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Ferroresonance on a Cable-fed Transformer
Francisco De La Rosa
Introduction
Ferroresonance phenomena may affect low voltage installations involving an underground cable
or a capacitor bank on the source side of a service transformer. When one or two phases on the
source side open as a result of a downstream fault, oscillatory voltage behavior can get started.
Figure 1 illustrates a single-phase blown fuse when a 150 kVA service transformer fed through a
3-mile 35 kV underground cable supplies power to a 600 V DC drive. The drive feeds a 40 HP
DC motor providing the voltage control function shown in figure 2. The 3-mile underground
cable assumed in the simulation is a 1/0 35 kV XLP whose cross section used in PSCAD to
calculate the admittance matrix is shown in figure 3. The conductor configuration in the
underground system assumes 5 cm distance between the two outer conductors relative to the
central conductor.
Figure1. 1/0 35 kV XLP 3-mile underground cable connecting to 5 % loaded 150 kVA 12.5 /
0.480 kV Transformer with a 3.5 % impedance that feeds a 40 HP DC drive
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g
a
( M
e
F E R R > V d c L in k -M a in
i t u
d
800
n
600
a
g
400
M
200
0
100
200
300
400
T im e ( m s )
E le c tr o te k C o n c e p ts ®
500
600
700
T O P , T h e O u tp u t P r o c e s s o r ®
Figure 2. Voltage control function provided by the DC drive
Figure 3. Cross section of the 35 kV XLP cable
The PSCAD simulation for the service transformer involves a five-limb model. The UMEC
(Unified Magnetic Equivalent Circuit) transformer model used in PSCAD is based mostly on
core geometry. This allows taking proper account of magnetic coupling between windings of
different phases, in addition to coupling between windings of the same phase. The model is
comprehensively described in [1].
Waveforms for the 40 kVA DC drive at the time phase A opens are shown in figure 4. Large
fluctuations on the Variable Frequency Drive output voltage are clearly observed on the bottom
plot. These represent a severe power quality issue which can produce damage to the DC motor if
this condition is not properly detected and action taken to disconnect the DC drive from the
power supply. Similar voltage fluctuations in customer’s sensitive equipment fed through
rectifiers can also become negatively impacted.
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Figure 4.Voltage fluctuations under ferroresonance between cable and transformer when phase A
opens
Tables 1 and 2 depict the Total Harmonic Distortion (THD)and RMS values of the phase to
neutral voltage and line harmonics at the high voltage (HV) and low voltage (LV) sides of the
service transformer, respectively. These are calculated starting from the time phase A is opened.
Notice that the THD on the voltage waveforms on phases B and C remains low while the THD
on the LV current on the open phase can reach levels 2-3 times those in the other phases.
Figures 5 and 6 show the harmonic spectra of phase to neutral voltage harmonics on the HV side
(33 kV) and the currents on the LV (480V) side of the service transformer.
Table 1. 33 kV side Phase to Neutral voltage harmonics
Table 2. 480 V side current harmonics
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Figure 5. Source side Ph-N voltage harmonic spectrum under phase A open
Figure 6. Load side line current harmonic spectrum under phase A open
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The subharmonic components of the 33 kV phase to neutral voltage and the harmonic
components of the DC voltage are further examined in figure 7. As illustrated in the figure, the
maximum subharmonic component on the opened phase (Ea) voltage waveform reaches
[900/(33000/√3)]100, or around 5% at 16 Hz, just on the limit recommended in the IEEE Guide
for Applying Harmonic limits on power systems [2], as shown in figure 8. The DC voltage on
the other hand, does not show any subharmonic component as depicted in the lower plot in figure
7, where only even harmonics 2 (120 Hz) and 4 (240 Hz) show up.
Figure 7. Source side voltage subharmonics and DC voltage harmonics under phase A open
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Figure 8. Recommended subharmonic levels, according to IEEE P519; adapted from [1]
On the low voltage side of the service transformer, some subharmonic content is also observed
both on the voltage and on the current waveforms. The phase-to-phase voltage shows a
maximum of around 10% level at around 15-17 Hz and the maximum subharmonic current hits
around 16%, as illustrated in figures 9 and 10. These are levels likely to produce light flicker.
Figure 9. Load side voltage subharmonics under phase A open
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Figure 10. Load side line current subharmonics under phase A open
Conclusions
1. The voltage oscillations under ferroresonance on a cable-fed transformer can represent a
severe power quality issue which can produce damage to LV DC motors if this condition
is not properly detected and action taken to disconnect the DC drive from the power
supply. Similar voltage fluctuations on customer’s sensitive equipment fed through
rectifiers can also become negatively impacted
2. THD on the voltage waveforms on phases B and C remains low while the THD on the
LV current on the open phase can reach levels 2-3 times those in the other phases
3. The maximum subharmonic component on the opened phase (Ea) voltage waveform
reaches around 5% at 16 Hz, just on the limit recommended in the IEEE Guide for
Applying Harmonic limits on power systems. The DC voltage on the other hand, does not
portray any subharmonic component but it shows even harmonics 2nd and 4th .
4. On the low voltage side of the transformer, the phase-to-phase voltage shows a maximum
of around 10% level at around 15-17 Hz and the maximum subharmonic current hits
around 16%.
5. The obtained levels of interharmonics during phase A open are likely to produce light
flicker
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References
[1] W. Enright, O. B. Nayak, G. D. Irwin, J. Arrillaga, An Electromagnetic Transients Model of
Multi-limb Transformers Using Normalized Core Concept, IPST '97 Proceedings, Seattle, pp.
93-98, 1997
[2] Draft: Guide for Applying Harmonic Limits on Power Systems, IEEE P519.1™/D10,
January 2005
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