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Long-Duration Voltage Variations. X. R. I. Load. V 1. V 2. V 1. jX I. V 2. RI. I. At given pf at full load, nominal V 2. Voltage Regulation. Definition: Voltage regulation (at point x) is the percent voltage rise caused by unloading a power system (at point x) - PowerPoint PPT Presentation
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Long-Duration Voltage Variations
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LoadV2V1
V2
I
I
RI
jX IV1
At given pf at full load, nominal V2
R X
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Voltage Regulation• Definition: Voltage regulation (at point x)
is the percent voltage rise caused by unloading a power system (at point x)– Assumption 1: The original power factor at
point x is given– Assumption 2: The original voltage is the
nominal value at point x, or a given value if not nominal; the source voltage is fixed
– Assumption 3: Original system is at full load, or a given value if not full load
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V2,NL = V1
At no load, V2 normally rises to equal V1
V2FL
%100V
VV%100
VVV
RegR2
R2NL2
FL2
FL2NL2
Last equality assumes full-load voltage is nominal or rated value for the system
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• The system inductive reactance usually causes voltage drops under normal loading
• If the load pf is leading or if very long transmission lines at EHV (345 kV and up, the line charging current may be very large), then regulation may be negative
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Root Cause
• Most long-duration voltage variations are caused by too much impedance (Zth) in the power delivery system
• The power system is too weak for the load– voltage drops to a low value under heavy
loads (lagging pf)– voltage rises to a high value under light loads
(more leading or less lagging pf)
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Solutions to Improve Voltage Regulation
• Add shunt capacitors to increase the load power factor (not leading however) tending to decrease the load kVA by decreasing the load kVAr
• Add static var compensation or other dynamic reactive power compensation (same reason as shunt capacitor addition, but better control)
• Add series capacitors to lines to cancel part of the jXI voltage drop (long transmission lines and (rarely) short lines with impact loads)
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Solutions to Improve Voltage Regulation
• Add voltage regulators to boost V under heavy load and buck voltage under light load
• Increase the size of conductors to reduce Z
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Loads…
Step voltage regulators
RaiseLower
Load side
Source side
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V sensing and gate control
Source side
…Load side
…Electronic tap-switching voltage regulator
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Loads…
voltage regulator set at 105% without line-drop compensationV(x)
x120 V
126 V
114 V
voltage profile for light load
voltage profile for heavy load
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Loads…
voltage regulator set at 100% with line-drop compensationV(x)
x120 V
126 V
114 V
voltage profile for light load
voltage profile for heavy load
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V(x)
x120 V
126 V
114 V
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V(x)
x120 V
126 V
114 V
Voltage profile after load rejection
Needs rapid runback controls
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Flicker
Sources of flicker-Load change-Induction motor starting -Variable power generation
Observable flicker is dependent on the following:-Size (VA) of potential flicker-producing source -System impedance (stiffness of utility)-Frequency of resulting voltage fluctuations
Example of Flicker
Power system model at Ulleung Island of South Korea.
YDG
4.5[MW]/0.5[MW]
4.5[MVA]/0.5[MVA]3.3[kV]/6.6[kV]
DG
1.5[MW]
1.5[MVA]3.3[kV]/6.6[kV]
WG
0.6[MW]0.6[MVA]
0.48[kV]/6.6[kV]
1.53+j0.790 [Ω]
1.16+j0.600 [Ω]
SMESPs, Qs
C 0.305 MVAR
Load 6[MW]/2[MW]
6.0[MVA]/2.0[MVA]0.23[kV]/6.6[kV]
1.16+j0.599 [Ω]Y
0.6[MVA]/0.1[MVA]6.6[kV]/3.3[kV]
HG
HG
0.6[MW]/0.1[MW]
0.1[MW]
Y
Y
0.378+j0.195 [Ω]
PL, QL
VGPWG Unit
System Responses
Wind speed data.
0 10 20 30 40 50 606
8
10
12
Win
d sp
eed
[m/s
]
Time [sec]
0 10 20 30 40 50 6058
59
60
61
62
0 10 20 30 40 50 600.0
0.5
1.0
1.5
2.0
2.5
Freq
uenc
y [H
z]
Time [sec]
Without Wind generator With Wind generator
Activ
e pow
er [M
W]
Time [sec]
Wind generator Diesel generator 1&2 Hydraulic generator 1&2 Load
Responses of active power and system frequency.
System Responses with SMES
0 10 20 30 40 50 6058
59
60
61
62
0 10 20 30 40 50 600.0
0.5
1.0
1.5
2.0
2.5
Freq
uenc
y [H
z]
Time [sec]
Without SMES With SMES
Act
ive
pow
er [M
W]
Time [sec]
Wind generator Transmission line Diesel generator 1&2 Hydraulic generator 1&2 Load
Responses of active power and system frequency with SMES .
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Source side Loa
d
Thyristor-controlled reactorOne type of static var compensator
3rd 5th 7th
capacitors configured as harmonic filters
Flicker Mitigation Techniques
-Adding series reactor
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Source side Loa
d
Thyristor-switched capacitorAnother type of static var compensator
capacitors are gated fully on in sequence