Course 11 Distribution Transformer Applications Instructor: David R. Smith, PE Due: April 24, 2017 (EV), April 25, 2017 (LC)

Transcription

1 Name: Course 11 Distribution Transformer Applications Instructor: David R. Smith, PE Due: April 24, 2017 (EV), April 25, 2017 (LC) 1. T F In three-phase four-wire delta systems rated 240/120 volts, sometimes called red leg delta systems, the nominal voltage from the red leg (sometimes called the wild leg ) to the secondary neutral conductor is 208 volts. 2. T F The primary windings of three-phase distribution transformers, or three single-phase distribution transformers (transformer bank) can be connected in grounded wye only if the primary system supplying the three-phase transformer or transformer bank is an ungrounded (delta) primary system. 3. T F When three single-phase distribution transformers are connected to the multi-grounded neutral primarysystem with the primary (HV) windings connected in grounded wye (one end of each HV winding connected to the neutral conductor), and the secondary (LV) windings connected in delta, one or more of the fuses in the primary leads of the distribution transformer bank cannot blow for single lineto-ground faults that are on the primary feeder, either upstream or downstream of the transformer bank. 4. T F A single-phase distribution transformer with a high voltage winding designated 7620/13200 Y volts on the nameplate has one high voltage bushing, with the rated voltage of the high voltage winding being 7620 volts. 5. T F A single-phase distribution transformer with a high voltage winding designated grd Y/7620 volts on the nameplate has two high voltage bushings, with the rated voltage of the high voltage winding being 7620 volts. 6. T F When paralleling two single-phase distribution transformers, the transformers should have the same turns ratio between the primary and secondary windings, and the same nameplate impedance in percent so that the kva rating of the parallel combination is the sum of the kva rating of the two units being paralleled. 1

2 7. T F A 50 kva single-phase transformer is paralleled with a 100 kva single-phase transformer. The impedance on the nameplate of the 50 kva transformer is 1.6 %, and the impedance on the nameplate of the 100 kva transformer is 2.2 %. The derating factor for the parallel combination is T F When three single-phase distribution transformers are connected in a delta-delta bank (primary windings connected in delta, secondary windings connected in delta), it is not necessary that each transformer have the same primary winding to secondary winding turns ratio. This is because the fuses installed in the primary lines for the transformers will always thermally protect the transformers if they have different turns ratios. 9. T F Con Edison has single-phase distribution transformers in open-wye open-delta banks on Staten Island to supply three-phase four-wire delta loads. In these banks, the primary (HV) winding of each transformer is connected from phase to ground (neutral), and the secondary (LV) windings are in open delta to supply 240-volt three-phase three-wire load and 120/240-volt single-phase three-wire load. If the line terminal of the high-voltage winding of each transformer is inadvertently connected to the same high-voltage phase, two of the secondary phase-tophase voltages will be normal (240-volts), and the other phase-to-phase secondary voltage will be one half of the normal value. 10. T F In three-phase distribution transformers having the primary windings connected in grounded-wye and the secondary windings connected in grounded-wye, placing the transformer windings on a three-legged core assures that the transformer will not experience tank heating during a single phase-to-ground fault at the HV terminal (From H1 to H0) with a blown fuse between the primary source and the faulted HV terminal (H1), with terminals H2 and H3 energized at rated voltage. 2

3 11. T F In three-phase distribution transformers having the primary windings connected in grounded-wye and the secondary windings connected in grounded-wye, placing the transformer windings on a four-or five-legged core assures that the transformer will not experience tank heating. This applies to all types of faults and unbalances that can occur on the primary system to which the transformer is connected. 12. T F Three-phase distribution transformers for supplying 208Y/120 volt or 480Y/277-volt services have the secondary windings connected in grounded wye, with the primary windings connected in either delta or groundedwye. With the grounded-wye grounded-wye connections for the transformer primary and secondary windings, fundamental frequency currents and harmonic currents from the secondary load which are zerosequence in nature (in-phase in each phase on the secondary) will flow in the neutral conductor of the primary feeder supplying the transformer. 13. T F Fuses applied at the high-voltage HV) terminals of a transformer with the delta connected HV winding and grounded-wye connected LV winding should have a voltage rating equal to or greater than the system line-toline voltage, whereas fuses applied at the high-voltage terminals of a transformer with the grounded-wye grounded-wye connections can have a voltage rating that is less than system line-to-line voltage. 14. T F When a fault occurs at the service entrance in a typical single-phase three-wire 120/240-volt service, the current for the 120-volt fault (phase-to-neutral fault) is higher than the current for the 240-volt fault (phase-to-phase fault). 15. T F When a fault occurs at the low-voltage (secondary) terminals of a single-phase distribution transformer with the low-voltage winding rated 120/240 volts, the current for the 120-volt fault (phase-to-neutral fault) is higher than the current for the 240-volt fault (phase-to-phase fault). 3

