7 power quality

• 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

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

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

Flicker is caused by repetitive variations in voltage – can be

irritant to utility customers

15 1

House Pumps Sump Pumps Air-Conditioning Equipment Domestic Refrigerators Oil Burners

Single Elevator Heights Y-delta Changes

on Generator Sets X-Ray Equipment

Elevator-Motor-Arc Furnace Flashing Signs Arc-W elders Manual Spot-W elders Sews Group Elevators

Reciprocating Pumps Compressors Automatic Spot-W elders

1 3 6 10 20

Border Line of Visibility

Border Line of Irritation

Flicker in Wind Plant Applications

– Wind turbulence, gusting

– Drive-train oscillations

– Blades passing tower

– Induction generator close-in inrush

eliminates issue

could be required to meet tight grid specs

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

Non-Repetitive Voltage Change

– Typically 1% - 2% for frequent events – Some codes relax limits to 3% for infrequent events

– Capacitor/reactor bank switching – Transformer energization

– Feeder switching – Interconnecting HV cable switching

Voltage Transients Affecting Grid

– Ringing oscillations, usually at several hundreds of Hz

– Severity depends on point-on-wave of energization

– 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)

What are Harmonics?

voltages superimposed on the system

– Frequencies are multiples of normal system frequency

– Interharmonics are non-integral multiples

Harmonic Sources in Renewable Plants

controlled rotor resistance induction generators)

– High frequency; are easily filtered

– Random phase superposition self-cancels much of the

harmonics produced in a plant with many WTGs/inverters

• All rotating machines produce a small amount of harmonics due to winding configuration

• WTG harmonics do not usually require action in collector

system design

PWM Waveforms

(Illustration only)

GE Test Data

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

Harmonic Performance Specifications

IEEE 519 specifies the following limits for “dispersed generation”:

I THD

• Voltage specifications might also be made (individual, and RSS)

• Need to consider impact on wind plant equipment as well:

– Transformers

– Capacitors

Harmonics Injected by Plant Into Grid

Zgrid

PCC

I PCC

“Infinite” Bus

Impedances at Harmonic Frequencies

– Inductive impedance increases with frequency

– Capacitive impedances (negative) decrease in absolute value with

frequency

components affected by eddy current losses

Inductive Impedance Capacitive Impedance

L f j

R jX

R

Z   L    2   

C f

j jX

L

f I

Z Z

Z

Z I

C L

C L

j

I Z

Z

Z I

C L

C inj

grid

Resonant Amplification, with Damping

• Resonant frequency unchanged

• Magnitude of resonant amplification is reduced

• In a realistic system, harmonic voltages and currents can be greatly amplified

j R C f

jR L

f I

j R C f

j I

f

0 5 10 15

If

f

Current amplification factor

The Grid as a Harmonic Source

– 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:

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Vh VTHD

69 – 161 kV 1.5% 2.5%

> 161 kV 1.0% 1.5%

Harmonics Injected by Grid Into Plant

ZgridPCC

V Harm



I PCC

I WTG

Resonant Amplification of Grid Harmonics

– Substation transformer + capacitor banks & cable

charging

– Can amplify grid distortion

• Problems caused by excess harmonics in collector system

h cap

Z Z

V I





C L

c h

coll

Z Z

Z V

HARMONIC ANALYSIS

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

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

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.

WTG representation and field WTG measurements

• Machine presents a shunt impedance to the grid that will

absorb distortion currents

• Most of measured distortion currents are due to external

sources

• Harmonic current measurements are severely affected by

distortion of external sources

Harmonic measurement

point

WTG representation – Norton Equivalent

• Study assumptions based on

physics of the system

• Realistic system distortion

Grid Impedance at Harmonic Frequencies

• Transmission grid impedance is important

to both studies of:

– Ambient distortion amplification in the

MITIGATING HARMONICS

Mitigating Grid Harmonic Amplification

– Selection of transformer impedances and capacitor bank ratings

– This option is limited by the wide range of tunings possible

– Avoid certain operating configurations

– E.g., start up WTGs on one feeder before switching in another feeder, on startup

– Detune shunt capacitors

– Add inductor in series with capacitor – Capacitor cannot resonate at a frequency higher than the frequency defined by the L-C combination

– Typical to select L-C to resonant at 4.8 th harmonic

• Filter out harmonics

– Complex design

Harmonic Filter Topologies

Single-Tuned

Filter

1 10 10 0 1 103 1 10410

0.1 1 10

Practical Filter Design Considerations

– Tuning taps on inductors

– Variable inductors

– Temperature variations (capacitors)

– Component failures (capacitors)

– System frequency variation

– Component aging

Filter design must be sufficiently robust to meet design objectives for expected variations

• Harmonic system studies should be based on realistic

converter modeling guidelines

• Wind/Solar plant equipment can create resonances

(Shunt Caps), particularly to grid distortion If shunt

capacitors are required, consider de-tuning

• If filters are needed, design must be sufficiently robust

to meet design objectives for expected variations.