Electrical Power and Machines Depart.
Power System Protection Application
Tanta University Faculty of Engineering
Sheet 2: Transformer Protection
1. A single-phase transformer is rated at 110/33 kV, 50 MVA. It has an overload
capability of 120 %. The 110 kV side has a tap changer with (5/8) % step size, and
with a range of 99 to 121 kV. Determine CT ratios for a percentage differential relay
to protect this transformer. Assume a maximum CT error of 5 % in either of the two
CTs. What is the minimum slope of the percentage differential characteristic of the
relay? Assume that both the relay windings are provided with taps for 3.0, 4.0, 4.8,
5.0, 5.2, 6.0 A, and determine the appropriate taps which will minimize the required
slope of the relay characteristic. What pickup current setting for the relay would you
recommend?
2. If the transformer of problem 1 had no tap changer, could you accept a lower slope?
Explain your answer.
3. Consider the three-phase transformer bank. The transformer is rated 500 MVA at
34.5(delta)/500(wye) kV, assume that an internal three-phase fault develops at the
bushings on the wye side (within the zone of protection of the differential relay) of
the power transformer. On the transformer base, the system impedances on the wye
and the delta sides are (0.0 + j0.3) pu and (0.0 + j0.2) pu, respectively. The leakage
impedance of the transformer is (0.0 + j0.08) pu on its own base. What are the currents
in the restraining and operating windings of the differential relays? Will the relay
operate?
4. For a phase-to-phase fault (outside the zone of the differential relay) on the wye side of
the transformer of problem 3, calculate the restraining and operating currents in each of the
relays. Will the relay operate for this fault? How do the fault currents compare with the
three-phase fault currents determined in problem 3?
5. For a three phase- fault (inside the zone of the differential relay) on the wye side of the
transformer of problem 3, Calculate the maximum fault resistance Rf that make the relay
will not operate for this internal fault ?
6. Assume for this problem that the 69 kV system (Figure 1) is open and make the
following calculation:
Draw the differential protection on the main transformer
Select CT ratio and relay setting
Electrical Power and Machines Depart.
Power System Protection Application
Tanta University Faculty of Engineering
Calculate the internal fault current in the three phases for phase-a to-ground fault
on the 69 kV terminals through 10 Ω. Will the relay operate or not?
For the internal phase-a to-ground fault on the 69 kV terminals through 10 Ω, Find
the percentage of unprotected winding.
Figure (1)
