Experiences with Wind Power Plants with Low SCR

Lessons learned from the analysis, design and connection of wind power plantsto weak electrical grids

IEEE PES General Meeting – Denver CO, 26 March 2015Vestas Wind Systems A/S: Philip Carne Kjaer, Manoj Gupta, Antonio Martinez, Steven Saylors

Experiences with Wind Power Plants with Low SCR

1

Agenda

2

1. Characteristics of a weak grid.

2. Weak grid challenges.

3. Power system study.

4. Wind Power Plant solutions.

5. Questions?

Characteristics of a weak grid3

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].

Wind Power Plant (WPP)

WPP MV Bus Point of Connection(PoC)

Rgrid Xgrid

Grid Impedance


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.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0 50 100 150 200 250

SCR

Swpp (MW)

SCR vs Swpp

Smin=200 MVA

Smin=300 MVA

Smin=400 MVA

Weak Grid Boundary


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:

*at Derby

WPP Swpp (MW) SCRMusselroe 168 1.74*

Collgar 250 2.65Silverton (stage1) 300 1.24


Weak grid connections

250km+ Transmission Line

Silverton WPPMusselroe WPP

100km+ Transmission Line to Norwood

SCR = 1.24 SCR = 1.74

Weak grid challenges7

Weak grids present technical challenges to WPP connections

Steady State Issues Voltage Stability is 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 being 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 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.


WPP MW rating limitation

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

0 0.5 1 1.5 2

VS (p

u)

P (pu)

PV Curves

X=0.6 pf=0.95; X/R= 10

X=0.3; pf=0.95; X/R= 10

X=0.6 pf=0.95; X/R= 2

X=0.3; pf=0.95; X/R= 2

Pmax = 1.2pu Pmax = 0.6pu

SCR↓→Pmax↓

X/R↓→Pmax↓

Note:X=0.6 represents weaker gridX=0.3 represents stronger grid

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%.

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2Q (p

u) Vs (pu)

QV Curves

X=0.3; P=0.5; X/R= 10

X=0.6; P=0.5; X/R= 10

X=0.3; P=0.5; X/R= 2

X=0.6; P=0.5; X/R= 2


Note:X=0.6 represents weaker gridX=0.3 represents stronger grid

Slope~1 for weak grid

Slope~2.85 for stronger grid

Stronger grid has reactive power marginWeaker grid has NO reactive power marginX/R↓→Qmargin↓

Weak grid challenges

-250

-200

-150

-100

-50

0

50

100

150

200

0 50 100 150 200 250 300

Rea

ctiv

e C

apab

ility

(MV

AR

)

Active Power Output (MW)

Required PQCapability

Q_PCC, V=0.90pu

Q_PCC, V=1.00pu

Q_PCC, V=1.10pu

10

Reduced Reactive Power Capability

-150

-100

-50

0

50

100

0 50 100 150 200 250 300

Rea

ctiv

e C

apab

ility

(MV

AR

)

Active Power Output (MW)

Required PQCapability

Q_PCC, V=0.90pu

Q_PCC, V=1.00pu

Q_PCC, V=1.10pu

Typical/Stronger Grid – Grid doesn’t affect WPP reactive power capability

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.


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.


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, damping, or settling time requirements of the grid code.


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!

Power system study14

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.

Power system study15

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 equipment is required.

Correct protection settings at various locations in grid

The site specific parameter settings for WTG, PPC and all reactive plant equipment derived from the PSCAD study can then be used (as applicable) to setup the equivalent PSSE model.

Wind Power Plant Solutions16

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 equipment: STATCOM, synchronous condensers, cap/reactor banks, etc;

WPP active power derating when the grid voltage goes outside the continuous operating range;

WTG transformer tap selection;

Substation transformer OLTC performance.


Coordinated WPP Control System

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.

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.

Typical WPP control concept for weak grid:


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 can be reducedto 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.


Reactive Plant Equipment 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 synchronous condenser.


Example - WPP -Overview

SCR at PoC (Port Hardy) is 1.64, and 1.28 at MV bus

Reactive Plant Equipment:

3× 5 MVAr STATCOMs

5× 9 MVAr Cap Banks

1× 20 MVA Synchronous Condenser

BCTC: Vancouver Island regional system21


Example – WPP – Voltage Angle Shift issue

Large and fast voltage angle shift can result in pole slip of synchronous machines, including the synchronous condenser, and create WTG PLL controller instability. Reverse power and angle shift→

pole slip


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)

angle shift limited to ~30degrees→ no pole slip

WPP solutions can be connected to a weak grid and successfully comply with the grid code.

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Experiences with Wind Power Plants with Low SCR