3 wind and solar plant performance

Power Systems & Energy Course:
Wind and Solar Plant Performance

Jason MacDowell


Advanced

Higher functionality is required for:
a) Bigger farms
b) Weaker systems
c) Grids with higher wind penetration

Basic

Performance Requirements

Grid Requirements Evolution

Protection

Volt/VAR
Control

LVRT

Primary Frequency Fast Frequency
Response
Response

Application Characteristics

Single WTGs
Low Penetration

Large Farms

Multiple Farms
High Penetration

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2/


Load & Wind Measurements
July 17, 2006

30,000.

200.

Load
Wind

25,000.

150.

15,000.

100.

MW Wind

MW Load

20,000.

Feeder circuit trips

10,000.

50.
5,000.

0.
00:01:00

0.
04:01:00

08:01:00

12:01:00

16:01:00

20:01:00

Time
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3/


Interconnection Issues – Dynamic Performance
• Voltage Regulation

• Dynamic voltage response
• Flicker
• Fault Tolerance/Low-Voltage Ride-Through
• Stability
• Maintaining Synchronism
• Damping

• Voltage Stability
• Active Power Control
• Frequency Regulation
• Intertie Flow Regulation

• Unit commitment

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

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Grid Friendly Wind Power Plant

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5/


Wind Turbines and Reactive Power Control


Reactive Power…Voltage control
Current leads
voltage or
“overexcited”

Lightly Loaded
Overhead Lines

VIDEO

System Load Voltage

Cable Circuits

Generators
Capacitive
Compensators
Flow from
Other Areas
Generators
Consumer
Loads

The Reactive

Power Tank

Transformers
Inductive
Compensation
Heavily Loaded
Overhead Lines

The Sources and Sinks of Reactive Power
The Reactive Power Balance must be struck on a local basis
Courtesy of National Grid Co, UK

Flow to Other Areas

Current lags
voltage or
“underexcited”


Reactive/Voltage Requirement Variations
• Fixed power factor

Q

P

• Power factor range (permissive)

Q

Permissive
Range

P

• Dispatched reactive or pf, within pf range
Q

• Voltage regulation, within pf range
– May regulate local or remote bus
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

Required
Range

P

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GE 1.5 MW Reactive Capability

•
•
•
•

Full leading and lagging range over full power range
Faster reactive response than synch. generator
Capability of reactive compensation with no wind

No need for FACTS devices

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.


WTG Reactive Power Capability
Reactive Power for Voltage Support
•
•
•
•

Steady-state PF range - 0.90 under-excited/0.90 over-excited
Dynamic range meets or exceeds steady-state range
WTG reactive capability often sufficient to satisfy PF
requirements at POI
VAR capability reduced at low power due to units cycling off-line
Rating Point

Terminal Bus
P gen

WTG

Active Power

Q gen

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

Voltage Regulation
Hierarchical Control Philosophy

Individual WTGs have fast, autonomous, self-protecting regulation of
their terminal voltages
• Individual WTGs will always respond rapidly and correctly
for grid voltage events
WindCONTROL provides plant-level controls to meet performance
requirements (e.g., voltage regulation) at the point-of-interconnection
(POI)
• Sends supervisory reactive power commands to individual
WTGs to ‘trim up’ initial individual WTG response

• Coordinates other substation equipment (e.g., switched
shunt capacitors)
• Interfaces with utility SCADA
• Accepts commands (e.g., voltage reference setpoint) from
utility system operator
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.


WindCONTROL

Substation
HV Bus

Plant Level Control System
• Coordinated turbine and
plant supervisory control

structure
• Voltage, VAR, & PF control
• PF requirements primarily
met by WTG reactive
capability, but augmented
by mechanically switched
shunt devices if necessary
• Combined plant response
eliminates need for SVC,
STATCOM, or other expensive
equipment
• Integrated with substation
SCADA

QWP PWP

Point of Interconnection
(POI)
LTC

LV Bus
QL

Reactive
Power
Controller

QC

Reactive Compensation

(if required)

PWTG

PWTG

QWTG

QWTG

PWTG

PWTG

QWTG

QWTG

PWTG

PWTG

QWTG
© 2016 General Electric International, Inc. All rights reserved. Not for distribution
without permission.

QWTG


Voltage & Reactive Power Controls

• Regulates Grid Voltage at
Point of Interconnection

Actual measurements from a
162MW wind plant
Voltage at POI

Wind Plant Voltage

• Minimizes Grid Voltage
Fluctuations Even Under
Varying Wind Conditions
• Regulates Total Wind Plant
Active and Reactive Power
through Control of Individual
Turbines

Wind Plant Power Output

Average Wind Speed

Voltage and Reactive Power Regulation
Like A Conventional Power Plant
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.


Power Factor Specification
• Typical power factor requirement variations:
– Unity
– 0.95 lagging to 0.95 leading (will use the terms “over-excited and

under-excited to avoid ambiguities)

• Even where voltage regulation is required, a pf or reactive
range is specified

• Difficult to achieve some seemingly simple pf requirements
– Maintain pf range down to low power
– Maintain constant pf
– Each of the above can require additional compensation equipment

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

14 /


UK Grid
Code
A well thought-out code

Must supply
or absorb
reactive over
this entire
range

Permissive
Zone

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15 /


Wind Plant vs. Wind Turbine Reactive Capabilities
Wind Plant pf capability  wind turbine pf spec

Reactive Gains

Reactive Losses
• I2X of unit transformer
• I2X of collector lines and
cables
• I2X of substation transformer
• V2BL of shunt reactors
• QL of dynamic compensator

•
•
•
•

V2BC of collector cables
V2BC of harmonic filters
V2BC of shunt cap banks
QC of dynamic compensator

Extra compensation provided to make up the difference
• Switched caps and reactors all step-wise compensation
• Dynamic compensation needed for smooth control unless WTG has
variable reactive capability

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16 /


Steady-State Reactive / Voltage Analysis Inputs
• Reactive characteristics of WTG
– As a function of power
– As a function of voltage at WTG terminals

• Voltage range of point-of-interconnection
– Reactive or pf spec may vary with voltage
(see next slide)

• Operating voltage range of WTG
• Topology of collector system
• Impedances of substation and unit transformers,
collector cables, HV line

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

17 /


Voltage-Dependent Power Factor Spec

Type of Requirements

Main transformer operation

Capacitor
compensation

Nominal voltage

Fixed tap

unusual

2-corner

Likely to work with fix tap

possible

4-corner

OLTC likely required

possible

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18 /


Voltage-Dependent Power Factor Spec
Assumes less reactive power
injection when grid voltage
is above nominal

1.1500

[p.u.]

Requirement

1.1000

Performance inside
the plant (fixed
1.0000 tap)
WTGs at min
reactive power
0.9500

DIgSILENT

1.0500

1.1500

[p.u.]

Terminal voltage above 1.1 pu

1.1000

1.0500

WTGs at max
reactive power

1.0000

Terminal voltage
below 0.9 pu
0.9000
-75.000

0.9500

0.9000
-75.000

-45.000
-15.000
45.000
-45.000
-15.000
15.000
45.000
[Mvar] 15.000
75.000
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm V Wind
Q Diagram
(EN):
Reactive
reference

(at Pn) in Mvar / Voltage at PCC in p.u.
Farm V Q Diagram
(EN): Reactive
pow er at PCCpow
in Mvar er
/ Voltage
at PCC in p.u.
Wind Farm V Q Diagram (EN): Total Reactive Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.
Date: 8/19/2014
Wind Farm V Q Diagram (EN): Total Reactive Pow er of All Wind TurbinesV-QatQ_WTG
LV Level
in Mvar / Voltage at PCC in p.u. 19 /
Annex: /5


POI Reactive Power Range: effect of load level
1.1500

[p.u.]

• Higher I2X at high power
• WTG reactive power range can
be:
– function of active power
– function of voltage at WTG
terminals
1.1000

1.0500

[p.u.]

1.0000

1.1500

DIgSILENT

1.0 pu active power
DIgSILENT

0.5pu active power
1.1500

[p.u.]

1.1000

1.1000

1.0500

1.0500

0.9500
1.0000

1.0000

0.9500

0.9500

0.9000
-75.000

0.9000
-75.000

0.9000

[Mvar]
75.000
-75.000
-45.000
-15.000
15.000
45.000
[Mvar]
75.000
-45.000
-15.000
15.000
45.000
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn)
in Mvar / Voltage at PCC in p.u.

