[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Input with
Interruption
Ferro Output
Input with Oscillating
Transient
Ferro Output
Input with
Sag
Ferro Output
Input with Impulsive
Transient
Ferro Output
FIGURE 2.8.14 Output voltages resulting from oscillating and impulsive transients in the input voltages.
Input with
Phase Shift
Ferro Output
FIGURE 2.8.13 Output voltages (labeled ferro-output) resulting from voltage interruption, sag, and phase shift on
the input.
but it does slightly decrease the effect of an interruption by reducing it to a deep voltage sag. In
conclusion, a CVT can moderately mitigate, but not eliminate, the effects of momentary voltage
interruptions.
2.8.2.3.2 Performance: Voltage Sag
To simulate a fault in the load’s electric-service supply, the input voltage to the CVT was decreased to
90%, 70%, and 58% of nominal voltage for one, two, three, four, and five cycles. The variations in the
output voltages were measured and recorded for each of the applied voltage sags. Figure 2.8.13 shows
the CVT’s input and output voltage waveforms down to 58% nominal voltage for sags lasting three cycles.
In general, the input voltage sags produced output voltage sag having approximately the same duration
but smaller sag depth. For example, during the nominal voltage sag to 58%, the CVT stayed in the
regulation mode and reduced the effect of the sag to approximately 20% of nominal voltage lasting three
cycles. Again, the recharging of the resonating capacitor caused an overshoot when the input voltage
returned to normal.
2.8.2.3.3 Performance: Voltage Phase Shift
To simulate the effects of a large load being switched off near the end of a long electric-service supply
feeder, the input voltage of the CVT was shifted forward 10˚ while the input and output voltages were
monitored and recorded. The phase shift occurred at the positive peak of the input voltage (see
Figure 2.8.13). The resulting phase shift in the input voltage caused the output voltage to briefly swell.
The CVT is sensitive to voltage phase shifts in the electric-service supply.
2.8.2.3.4 Performance: Oscillating Transient
A transient caused by capacitor switching was simulated with a 500-Hz ring wave with a peak magnitude
of 1 per unit and duration of 10 ms. The ring wave was applied to the positive peak of the input voltage.
Figure 2.8.14 shows the input voltage with the ring-wave transient and the output voltage with a small
impulsive transient.
2.8.2.3.5 Performance: Impulsive Transient
To simulate a lightning strike, a 2-kV, 1.2/50-s combination wave (as described in ANSI C62.41-1991)
was applied to the positive peak of the input voltage. As shown in Figure 2.8.14, the CVT significantly
damped and filtered the surge.
2.8.2.4 Application Considerations — CVT Electrical Characteristics during Linear and
Nonlinear Loading
The objective of this application was to characterize the CVT as a load while the CVT supplied a simple
linear load and while it supplied a complex nonlinear load [11]. In the following tests, the CVT was
connected first to a simple linear load and then to a complex nonlinear load. An electric-service supply
source with an average total harmonic distortion in the voltage of 3% supplied power to the CVT during
all tests.
2.8.2.4.1 Performance: Line Current Distortion
A resistive linear load consisting of incandescent lamps was connected to the output of the CVT. The
load was increased in ten equal increments from 0 to 8.3 A (output current rating of the CVT). Next, a
bridge rectifier (such as the type that might be used in electric-vehicle battery chargers) was connected
to the CVT. The rectifier and its resistive load (incandescent lamps) were the complex nonlinear load of
the CVT. By adding lamps, this complex load was increased in ten equal increments from approximately
0.4 A (rectifier with no lamps connected) to 8.3 A. Figure 2.8.15 and Figure 2.8.16 show the line-current
distortion during these tests compared with the line-current distortion for the same loads connected
directly to the electric-service supply. At no load, the power consumption of the CVT was approximately
120 W (core losses only). With the full linear load, total losses increased to approximately 134 W (core
losses plus load losses); with the full nonlinear load, total losses dropped to approximately 110 W.
Notice in Figure 2.8.15 and Figure 2.8.16 that, while the y-axis current-distortion magnitudes are
significantly different, the absolute current-distortion values of the CVT’s input current with either linear
or nonlinear load is nearly identical. Current distortion at the CVT’s input terminals was practically
independent of the type of load connected to the output (approximately 40% at no loading to approximately
5% at full loading). When a linear, low-distortion load was connected to the CVT output, the
CVT contributed to the current distortion at its input terminals from the electric-service power source,
particularly during low loading. When a nonlinear, high-distortion load was connected, the CVT substantially
reduced load-current distortion. When fully loaded, the CVT had relatively small power
consumption and an efficiency of 85% to 90%. As opposed to most voltage regulators, the losses of the
CVT decreased as the nonlinear load increased. The CVT also significantly affected the power factor of
the load.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC