8/13/2019 Westinghouse - Electrical Trannd Distribution Reference Book 128

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Power Transformers and Reactors

ciency becom e less apparent as the transformation ratioincreases, so that autotransformers for power purposes areusually used for low transformation ratios, rarely exceeding2 to 1.

XI. GROUNDING TRANSFORMERS

A grounding transformer is a transformer intended solelyfor establishing a neutral ground connection on a three-

phase system. The transformer is usually of the star-deltaor interconnected-star (zig-zag) arrangement as shown inFig. 27.

The kva rating of a three-phase grounding transformer,or of a grounding bank, is the product of normal line-to-neuh-al voltage (kv) and the neutral orgrozbnd amperes thatthe transformer is designed to carry under fault conditionsfor a specified time. A one-minute time rating is often usedfor grounding transformers, though other ratings such asthose suggested in AIEE Standard for Xeutral GroundingDevices (No. 32, May 1947) can be specified dependingupon the probable duty to be imposed on the unit inservice.

Rated voltage of a grounding transformer is the line-to-line voltage for which the unit is designed.

When operated at rated three-phase balanced voltage,only exciting current circulates in the windings of a ground-ing transformer. Current of appreciable magnitude beginsto flow in the grounding circuit only when a fault involvingground develops on the connected system.

Grounding transformers, particularly the zig-zag type,normally are designed so that rated neutral current flowswhen a solid single-line-to-ground fault is applied at thetransformer terminals, assuming supply voltage to be fullymaintained. This is equivalent to loo-percent zero-sequencevoltage impressed at the transformer terminals resulting inthe circulation of rated neutral current. Transformers so

designed are said to have loo-percent impedance based onrated kva and rated voltage.Sometimes a resistor or other impedance is connected

in the transformer neutral, and in these cases it may bedesirable to specify that the grounding transformer shallhave less than the conventional 100 percent impedance.Equivalent circuits for star-delta and zig-zag groundingtransformers with external neu tral impedance are includ-ed in the Appendix.

Because a grounding transformer is a short-time device,its size and cost are less than for a continuous duty trans-former of equal kva rating. The reduced size can be estab-lished in terms of an equivalent two-winding 55 C kvaU, by applying a reduction factor K to the short-timerated kva of the grounding transformer, and this reducedkva can be used for a price estimate.

U, = Uo X KS for a three-phase grounding unit. (56)U, = 3 Uo X Kl for a bank of single-phase grounding

units (57)where

U, = equivalent two-winding ,55C kva, three-phase.UC= (line-to-neutral kv) X (rated neutral amperes).

Values for K are listed in Table 12 for various types and

Table 12-K FACTORS FOR DETERMINING EQUIVALTWO-WINDING 55 C KVA OF GROUNDING TRANSFORM

Star-Delta Zig-Zag ConnectionTime

Rating ;I;- ;:;tkq,3f~~v~ 46 kv 1 69 kv

Ka, For A Three Phase Unit

10 seconds 0.064 0.076 1 0.080 0.0851 minute 0.170 0.104 0.110 0.113 0.1182 minutes 0.240 0.139 0.153 0.160 0.1673 minutes 0.295 0.170 0.187 0.196 0.2044 minutes 0.340 0.196 0.216 0.225 0.2355 minutes 0.380 0.220 0.242 0.253 0.264

K1, For A Single Phase Unit (One of three in a ban

1 minute 0.057 0.033 0.03i 0.040 0.0432 minutes 0.080 0.046 0.051 0.055 0.0603 minutes 0.098 0.057 0.064 0.068 0.0744 minutes 0.113 0.065 0.073 0.078 0.0845 minutes 0.127 O.Oi.7 0.082 0.088 0.005

*These valuev are cnlculated on the basis that the initial uve~we windperature is not over 7.52. that the heat from load lossev is nil stored in former. and that the final temperature will not ruceed VZLIUBY permitted ards. The values are applicable only for groundinp trsnsform ers designed 100 percent impedance.

classes of grounding transformers; the table includes vfor both three-phase and single-phase units, thoughsingle-phase type is uncommon.

Conventional power transformers may be connecserve solely as grounding transformers, but the currentime ratings for grounding service are open to qudepending upon the form and details of construction. these modified ratings are desired, they should be obfrom the transformer manufacturer.

Star-Delta Impedances-The impedance tosequence currents in each phase of a solidly-grounded delta grounding bank made up of single-phase u

equal to Zrs, the ohmic leakage impedance betweeprimary (star) winding and the corresponding sec.(delta) winding:

ZO=ZPS

Percent zero-sequence impedance is normally expresterms of short-time kva and line-to-line voltage:

Zo = ZPS x UG10 x kv2

In a three-phase star-delta grounding transformer 2be smaller than Zps by an amount depending on theof core construction: a typical ratio of 20 to Zps though variation from this value for different is likely.

Zig-zag Impedances-The impedance to sequence currents in each phase of a solidly grouzig-zag bank can be derived on a theoretical bareference to Fig. 27(d).

Eo=IoXZp~--ep+ep.

ElT=Zo=Zps.0

Percent zero-sequence impedance for the zig-zag co