[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 2.8.20A Performance of a ride-through transformer (RTT) during a ten-cycle voltage interruption and<br />
voltage sag. Voltage regulation of an RTT during a single-phase voltage interruption (top: input; bottom: output).<br />
control components: ice-cube relays, motor starters, contactors, a programmable logic controller, a linear<br />
dc power supply, and a switch-mode power supply.<br />
The test results revealed that the prototype RTT protected the connected process controls from most<br />
of the applied voltage sags and interruptions. Besides, it was observed that RTT performance greatly<br />
depended on the phase configuration (that is, single-, two-, or three-phase) of the voltage sags or<br />
interruption and, to a much lesser extent, on the loading of the RTT output. It was observed that the<br />
RTT performed like a typical CVT during three-phase voltage sags. Figure 2.8.20a and Figure 2.8.20b<br />
show the response of an RTT to phase-to-neutral and phase-to-phase sags.<br />
To get the most out of a CVT with a three-phase input, the most trouble-free voltage phases of the<br />
electric-service supply will have to be determined. For example, if most voltage sags occur on phase A<br />
or B, then the center tap on the transformer primary should be connected to phase C. Although this<br />
prototype transformer promises to retail at a price substantially higher than the price of a traditional,<br />
single-phase CVT, the price differential can be greatly reduced by a reduction in size. Because the<br />
performance of a traditional CVT greatly depends upon loading, CVTs are often oversized for the<br />
connected load. A smaller but more loaded RTT should be able to perform as well as the derated,<br />
traditional CVT.<br />
2.8.3 Procurement Considerations<br />
This section describes what to look for when purchasing CVTs; typical prices of various CVT ratings;<br />
and typical sizes, weights, efficiencies, and service conditions of CVTs.<br />
FIGURE 2.8.20B Performance of a ride-through transformer (RTT) during a ten-cycle voltage interruption and<br />
voltage sag. Voltage regulation of an RTT during a two-phase ten-cycle voltage sag to 60% of nominal (top: input;<br />
bottom: output).<br />
2.8.3.1 What To Look for When Purchasing a Constant-Voltage <strong>Transformer</strong><br />
Most CVT manufacturers guarantee their products will meet their published specifications [2]. In general,<br />
most CVTs are specified to recognized industrial standards, and under well-defined conditions (agreed to<br />
by the CVT manufacturer) they may be capable of operating outside their published limits. Although CVTs<br />
are highly reliable and are effective at stabilizing variations in the electric-service supply, it would be prudent<br />
to review the following application issues with each CVT manufacturer before purchasing a CVT:<br />
2.8.3.1.1 Effect of High Input and High Output Voltages upon Operation of Input Protection Devices<br />
Establish the proper input fuse and/or circuit-breaker rating required for the application. Identify with<br />
the CVT manufacturer what the maximum primary rms current is under secondary short-circuit conditions<br />
for determining the rating of the fuse or circuit breaker. Be aware that sizing fuses and circuit<br />
breakers for conventional transformers is not applicable when sizing fuses and/or circuit breakers for<br />
CVTs. This is because CVTs are high-impedance units (typically ten times higher than a control transformer)<br />
compared with the equivalent-rated conventional transformer. Therefore, it is essential that the<br />
guidelines in the CVT manufacturer’s specification should be consulted before finalizing the fuse sizing<br />
and/or circuit-breaker rating requirements.<br />
With the correct input fuse or circuit breaker, the application should work fine until the protection<br />
opens at approximately 150% of the nominal input voltage. In some cases, after the circuit protection<br />
opens, the CVT’s output voltage will rise with increasing input at approximately 20% of the increase<br />
(that is, the output voltage may go up 1% for each 5% the input voltage rises). This situation may<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC