Long-Duration Voltage
Variations
1
X
R
I
V1
V2
Load
V1
I
V2
jX I
RI
At given pf at full load, nominal V2
2
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
3
V2,NL = V1
V2FL
At no load, V2 normally rises to equal V1
V2NL  V2FL
V2NL  V2R
Reg 
 100% 
 100%
V2FL
V2R
Last equality assumes full-load voltage is
nominal or rated value for the system
4
• 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
5
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)
6
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)
7
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
8
…
Loads
Step voltage
regulators
Raise
Source Lower
side
Load
side
9
…
…
Source
side
V sensing
and gate
control
Load
side
Electronic tap-switching voltage regulator
10
…
Loads
voltage regulator set
at 105% without linedrop compensation
V(x)
voltage profile
for
light
load
126 V
120 V
114 V
voltage profile
for heavy load
x
11
…
Loads
voltage regulator set
at 100% with linedrop compensation
V(x)
126 V
120 V
114 V
voltage profile
for light load
voltage profile
for heavy load
x
12
V(x)
126 V
120 V
x
114 V
13
Voltage profile
after load rejection
V(x)
126 V
120 V
x
114 V
Needs rapid runback controls
14
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
15
Example of Flicker
4.5[MVA]/0.5[MVA]
3.3[kV]/6.6[kV]
D
DG
0.6[MVA]/0.1[MVA]
1.16+j0.599 [Ω] 6.6[kV]/3.3[kV]
Y
Y
4.5[MW]/0.5[MW]
HG
0.6[MW]/0.1[MW]
1.53+j0.790 [Ω]
0.378+j0.195 [Ω]
1.5[MVA]
3.3[kV]/6.6[kV]
HG
Y
D
D
DG
D
1.5[MW]
6.0[MVA]/2.0[MVA]
0.23[kV]/6.6[kV]
Load 6[MW]/2[MW]
1.16+j0.600 [Ω]
0.6[MW]
WG
C
0.6[MVA]
0.48[kV]/6.6[kV]
PWG
D
Y
PL, QL
VG
SMES
Ps, Qs Unit
0.305 MVAR
Power system model at Ulleung Island of South Korea.
0.1[MW]
System Responses
Wind generator
Diesel generator 1&2
Hydraulic generator 1&2
Load
Active power [MW]
2.5
10
2.0
1.5
1.0
0.5
0.0
8
6
0
0
10
20
30
40
Time [sec]
Wind speed data.
50
10
20
30
Time [sec]
40
50
60
Without Wind generator
With Wind generator
62
60
Frequency [Hz]
Wind speed [m/s]
12
61
60
59
58
0
10
20
30
Time [sec]
40
50
Responses of active power and system
frequency.
60
System Responses with SMES
Wind generator
Transmission line
Diesel generator 1&2
Hydraulic generator 1&2
Load
Active power [MW]
2.5
2.0
1.5
1.0
0.5
0.0
0
10
20
30
Time [sec]
40
50
Without SMES
With SMES
62
Frequency [Hz]
60
61
60
59
58
0
10
20
30
Time [sec]
40
50
Responses of active power and system frequency with SMES .
60
Flicker Mitigation Techniques
-Adding series reactor
Source
side
Load
3rd 5th 7th
capacitors configured as
harmonic filters
Thyristor-controlled reactor
One type of static var compensator
19
Source
side
Load
capacitors are gated fully
on in sequence
Thyristor-switched capacitor
Another type of static var compensator
20