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EEE340 Protective Relaying
Assignment 2

Notice:
– The number in the column on the right indicates the approximate marks for the question.
– Only electronic copy (Word or PDF) submission is required.

Q1
a) A set of unsymmetrical three phasors can be decomposed as sum of a
set of symmetrical sequence components. What are these symmetrical
sequence components? Please explain in detail by their magnitude,
phase displacement and sequence.
(6)
b) For a set of unsymmetrical phasors ௔̇, ௕̇ and ௖̇, if the symmetrical
sequence components of phase A are given as ௔̇(ଵ), ௔̇(ଶ), ௔̇(଴), and
= ௝ଵଶ଴೚ , calculate ௔̇, ௕̇ and ௖̇ by ௔̇(ଵ), ௔̇(ଶ), ௔̇(଴).
(6)
c) If ௔̇, ௕̇ and ௖̇ are given, calculate the zero sequence component
௔̇(଴) of phase A.
(4)
d) In Figure 1.1, there is a single line-to-ground fault on phase a, the
symmetrical sequence components at fault location have following
relations:
௙௔(ଵ) + ௙௔(ଶ) + ௙௔(଴) = 0
௙௔(ଵ) = ௙௔(ଶ) = ௙௔(଴)
Draw the interconnected sequence network for the single line-to-
ground fault.
(4)
Total
20
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Figure 1.1




Q2
a) In Figure 2.1, if the input voltage and current to the directional
overcurrent protection 1 are phase voltage UA and current IA, what is
a dead zone for this protection?
(4)
b) For zero sequence current directional protection, is there any dead
zone for the directional element? Why?
(5)
c) In Figure 2.2, the fault current through line A-B is indicated as IAB,
the fault current supplied by A’ is indicated as ஺஻ᇱ , the fault current
through line B-C is indicated as IBC. The instantaneous protection
(zone I) of 1 is set as point d. How to calculate the setting value for
Zone II of protection 2? Include figures in your explanation.
(6)
d) What is an inverse time overcurrent protection? What is different
between its starting current and instantaneous operating current?
Include figures in your explanation.
(5)
Total 20

Figure 2.1
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Figure 2.2


Q3
a) The input voltage and current for an impedance relay are set as:
௠ = ஻஼, ௠ = ஻஼, can this relay be used to detect a line-to-line
fault on phase B and phase C? Why?
(6)
b) With a potential transformer PT, a current transformer CT, a reactance
transformer UR and a voltage transformer T given, please design a
circuit of impedance relay by amplitude comparison to get a
directional circular characteristic.
(6)
c) With the same conditions of (b), what is the operating characteristic
equation of this relay?
(4)
d) In Figure 3.1, the amplitudes of power sources EM and EN are equal,
their phase angle difference is δ, equivalent impedance between two
power sources is , the impedance phase angles of
the whole system are identical. Draw a phasor diagram to indicate
relations among voltage EM and EN of power sources and voltage of
any point F on line lM-N.
(4)




Figure 3.1

Total
20
NLM ZZZZ 
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Q4
a) What are the four communication technologies mainly used in pilot
protections?
(4)
b) What are the three working modes for Power Line Carrier Pilot
Protection?
(3)
c) If a distance pilot protection is the main protection for a transmission
line and a distance protection works as the backup protection, when
the backup protection is under maintenance, the main protection has
to be stopped, why?
(3)
d) If the differential relay for a pilot protection uses static setting value
to avoid the possible maximum unbalance current, the sensitivity may
not be enough. How can we utilize dynamic setting value to solve this
problem?
(10)


Total
20

Q5
a) Directional Blocking Pilot Protections are installed from protection 1
to protection 6 as shown in Figure 5.1, the construction of each
protection is given in Figure 5.2. A three-phase short circuit occurs at
point k on line B-C. Please explain the complete working process of
all protections in details.
(10)
b) The operating time for single pole autoreclosure is normally longer
than three pole autoreclosure. Please explain the reason in details.
(10)
Total 20

Figure 5.1
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Figure 5.2