ECE 504 Session 33, Page 1/1
Advanced Power Systems Protection Spring 2009
ECE 504‐AP Homework 6, Transformer Protection
Due session 36 (April 20 on campus and May 4 for Engineering Outreach)
1. Consider the power system in shown in Fig.1
a. Show the CT connections and relay connection if you were going to protect the transformer by
means of an old electro mechanical relay such as Westinghouse HU relay.
b. Select the HV and LV CT’s to perform differential current protection, (Turns ratios, C rating etc )
so that the CT’s do not saturate for the 1st ½ cycle (use realistic CT ratio for, example you don’t
get a 3150:5 CT standard, use a 3000/5)
c. If the relay was a microprocessor relay, what Matrix compensation would you use for the primary
and secondary windings?
d. If the relay you used did not calculate its own tap ratio how would you calculate the TAP ratio so
that the CT you selected combined with the TAP ratio for that winding would give a per unit value
of 1 at full load.
e. Assume the transformer is equipped with an OLTC tap changer how would you determine the
minimum operate current setting and Slope setting for the differential element
2. The System shown in Fig.1 is capable of single pole tripping.
a. Calculate the restraint and operate current as seen by the differential relay if the B-phase poles are
open on the 400 kV line.
b. Calculate the Restraint and operate current for an external single phase to ground fault 50% down
c. Repeat part by but this time assume one of the phase CT’s saturates by 100% (i.e. it produces no
d. Calculate the restraint and operate current for an internal 3-phase transformer fault.
e. Calculate the unrestraint differential element setting (87UP)
f. Calculate the overcurrent backup setting for the relay
3. Explain , with the aid of a sketch the operating principle for the Restricted earth fault element.
4. Explain the difference between Harmonic restraint and harmonic blocking as referred to transformer differential
5. Overfluxing in Transformers
a. Discuss why over-fluxing leads to a transformer drawing odd harmonic currents and why we need
to block the differential element when an excess of these harmonic are detected. Also discuss
which harmonic is used to detect over-fluxing and why this harmonic is used.
b. Discuss an alternative method for detecting overfluxing.
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