
50
Required PQ
Capability
0
Q_PCC, V=0.90pu
-50
100
Q_PCC, V=1.00pu
-100
Q_PCC, V=1.10pu
-150
-200
-250
0
50
100
150
200
250
300
Active Power Output (MW)
Weak Grid – It doesn’t take much +/-Q for
the power system voltage to reach +/-10%.
The WTG continuous operating voltages
(typ. +/-10%) limits the WPP reactive power
capability.
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Reactive Capability (MVAR)
Reactive Capability (MVAR)
100
Required PQ
Capability
0
Q_PCC, V=0.90pu
-50
Q_PCC, V=1.00pu
Q_PCC, V=1.10pu
-100
-150
0
50
100
150
200
Active Power Output (MW)
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250
300
Weak grid challenges
Dynamic Issues
 Inability of the power system to absorb the reactive current injection during the fault may
cause the WPP to trip on the transient overvoltage during the fault recovery period;
 Fast and large voltage angle shifts can make it difficult for the WTG Phase Lock Loop
(PLL) to track the voltage angle correctly, which may create instability of WTG fast
current control loops;
 WTG LVRT control retriggering may produce reactive power swings and voltage
instability if the WPP control system and the WTG level control is not coordinated.
Coordination can be challenging due to large voltage difference between the PoC and the
WTG;
 Poorly damped FRT response due to low system inertia amongst other weak grid
contributors.
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Weak grid challenges
Grid Code Issues
 In general grid codes have been written under the assumption that WPP connect to
strong grids;
 Some grid code technical requirements for WPP have no benefit and may
adversely impact the stability of the grid. For weak grids these requirements
should be modified or not be binding;
 Steady state reactive power requirements. Asking for +/- 0.93 power factor, for example,
may not be possible in a weak grid without exceeding the grid normal operating voltage
range of +/-10%;
 Steady state P and Q (pf, V) control requirements. The P and Q ramp rates can not be
too fast in a weak grid without exceeding the voltage change or damping or settling time
requirements of the grid code.
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Weak grid challenges
Grid Code Issues
 FRT requirements.
 Too much reactive power/current injection during the fault may lead to voltage
instability or overvoltage tripping after the fault is cleared.
 The P recovery can not be too fast in a weak grid without exceeding the damping or
settling time requirements of the grid code. Ramping P to pre-fault value too fast
may also produce transient overvoltage, LVRT retriggering and trip WPP.
CAUTION!
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Power system study
Dynamic Simulation Considerations
 Use the right tools for the job! PSSE alone is not the right tool. Both PSCAD (or
equivalent EMT software) and PSSE software is required for weak grid studies;
 PSSE WTG models do not represent the fast inner current control loops of the power
electronics and therefore the transient stability representation in PSSE is optimistic;
 PSSE time steps are typically in milliseconds, but microsecond time steps are required
for the fast inner current control loops;
 PSSE can experience numerical instability with SCR<3 and hence hard for a simulation
to converge;
 Asymmetrical grid conditions are more accurately represented in PSCAD than PSSE.
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Power system study
Dynamic Simulation Considerations
 Detailed PSCAD model is required.
 SMIB model is not sufficient. A full grid model (use E-TRAN) is required to represent the
grid response accurately.
 Accurate representation/aggregation of the WPP collector network is required.
 Source Code Integrated (SCI) PSCAD models should be used for WTG and PPC.
 Site specific voltage/reactive control scheme is required.
 Manufacturer’s specific models for STATCOM, synchronous condensers, and other
reactive plant is required.
 Correct protection setting at various locations in grid
 The site specific parameter settings for WTG, PPC and all reactive plant derived from the
PSCAD study can then be used (as applicable) to setup the equivalent PSSE model.
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Wind Power Plant Solutions
Overview
 The solution is tailored for each WPP according to the grid code requirements and the
SCR at the PoC. As such the solution will be different from WPP to WPP.
 The WPP solution consists of a combination of the following.
 Power system studies in PSCAD;
 Coordinated WPP voltage control system;
 Site specific tuning of the WTG FRT response;
 Reactive plant. STATCOM, Synchronous condensers, cap banks, etc;
 WPP active power derating when the grid voltage goes outside the continuous
operating range;
 WTG transfomer tap selection;
 Substation transformer OLTC performance.
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Wind Power Plant Solutions
Coordinated WPP Control System
Typical WPP control
concept for weak grid:
 Power Plant Controller®
(PPC) is master controller
and STATCOM is the slave
controller for V control.
 The PPC sends Qref to
STATCOM.
 The PPC controls the cap
banks.
 Synchronous condenser is
left to control its own terminal
voltage.
 STATCOM is used for fast
dynamic voltage control
during and post fault.
 Capacitor banks plus WTG
Q support is mainly used for
steady state voltage control.
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 Standard synchronous
condenser AVR response
time is used.
 PPC Q control should use
a rise time according to
grid code or contingencies
analysis. .
 PPC controls the WTG P
dispatch.
Wind Power Plant Solutions
Tuning WTG FRT response
 During the fault the WTG reactive current injection is
limited to avoid overvoltage tripping on fault clearance
or voltage instability during the fault recovery period.
 The WTG active current injection ramp rate is reduced
to limit the voltage change and to allow enough time for
the STATCOM to stabilise the voltage during the fault
recovery period. No WTG LVRT control retriggering.
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Wind Power Plant Solutions
Reactive Plant
 STATCOM.
 Provides steady state and dynamic voltage regulation.
 STATCOM is used for fast dynamic voltage control during and post fault for a smooth
fault recovery.
 Synchronous Condenser.
 Provides steady state and dynamic voltage regulation.
 Used to increase the fault level and inertia, and to reduce the voltage angle shifts to
ensure the WTG stays “synchronised” for the FRT event.
 H as high as possible, H>3 secs; Xd” as low as possible <10%, Xd’ < 15%.
 Capacitor bank.
 Provides steady state voltage support.
 Typically under normal operation Q losses are compensated with 10% by STATCOM,
50% by cap bank, and the rest by Syncon.
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Wind Power Plant Solutions
Example - WPP - Overview
 SCR at PoC is 1.7.
 Reactive plant:
 3× 5 MVAr STATCOMs
 5× 9 MVAr cap banks
 1× 20 MVA Syncon
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Wind Power Plant Solutions
Example – WPP – Voltage angle shift issue
 Large and fast voltage angle
shift can result in pole slip
of synchronous machines
including the syncon and
WTG PLL controller
instability.
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Reverse power and angle shift→
pole slip
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Wind Power Plant Solutions
Example – WPP – Voltage angle shift solution
 Increase the inertia for the
synchronous condenser to
reduce the angle shift. The
inertia constant (H)
increased from 3 to 3.93 s
 Within the timeframe before
pole slip, P can be reduced
by advancing the WTG
LVRT control activation
voltage to 0.89 pu (default is
0.85 pu)
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angle shift limited to
~30degrees→ no pole slip
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Thank you for your attention. Questions?
Vestas WPP solutions can be connected to a weak
grid and successfully comply with the grid code.
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