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