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7 power quality

Power Systems & Energy Course:
Power Quality Issues with Large-Scale
Renewable Plants

Jason MacDowell


Power Quality – Renewable Plant Perspective
• Power quality is a two way street!
• Plant effect on grid
• Grid effect on plant
• Interconnection requirements are typically imposed
at the PCC (Point of Common Coupling)

• Impacts at the PCC are a function of:
• Diversity of the individual WTGs/PV arrays
• Characteristics of the collector system
• Characteristics of the transmission grid

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


11-2


Power Quality Basics
Undervoltages:
Notch – less than one cycle duration
Dip – duration of a few cycles
Sag – duration of seconds to minutes
Interruption – reduction to zero for seconds to minutes

Overvoltages:
Spike – less than one cycle duration
Swell – duration of cycles or more

Waveform Distortion:
Harmonics – steady-state distortion affecting every cycle
integral multiples of operating frequency
Interharmonics – non-integral multiples of operating frequency
Noise – random high-frequency signals
Frequency Shift – variations in frequency through time
Flicker – cyclic or random variations in voltage magnitude
Unbalance – phase asymmetry
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-3


Flicker

Flicker is caused by repetitive variations in voltage – can be
irritant to utility customers
House Pumps
Sump Pumps
Air-Conditioning
Equipment
Domestic
Refrigerators
Oil Burners

Arc Furnace

Flashing Signs
Arc-W elders
Manual Spot-W elders
Sews
Group Elevators

Single Elevator
Heights
Y-delta Changes
on Elevator-MotorGenerator Sets
X-Ray Equipment

Reciprocating
Pumps
Compressors
Automatic
Spot-W elders

5

% Voltage Fluctuation

4
Border Line of
Irritation
3

2
Border Line of
Visibility
1

0

1

2

3

6

10

Fluctuation per Hour

20 30

1

2

4

6

10

Fluctuation per Minute

20 30

60

2

3 4

6

10 15

Function per Second

Measurement of flicker has been standardized by IEC,
using “flickermeter” algorithm (IEC 61000-4-15)
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-4


Flicker in Wind Plant Applications
• Sources of flicker:





Wind turbulence, gusting
Drive-train oscillations
Blades passing tower
Induction generator close-in inrush

• Diversity reduces impact on a large wind plant

• DFAG and full-conversion reduces impact
• GE WTG voltage regulation capability virtually
eliminates issue

• For induction generators, dynamic compensation
could be required to meet tight grid specs
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-5


Flicker in Large PV Plants
• Cloud shadow passage potentially can cause significant
voltage variation
– Frequency of variations usually not sufficient to be technically
considered “flicker” unless variations are very large
– Very large voltage variations would be unacceptable for other reasons

• Output of PV plants becomes smoother as plant rating
increases
– Finite size of cumulus clouds that cause the most intermittent
shadowing
– Output of very large plants approaches (1 - %overcast)*Pclear_sky

• Plant level voltage regulation can readily mitigate voltage
variation

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

11-6


Non-Repetitive Voltage Change
• Some grid codes limit step voltage changes
– Typically 1% - 2% for frequent events
– Some codes relax limits to 3% for infrequent events

• Causes:





Capacitor/reactor bank switching
Transformer energization
Feeder switching
Interconnecting HV cable switching

• Solutions:
– Limit size of individually switched banks
(Qmax = DV x MVASC)
– Controlled transformer energization
– Resistor preinsertion
– Point-on-wave switching
– Compensation of HV cables

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

11-7


Voltage Transients Affecting Grid
• Transients are sub-cycle overvoltages
• In wind plant, usual cause is capacitor switching
– Ringing oscillations, usually at several hundreds of Hz
– Severity depends on point-on-wave of energization

• Solutions
– Synchronized switching -switch closes near voltage zero

– Ideally, eliminates transient
– Experience has been that switchgear must operate often to “keep
in training”

– Impedance preinsertion

– Resistor preinsertion (breakers)
– Lossy inductor (circuit switchers)

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

11-8


What are Harmonics?
• Non-fundamental-frequency AC currents and
voltages superimposed on the system
– Frequencies are multiples of normal system frequency
– Interharmonics are non-integral multiples

• Results in repeated distortion of the sine wave
• System impacts:





Capacitor overload
Excess heating
Device misoperation
Telecommunication interference

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

11-9


Harmonic Sources in Renewable Plants
• Power converters (used in DFAG, full conversion, and
controlled rotor resistance induction generators)
• PV inverters
• Generators
• Dynamic compensation equipment
– SVCs, STATCOMS
• The grid!

