Question

A three-phase transformer has a nameplate rating of 20 MVA,
and the voltage rating of 345kV wye /34.5kV delta with a leakage
reactance of 12% and transformer connection is wye-delta. Select a
base of 20 MVA and 345 kV on the high-voltage side. Please
determine the following:

(a) Turn ratio of windings

(b) Transformer reactance referred to low-voltage side in ohms
(c) Transformer reactance referred to low-voltage side in per
units

Answer #1

a) A three phase transformer rated at 30 MVA 10 kV(Δ)/1 kV(Y) is
supplying a load at the low voltage side. The load absorbs 5 MVA,
0.7 power factor lagging at line to line voltage 0.9 kV. The
transformer has an equivalent reactance 0.05 Ω/phase. Assuming 10
MVA and 1 kV as base value, draw the per unit diagram for the
system. Find the Line to Line voltage at the primary side of the
transformer.
b) A three-phase synchronous generator...

Use MATLAB:
A three-phase Y-A transformer is rated 225-kV:24-kV, 400 MVA and
has a series reactance of 6.08 Ohms as referred to its high-voltage
terminals. The transformer is supplying a load of 375 MVA, with
0.89 power factor lagging at a voltage of 24 kV (line-to-line) on
its low-voltage side. It is supplied from a feeder whose impedance
is 0.17 + j 2.2 Ohms connected to its high-voltage terminals. The
total load in the system remains constant at 375 MVA....

Problem 2. A three phase transformer rated at 5 MVA, 115/13.2 kV
has a per phase series impedance of 0.007 + j0.075 per unit (shunt
parameters are ignored). The low voltage side is connected to a
short distribution line which modeled by a series per phase
impedance of 0.02 +j0.1 per unit on the base of 10 MVA. The line
supplies a balanced three phase constant impedance load at 4 MVA,
13.2 kV with power factor of 0.85 lagging
•...

The transformer is protected by a differential relay. The relay
is connected to three phase transformer, 50 MVA, 22 kV/33 kV delta
– star connection. The relay is connected in delta. The current
transformer ratio is 1500/5 Amp at the lower voltage and 1000/5 Amp
at the high voltage. Calculate the primary and secondary current of
the relay and it’s the primary turns. State any assumption.

•The 3-phase system consists of a generator, circuit breaker, power
transformer and extra high voltage transmission line.
•The generator is rated at 60Hz, 1559Mva and 24kV. The
sub-transient reactance is 2*%. The winding connection is wye with
the neutral grounded.
•The transformer is a 1550Mva, 3-phase, shell type, 2-winding
transformer with a 24kV delta winding and a 525kV grounded wye
winding. Neglect saturation and core losses. The exciting current
is 1.0% of the rated current. The capacitance of each low...

A three-phase wye-connected (Y) synchronous generator is
operating at Ea = 8.OLO° kV (lineneutral) and has synchronous
reactance of X~ = 1.0 fl . The generator is connected to a
threephase delta-wye connected step-down transformer that is
constructed from three individual single-phase transformers. Each
the single-phase transformers has rated power of 200 kVA has a
turns ratio of 4:1 where the rated primary voltage of 8.0 kV and a
rated secondary of 2.0 kV and a per-unit series reactance of...

A 100 MVA, 13.2 kV three phase generator (G) having a
synchronous reactance of 10% is connected to a three phase Y-Y
transformer T1 which feeds a 132 kV 10 miles transmission line
having an impedance per phase of 2 + j5 ?/mile. At the receiving
end of the transmission line is a Y-Y step down transformer T2.
Three loads are connected to the secondary side of the transformer
T2. Loads are as follows:
Load#1 40 MVA at 0.8 pf...

A single phase 3kVA 480/240 V 50 Hz transformer has the
following parameters:
High voltage side: r1 = 0.25 Ω Xl1 =
0.75
Low voltage side: r2 = 0.05 Ω Xl2 =
0.18
a) Draw the equivalent circuit with
quantities referred to the low voltage side;
b) Determine the voltage regulation when the transformer
is supplying full load at 240 Vrms
and 0.9 power factor lagging;
c) If the load terminals are accidentally
short-circuited determine the currents in the high...

3) A single phase, 100 MVA, 200/80 kV transformer has the
following test data:
open circuit: 20 amps, with 10 kW losses
short circuit: 30 kV, with 500 kW
losses
Determine the model parameters. Part of
the problem is to determine on which side of the transformer the
measurements are made on.

A Wye-Delta transformer is connected to a 66 kV input
three-phase transmission line. Each transformer is rated at 1000
kVA and has a step-down ratio of 8:1. The bank supplies a balance
load having a power factor of 0.8 lag with a load current of 70 A.
Assume ideal transformer, determine.
The primary line and phase voltages
The secondary line and phase voltages
The secondary line and phase currents
The apparent and active power at the load
Total kVA capacity...

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