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Q: Star transformer, 3 phase system ( No Answer,   1 Comment )
Subject: Star transformer, 3 phase system
Category: Science > Physics
Asked by: pkuanko-ga
List Price: $5.00
Posted: 14 Aug 2006 18:16 PDT
Expires: 03 Sep 2006 17:50 PDT
Question ID: 756034
A 66kV/11kV 10MVA star transformer with impedance of 0.06p.u. is
connected to a solid voltage source of 69 kV. A delta connected
balance load of impedance R = 60 ohms. Determine the total power to
the load by
(a) converting all elements to single phase equivalents
(b) convert impedance to per unit
(c) solve for current/power single phase per unit
(d) convert to 3 phase MVA.
Please give me full working, I will give you a good tip.
There is no answer at this time.

Subject: Re: Star transformer, 3 phase system
From: neilzero-ga on 15 Aug 2006 11:28 PDT
Perhaps I can help by translating some of the abrieviations:
66 kv = 66,000 volts primary voltage, applied to each of the 3 points
of the star = Y. This is the design specs of the transformer, which is
likely three separate transformers inside a box
11 kv secondary voltage. More transformer design specs
10MVA = 10,000,000 volt amperes = volts times amps: divide by 11,000 =
910 amps but that is not exact as the 3 phases add vectorally at 120
degrees of phase shift.
Impediance is normally measured in ohms resistive and ohms reactive. I
have no idea of the meaning of 0.06p.u.
60 ohms is likely the resistance connected to each of the three points
of the delta, so there are 3 separate resistors (more likely it
appears as if it is three separate resistors of 60 ohms each) We can
assume the power factor of the load is 100% since no reactive power is
suggested, unless it is suggested by p.u.
p.u. may mean per unit which could refer to the 3 units of the load or
to the 3 separate transformers in the Star transformer box
MVA = megavolt amps which is likely about 1/4 of the star transformer
rating of ten million VA = 2.5 MVA This is because slightly more than
the rated input voltage is applied, but the load is only about 1/5 of
the Star transformer rating. Sorry I do not know how to get the
correct answers.  Neil

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