4 16. T F When a fault occurs at the low-voltage terminals of a particular distribution transformer, with low-voltage windings rated 120/240 volts, the current in the fault path is 10,000 amperes rms when the two halves of the lowvoltage (secondary) winding are in series and the fault is across the entire secondary winding. For practical purposes, if the two halves of the secondary winding, each rated 120 volts, are paralleled, and a fault placed across the terminals of the paralleled windings, the current in the fault path will be 20,000 amperes. 17. T F In ungrounded secondary systems supplied from open delta transformer banks, such as a 480-volt delta secondary system, voltages of 1000 volts or higher may appear from phase-to-ground in the secondary system under normal operating condition when the transformer bank is energized from the primary system. 18. T F When ferroresonance occurs in radial primary distribution circuits during single-pole switching of a cable circuit and an unloaded three-phase distribution transformer, the waveshapes of the phase currents, phase-to-phase voltages, and phase to ground voltages are perfect sine waves. 19. T F When ferroresonance occurs in radial primary distribution circuits during single-pole switching of a cable circuit and an unloaded three-phase transformer connected to the end of the cable circuit, there are very high currents in the energized phase(s) [phases with closed switch at the sending end], and the line-to-ground voltages on the open phase(s) [phases with open switch at the sending end] are always less than the rated line-to-ground voltage of the system. 20. T F Transformers with triplex construction use three singlephase core-coil assemblies in a common tank with no magnetic coupling between the three phases within the transformer tank. 4

5 21. T F Ferroresonance is very unlikely during energization or deenergization of a three-phase cable circuit and an unloaded three-phase transformer connected to the circuit, using single-pole switches located at the source end of the cable circuit, if the transformer primary windings are connected in grounded wye and the three-phase transformer employs triplex construction. 22. T F Ferroresonance is very unlikely during energization or deenergization of a three-phase cable circuit and an unloaded three-phase transformer connected to the circuit, using single-pole switches located at the source end of the cable circuit, if the transformer primary windings are connected in delta. 23. T F Ferroresonance can not occur when energizing, with single-phase fused cutouts (switches), unloaded transformers banks made with three single-phase pole-top transformers with ungrounded primary windings (connected in floating wye with secondary windings in delta) in the overhead system when the fuse cutouts are on the same pole as the transformer bank. 24. T F When energizing with single-pole switches a floating-wye delta transformer bank with unbalanced load connected to the secondary (delta) side, the voltage from the primary terminal to ground at the transformer whose cutout has not yet been closed can never exceed times the system nominal phase-to-ground voltage. 25. T F When a current-limiting fuse is applied at the HV terminals of a single-phase single-bushing distribution transformer with primary (HV) winding connected from phase-toground (neutral), rather than a fused cutout (expulsion fuse), the transformer is less likely to rupture should a high-current fault occur internally from a primary (HV) winding lead to the grounded tank of the transformer. 26. T F In a secondary system with a nominal phase-to-phase voltage of 240 volts, the three measured phase-to-phase voltages are 235 volts, 230 volts, and 222 volts. Using the NEMA definition for voltage unbalance, the percent voltage unbalance is %. 5

6 27. T F Ferroresonance will not occur in Con Edison network systems supplied from 13.8 kv or 27 kv primary feeders because the network transformers use the grounded wye connections for both the primary (HV) and secondary (LV) windings. 28. T F Con Edison three-phase pole-top distribution transformers use the delta connection for the primary (HV) windings (13.8 kv and 27 kv systems), and the grounded-wye connection for the secondary (LV) windings to supply either 208Y/120-volt or 480Y277 volt secondary systems. 29. T F Con Edison three-phase pole-top transformers in the cylindrical tank utilize a four-loop five-legged core design, and three-phase pole-top transformers in the rectangular tank utilize triplex construction (three single-phase corecoil assemblies). March