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

11-10


WTG/Inverter Harmonics

• PV inverters, and power converters used in most WTGs use
PWM technology
– Harmonics are clustered around multiples of the switching
frequency
– High frequency; are easily filtered
– Random phase superposition self-cancels much of the
harmonics produced in a plant with many WTGs/inverters
PWM Waveforms
(Illustration only)

• All rotating machines produce a small amount of harmonics
due to winding configuration
• WTG harmonics do not usually require action in collector
system design
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-11


GE Test Data

Actual values may be less; it is hard to
discriminate harmonics from WTG
from harmonics flowing into WTG
from grid distortion

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

11-12


Harmonic Performance Specifications
IEEE 519 specifies the following limits for “dispersed generation”:
Harmonic Order
h < 11

12 - 17 18 - 23 23 - 35 35 - 50

Odd

2.0%

1.0%

0.75%

0.3%

0.15%

Even

0.5%

0.25%

0.187%

0.075%

0.037%

THD

THD 

2.5%

50

 Ih

2

h2

• Voltage specifications might also be made (individual, and RSS)
• Need to consider impact on wind plant equipment as well:
– Transformers
– Capacitors

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

11-13


Harmonics Injected by Plant Into Grid
“Infinite”
Bus
Iharm

Iharm

Iharm

Iharm

PCC
Zgrid
IPCC

Iharm

Iharm

Iharm

Iharm

Iharm

Iharm

Iharm

Iharm

IPCC not equal to n  Iharm
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-14


Impedances at Harmonic Frequencies
Inductive Impedance
Z  R  jX L  R  j  2  f  L
• Transformers
• Lines and cables
•Shunt reactors

Capacitive Impedance

Z   jX C 

j
2  f  C

• Capacitor banks
• Line and cable charging

• Impedances change with frequency
– Inductive impedance increases with frequency
– Capacitive impedances (negative) decrease in absolute value with
frequency

• Damping (resistive) component can also change in
components affected by eddy current losses

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

11-15


Resonant Amplification

Igrid
Iinj

Z L  ZC
2  f  L
V  I inj 
 I inj 
2
Z L  ZC
j 2  f   L  C  1



I grid



ZC
1
 I inj 
 I inj 
2
Z L  ZC
j 2  f   L  C  1





Harmonic voltage and grid current go to infinity when
(2f) 2-1 = 0
This occurs when f =1/(2 (LC) 1/2)

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

11-16


Resonant Amplification, with Damping
V  I inj 

2  f  L  jR





2  f  C  R  j 2  f   L  C  1

I grid  I inj 

2

j





2  f  C  R  j 2  f   L  C  1
2

• Resonant frequency unchanged
• Magnitude of resonant amplification is reduced
• In a realistic system, harmonic voltages and currents can be
greatly amplified

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

11-17


Resonance Example
0.15

15

Per-unit
harmonic
voltage
0.1

Current
amplification
factor
10

Vf

If

0.05

0

5

0

200

400
f

600

0

0

200

400

600

f

Typical wind plant parameters, 1% harmonic current injection
from equivalent WTG
Harmonic voltage and current amplification can be great if
resonance coincides with an injected frequency
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-18


The Grid as a Harmonic Source
• Most grids have significant background distortion
– Due to nonlinear loads, P-E devices, transformers
– Greatest at 3rd, 5th, and 7th harmonic
• Good data requires extended-duration monitoring
• IEEE-519 sets “recommended practice” for utilities:
69 – 161 kV
> 161 kV

V THD

1.5% 2.5%
1.0% 1.5%

1.6
1.4
1.2
Voltage (%)

Vh

Measurements made
on an actual utility
system

1
0.8
0.6
0.4
0.2
0
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic

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

11-19


Harmonics Injected by Grid Into Plant

PCC



Zgrid
IPCC

VHarm

IWTG

Harmonics from the grid are
a major portion of IPCC
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-20


Resonant Amplification of Grid Harmonics
• Wind plant presents a series resonant circuit
– Substation transformer + capacitor banks & cable
charging
– Can amplify grid distortion
• Problems caused by excess harmonics in collector system
– Capacitor overload
– Component heating (WTG, transformers, cable)
– Misoperation
Vcoll

Vh

Icap

I cap 

Vh
Z L  ZC

Vcoll  Vh 

Zc
Z L  ZC

A lightly damped resonance can cause severe
voltage distortion on collector, and high currents in
shunt capacitors

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

11-21


HARMONIC ANALYSIS

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

11-22


Harmonic Analysis Software Tools
Steady-state phasor analysis – one frequency at a time
• Time-domain modeling tools are not needed
• Some time-domain tools also perform phasor analysis (e.g., EMTP, ATP)

Capability to represent frequency-dependent characteristics of
network components
• XL = 2fL XC =-1/(2fC)
• Long-line characteristics of cables and overhead lines
• Frequency-dependent damping characteristics are extremely important

Must handle a large
number of configurations

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

11-23


Source Characteristics of WTGs/Inverters
Modern PV inverters and wind turbines do not look like an ideal
harmonic current source
• Internal filters are in shunt
• VSC bridge appears like:
– Ideal current source at low frequency (within controller bandwidth)
– Ideal voltage source at high frequency (f >> controller bandwidth)
– Complex transition of characteristics in between

• Shunt impedance of machine

Norton equivalent is the preferred representation
• Source magnitude spectrum depends on operating point
• Equivalent shunt source impedance is a complex function of frequency –
does not conform to simple models

Characteristics best determined by detailed time-domain
simulation with controls modeled
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-24


WTG representation (Internal Distortion)
• Doubly-fed asynchronous generator machine
• Low distortion energy:
– Converter operated with high switching frequency
– Converter rating is a fraction of turbine rating
• Small distortion filter within the turbine to absorb most of the
distortion energy created by the converter.

Conceptual converter harmonic current distortion flows
© 2016 General Electric International, Inc. All rights reserved. Not for distribution without permission.

11-25


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