Wind Farm V Q Diagram (EN): Total Reactive Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
V-Q Q_WTG Date: 8/19/2014
V-Q Q_WTG Date: 8/19/2014
Wind Farm V Q Diagram (EN): Total
Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.
Annex: Reactive
/5
Annex: /5

-45.000
-15.000
15.000
45.000
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Total Reactive Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

/
V-Q20Q_WTG


POI Reactive Power Range: effect of compensation
1.1500

[p.u.]

• Capacitors usually not required

1.1000

• If required, typically at 34.5kV substation
1.0500

• Capacitors can be controlled with WC
DIgSILENT

1.1500

[p.u.]

1.0000

DIgSILENT

With capacitors

Without capacitors
1.1500

[p.u.]

1.1000
1.1000

1.0500

1.0500

0.9500
1.0000

1.0000

0.9500

0.9500

0.9000
-75.000

0.9000
-45.000
-75.000

0.9000
[Mvar]
75.000
-45.000
-15.000
15.000
45.000
-75.000
-45.000
-15.000
15.000
45.000
[Mvar]
75.000 [Mv

Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm
Q Diagram (EN):
Reactive pow er at
in Mvar / in
Voltage
at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn)
in VMvar
/ Voltage
atPCCPCC
p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
V-Q Q_WTG Date: 8/19/2014
V-Q Q_WTG Date: 8/19/2014
Annex: /5
Wind Farm V Q Diagram (EN): Total
Reactive
Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.
Annex: /5

-15.000
15.000
45.000
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Total Reactive Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.

© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

Figure 5-1: VQ curve with neutral tap positions at 0.5pu active power

21 /

V-Q Q_WTG

D


POI Reactive Power Range: effect of OLTC
1.1500

[p.u.]

• On Load Tap Changer (OLTC) on if really needed
• Considered for “4-corner” requirements
• Wide voltage control band preferred to reduce
operations
1.1000

1.0500

1.0000

1.1500

DIgSILENT

1.1500

[p.u.]

OLTC on main
transformer
DIgSILENT

Fixed tap on main
transformer
[p.u.]

1.1000

1.1000

1.0500

1.0500

0.9500
1.0000

1.0000

0.9500

0.9500

0.9000
-75.000

0.9000
-45.000
-75.000

0.9000

-75.000
-45.000
-15.000
15.000
45.000 45.000
[Mvar]
75.000
[Mvar]
75.000
-45.000
-15.000
15.000
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Wind
Pn)FarminV QMvar
/ Voltage at PCC in p.u.
Diagram (EN): Total Reactive Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
V-Q Q_WTG Date: 8/19/2014
V-Q Q_WTG Date: 8/19/2014
Wind Farm V Q Diagram (EN): Total
Reactive
Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.

Annex: /5
Annex: /5

-15.000
15.000
45.000
Wind Farm V Q Diagram (EN): Reactive pow er reference (at Pn) in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Reactive pow er at PCC in Mvar / Voltage at PCC in p.u.
Wind Farm V Q Diagram (EN): Total Reactive Pow er of All Wind Turbines at LV Level in Mvar / Voltage at PCC in p.u.

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22 /

V-Q Q_WTG

[Mv

D


Voltage Stability
– Classical Theory –
Vr
P L + jQ L

Es

Vr

QL
= const.
PL

V crit

PL
P max
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23 /


Classical Power Limit
“Nose Curve”
V

Operating
Point

Normal Range

Vo
V Critical

V Margin

Stable

Unstable

P Margin
P oper

Power
Limit

P

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24 /


System Strength

What is it?
• Usually measured in short circuit MVA
• MVAsc = kVb2/Xsc = 3½kVbkIsc
Why is it the single most important factor?
• Maximum short circuit (I.e. max kIsc or min Xsc)
dictates breaker duties, many equipment ratings
(later lecture)
• Minimum short circuit (I.e. min kIsc or max Xsc)
dictates worst sensitivities, e.g. dV/dC, dV/dP, etc.
(we’ll look at this some more below)

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