Grid integration to weak grids

7/17/2019 Grid integration to weak grids

http://slidepdf.com/reader/full/grid-integration-to-weak-grids 1/23

Connecting Wind Power Plants to Weak GridsLessons learned from the analysis, design and connection of wind power pl

to weak electricity grids

Wind Industry Forum, 26 March 2015 Antonio Martinez | Manager, BoP Engineering APAC | Vestas Wind Systems A/S

Connecting Wind Power Plants to Weak Grids, Vestas Wind Systems1

PUBLIC

PUBLIC



2

Agenda

1. Characteristics of a weak grid.

2. Weak grid challenges.

3. Power system study.

4. Wind Power Plant solutions.

5. Questions?

Connecting Wind Power Plants to Weak Grids, Vestas Wind Systems PUBLIC



3

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 powesystem 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




4


Weak grid definition

Both the fault level at the point of connection (PoC) and WPP MW rating determin

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

S C R

Swpp (MW)

SCR vs Swpp

Smin=200 MVA

Smin=300 MVA

Smin=400 MVA

Weak Grid Boundary




5


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, includi

Australia.

Examples in Australia:

*at Derby;

WPP Swpp (MW) SCR

Musselroe 168 1.74*

Collgar 250 2.65

Silverton (stage1) 300 1.24




6


Weak grid connections

250km+

Transmission Line

Silverton WPPMusselroe WPP

100km+

Transmission Lin

to Norwood

SCR = 1.24 SCR = 1.74




7

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 anal

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

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 iss

with P control and Q control requirements from grid codes;

N-1 (element put of service) power system amplifies the weak grid issues by lowe

further the SCR.




8


WPP MW rating limitation

Connecting Wind Power Plants to Weak Grids, Vestas Wind Systems

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

V S ( 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 = 1.2puPmax = 0.6pu

SCR↓→Pmax↓

X/R↓→Pmax↓

Note:

X=0.6 represents weaker grid

X=0.3 represents stronger grid

PUBLIC



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 1.2 Q

( 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

9



Note:

X=0.6 represents weaker grid

X=0.3 represents stronger gridSlope~1 for

weak grid

Slope~2.85 for

stronger grid

Stronger grid has reactive power margin

Weaker grid has NO reactive power margin

X/R↓→Qmargin↓

PUBLIC



10


Reduced Reactive Power Capability


-250

-200

-150

-100

-50

0

50

100

150

200

0 50 100 150 200 250 300

R e a c t i v e C a p a b i l i t y ( M V A R )

Active Power Output (MW)

Required PQCapability

Q_PCC, V=0.90pu

Q_PCC, V=1.00pu

Q_PCC, V=1.10pu

-150

-100

-50

0

50

100

0 50 100 150 200 250 30

R e a c t i v e C a p a b i l i t y ( M V A R )

Active Power Output (MW)

Typical/Stronger Grid – Grid doesn

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 powercapability.

PUBLIC



11


Dynamic Issues

Inability of the power system to absorb the reactive current injection during the fau

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 L

(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

WTG;

Poorly damped FRT response due to low system inertia amongst other weak grid

contributors.




12


Grid Code Issues

In general grid codes have been written under the assumption that WPP connect t

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 exa

may not be possible in a weak grid without exceeding the grid normal operating vo

range of +/-10%;

Steady state P and Q (pf, V) control requirements. The P and Q ramp rates can no

too fast in a weak grid without exceeding the voltage change or damping or settlin

requirements of the grid code.




13


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 damp

settling time requirements of the grid code. Ramping P to pre-fault value too

may also produce transient overvoltage, LVRT retriggering and trip WPP.


CAUTION!

PUBLIC



14

Power system study

Dynamic Simulation Considerations

Use the right tools for the job! PSSE alone is not the right tool. Both PSCAD (o

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 pow

electronics and therefore the transient stability representation in PSSE is optimisti

PSSE time steps are typically in milliseconds, but microsecond time steps are req

for the fast inner current control loops;

PSSE can experience numerical instability with SCR<3 and hence hard for a simuto converge;

Asymmetrical grid conditions are more accurately represented in PSCAD than PS




15

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 represegrid 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 othe

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 fr

PSCAD study can then be used (as applicable) to setup the equivalent PSSE mod




16

Wind Power Plant Solutions

Overview

The solution is tailored for each WPP according to the grid code requirements and

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 continuou

operating range;

WTG transfomer tap selection;

Substation transformer OLTC performance.




17


Coordinated WPP Control System


Typical WPP control

concept for weak grid:

Power Plant Controller ®

(PPC) is master controllerand 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 terminalvoltage.

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 sy

condenser Atime is used

PPC Q contr

a rise time agrid code or

analysis. .

PPC controls

dispatch.

PUBLIC



18


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.

PUBLIC



19


Reactive Plant

STATCOM.

Provides steady state and dynamic voltage regulation.

STATCOM is used for fast dynamic voltage control during and post fault for a

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 sh

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 STATCO

50% by cap bank, and the rest by Syncon.




20


Example - WPP - Overview

SCR at PoC is 1.7.


Reactive plant:

3× 5 MVAr STATCOMs

5× 9 MVAr cap banks

1× 20 MVA Syncon

PUBLIC

Wi d P Pl S l i



21


Example – WPP – Voltage angle shift issue


Large and fast voltage angleshift can result in pole slip

of synchronous machines

including the syncon and

WTG PLL controller

instability. Reverse power and angle shift

pole slip

PUBLIC

Wi d P Pl t S l ti



22


Example – WPP – Voltage angle shift solution


Increase the inertia for thesynchronous 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)

PUBLIC

angle shift limited to

~30degrees→ no pole slip



Thank you for your attention. Questions?

Vestas WPP solutions can be connected to a weak

grid and successfully comply with the grid code. © Vestas Wind Systems A/S. All rights reserved.

This document was created by Vestas Wind Systems A/S on behalf of the Vestas Group and contains copyrighted material, trademarks and other proprietary information. This document or p arts thereof may not be reprod

copied in any form or by any means without the prior written permission of Vestas Wind Systems A/S. All specifications are fo r information only and are subject to change without notice. The use of this document by you,

authorized by you, is prohibited unless specifically permitted by Vestas Wind Systems A/S. You may not alter or remove any tr ademark, copyright or other notice from the documents. The document is provided “as is” an

Systems A/S shall not have any responsibility or liability whatsoever for the results of use of the document by you. Vestas W ind Systems A/S does not make any representations or extend any warranties, expressed or im

adequacy or accuracy of this information. Certain technical options, services and wind turbine models may not be available in all locations/countries.

PUBLIC

Documents

Grid integration to weak grids