[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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