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Assessment of Harmonic Distortion and
Voltage Unbalance
Zia Emin
National Grid Transco
Network DesignSystem Policy & Support
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What is Quality of Supply? (1)
Ideal 3-Phase Balanced Voltages
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.01 0.02 0.03 0.04
Time(Seconds)
V(pu)
V(Red)
V(Yellow)
V(Blue)
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What is Quality of Supply? (2)
Actual measurement 3-phase measurement
-1.5
-1
-0.5
0
0.5
1
1.5
V(pu)
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Todays Talk
Will cover harmonic voltage distortion and voltage unbalanceassessment in some detail
Will not cover voltage dips, voltage steps, flicker or transients
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Voltage Waveform Quality: the Grid Code
National Grid is bound by Grid Code CC6.1.5 to:Apply G5/4 planning criteria for new connections to limit harmonicemissions
Take existing and prospective users into account
Comply with the compatibility levels of G5/4
To ensure, under planned conditions, a maximum continuous PhaseVoltage Unbalance of
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ER G5/4 Stage 3 Assessment
All new connections at 33 kV Consider voltage distortion at the connection point andother nodes(not necessarily owned by NGT)
Detailed harmonic impedance model of the network Resonance conditions must be considered
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Stage 3 Assessment Process
BACKGROUND
MEASUREMENTS
HARMONIC
STUDIES
ESTIMATE
TOTAL HARMONICVOLTAGES
EMISSION
LIMITS
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Harmonic Studies (2)
For each system condition, calculate up to the 50
th
harmonic: Harmonic self-impedance (ZAh) at the connection point (node A)
Voltage gain to other nodes (GAXh)
Harmonic voltages: At the connection point: VAh= IhZAh
At a remote node X: VXh= VAhGAXh
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Harmonic Study Example
400kV
132kV
MV Node C
Node A
Node B
Polluting Load
Node E
Node H
Ih
VAh=Ih . ZAh(ZAh)
GAE
GAH
GACVHh=GAHh . VAh
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Self-Impedance at Node A (ZAh)
0
0.1
0.2
0.3
0.4
0.5
0 2 4 6 8 10 12 14 16 18 20 22 24
harmonic number
Zmag(p
u)
intact system
system condition 1
system condition 2
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Voltage Gains
Intact System Conditions
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0 2 4 6 8 10 12 14 16 18 20 22 24
harmonic number
voltagegain
GAE
GAH
GAB
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Background Measurements
Minimum period of 7 days (no bank holidays!)
95th percentile assumed for background
Preferably no outages
Choose a few representative sites Not possible at some locations (no suitable transducer)
Access can be a problem (3rd party sites)
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Example Background Measurement
5th
Harmonic Voltage
0.0%
0.5%
1.0%
1.5%
2.0%
2.5%
3.0%
Wed Thu Fri Sat Sun Mon Tue Wed
Magnitude
95thPercentile (2.35%)
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Estimate Total Individual Harmonic Voltages
Combine study results and measured background: Assume 0 phase shift for the harmonic with the highest magnitude
90 for the rest
meashcalchtotalh VVV +=
22
meashcalchtotalh VVV +=
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Estimated Total Harmonic Voltage (5th)
0.00%
0.50%
1.00%
1.50%
2.00%
2.50%
3.00%
3.50%
A (400kV) B (132kV) E (11kV)
Nodes
5thharmonicvolt
agemagnitude
new load
background
G5/4 Planning Level
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Harmonic Emission Limits
Set to declared emission level, unless planning levels are exceeded
Otherwise, reduce emission until Vtotalh
is below planning level at all
nodes
What if background already exceeds planning level?
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Phase Unbalance
Under unfaulted system condition, caused by:
Untransposed transmission lines
Unbalanced loads (e.g. traction supplies)
Inverse relationship to system strength eg 50 MVA phase to phase load atfault level of 5000 MVA gives approx 1% NPS voltage
Highest risk is generator NPS relay operation
Low NPS levels when the network is intact Unfavourable outages could lead to higher levels
Balanced Phases Unbalanced Phases
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Where does OHL NPS come from?
seqseqseq IZV *= For a double-circuit one can write
where
if the phasings are RYB/BYR then
=
zps
nps
pps
zps
nps
pps
seq
V
V
V
V
VV
V
2
2
2
1
1
1
=
zps
nps
pps
zps
nps
pps
seq
I
I
I
I
II
I
2
2
2
1
1
1
=
666564636261
565554535251
464544434241
363534333231
262524232221
161514131211
ZZZZZZ
ZZZZZZ
ZZZZZZ
ZZZZZZ
ZZZZZZ
ZZZZZZ
Zseq
zpsnpsppszpsnpsppsnps IZIZIZIZIZIZV 2262252241231221211 +++++=
=
82778.0161023.011025.078436.078010.0125010.0
11025.086256.015027.043010.0101013.0151015.0
161023.0163027.086256.0114010.031015.081013.0
78436.0114010.043010.082778.0108025.078023.0
125010.081013.0151015.078023.086256.043027.0
76010.031015.0101013.0108025.0135027.0860.256
seqZ
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Configurations Leading to System Unbalance
outaged
A B
C
outaged
A B
C
outaged
A B
C
Loop in of a demand point creating mismatch in flows Loop in of a demand point creating opposite flows
Loop in of a generation point creating opposite flows
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NPS Voltage due to Flow Mismatch
outaged
A B
C
0
100
200
300
400
500
600
700
800
900
1000
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 00
time (hr)
powerflow(MW)
0.00%
0.10%
0.20%
0.30%
0.40%
0.50%
0.60%
0.70%
0.80%
0.90%
1.00%
NPSvoltage(%offundam
ental)
outage of C - B
line taken at this
point flow from A to B
flow from A to C
flow from C to B
NPS voltage at C
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NPS Voltage due to Counter Flow
Approximate NPS levels for generation connected to Quad Circuit
Single circuit export with counter flow in the parallel circuit
0
1
2
3
4
5
0 25 50 75 100
Line Length kM
NPS
Volts
%
400 MW gen/1400 MVA flow
400 MW gen/2400 MVAflow
400 MW flow/3400 MVA flow
800 MW gen/1800 MVA flow
800 MW gen/2800 MVA flow
800 MW gen/3800 MVA flow
Flow
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NPS Voltage Measurement
0
0.1
0.2
0.3
0.4
0.5
Tue Wed Thu Fri Sat Sun Mon Tue
NPSVoltage(%)
95th percentile
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Effects of Phasing
NPS/ZPS
Voltages
Maximised
RYB YBR BRY
NPS/ZPS
Currents
MaximisedRYB
YBR
BRY
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Distribution of NPS Voltage with Proper Phasing
RYB
BYR
RBY
YBR
YRB BRY
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Unbalanced Loads
Optimum Phase-Pair Allocation
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2
0o
90o
180o
270o
Y-B load
R-Y load
B-R load
-------- 0.9pf
-------- 1.0pf
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2
0o
90o
180o
270o
Y-B load
R-Y load
B-R load
-------- 0.9pf
-------- 1.0pf
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Controlling NPS Voltages
Introduce phase transpositions
Create parallel lines
Install phase balancer
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Transposition Tower (Its nothing new)