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Power System Protection Dr.Prof. Mohammed Tawfeeq Alzuhairi

Tutorial Nos. 4 & 5

1.A radial distributor line consists of four substations designated A,B,C,and D of powers 40

, 30, 25, and 20 MW respectively . The substations are connected by transmission lines

having the following impedances per phase (p.u.):

AB = j 0.20 ; BC = j 0.10 ; CD = j 0.10 .

The system is fed at A with 33 kV from a source of negligible impedance. All circuit breakers

having operating time of 0.1 second and are controlled by relays fed from current

transformers with ratios: 400/5A at D, 800/5A at C, 1000/5A at B and 1500/5A at A. The

characteristic of the overcurrent relay used is given in Fig. 1. Sketch the arrangement of the

system and determine the satisfactory relay settings of breakers at D and C considering only

3-phase fault with short circuit MVA = 75.

Fig.1

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2. Figure 2 shows a simple radial transmission line system. Lines AB , BC and BD have an

impedances of 0.8 Ω / km , 1.0 Ω / km and 0.8 Ω / km respectively. The distance relay at bus A is fed

by current transformers rated at 2000/5 A and a voltage transformers rated at 354 kV/ 120V. Find

zone 1 and zone 2 setting of the relay.

A B

100km D

100 km

C

40 km

Fig.2

3. Consider the 230 kV transmission system shown in Fig.3 Assume that the positive –

sequence impedances of lines LAB and LBC are 1+j 10 and 1.5 + j 15 Ω respectively. If the

maximum peak load supplied by the line LAB is 100 MVA with a lagging power factor of 0.9,

design a three – zone distance relaying system for RAB impedance relay by determining the

following:

(a) Maximum load current.

(b) CT ratio.

(c) VT ratio.

(d) Impedance measured by relay.

(e) Load impedance base on secondary ohms.

(f) Zone 1 setting of relay RAB.

(g) Zone 2 setting of relay RAB.

(h) Zone 3 setting of relay RAB.

(i) Draw the zones of protection for the system and suggest time setting for each

zone.

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A B C

RAB RBC RCD

Fig.3

4. A single –phase two – winding 100kVA 2400 / 240 V step down transformer is to be

differentially protected as shown in Fig.4.

(a) Choose appropriate CT ratios.

(b) Verify that the relay will not operate with the chosen CT ratios.

(c) Determine the ratio k= Nr: No such that the relay will tolerate a mismatch in current up to

20% of I1

Fig.4

5. (a) Consider a three-phase Delta-Wye-connected two-winding transformer bank that is to

be protected by using percentage differential relays. Assume that the high-voltage side is

connected in delta and sketch the necessary wiring diagram for connecting three differential

relays one for each phase.

(b) For a 1000kVA, 11kV/400V, delta/star, three-phase transformer, (i) calculate the turns

ratio of the current transformers of a nominal 5A secondary current.

(ii) Determine the ratio k if the relay is to tolerate a mismatch in current of up to 30 %.

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6. Fig. 5 shows a 345kV transmission loop (ring system). Table 1 gives positive – sequence

line impedances as well as CT and VT ratios at B12 for the system shown.

(a) Determine the impedance relay settings (Z1, Z2, Z3) for the B12 three-zone, directional

impedance relay connected as shown in Fig.3B. Consider only solid, three-phase fault.

(b) The maximum current for line 1-2 during emergency loading conditions is 1500 A at a

power factor of 0.95 lagging. Verify that B12 does not trip during normal and emergency

loadings.

Table 1

Fig.5

7. (a) Consider a three-phase 138 / 69 kV ,75 MVA , delta-wye – connected two – winding

transformer bank that is to be protected by using percentage differential relays. Assume that

the high voltage side is connected in delta and sketch the necessary wiring diagram for

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connecting three differential relays one for each phase. indicate in your diagram all the

currents flowing through the restrain windings of all relays.

(b) For the above transformer, choose CTs ratio of the relays for both the primary and

secondary sides. Also find the ratio of the CT secondary of the HV side to the restrain

winding current at LV side.

(Ans: HV side 400/5 , LV side : 700/5 ,

)

8. Consider the system shown in Fig. 6. Assume that the three lines are identical with positive

sequence impedance Z1 = j 0.1 p.u. Assume that the six line breakers are controlled by zone

distance and directional relays. Consider only three-phase faults:

(a) Determine zone 1, Zone 2 and Zone 3 settings for all distance relays in per-unit.

(b) If the CTs are rated 400:5 A , and the VTs 133kV:115 V, convert the settings into ohms .

( c) Discuss relay operations for a fault at point X, assuming X is 10% down the line L31

from bus 3.

Fig.6

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Power System Protection

Tutorial 4 Typical problem solutions

Problem-1

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8

Problem -2

Solution

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Problem -3

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Problem 4

Solution

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Problem 5

Solution

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13

Problem 6

Solution

(a)

14

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Problem 7

Solution

The solution is shown in figure below:

(b)

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Problem 8

Solution

(a) The reach are given by :

Zone 1 : Z1 = 0.1 x 80% = 0.08

Zone 2 : Z2 = 0.1 X 120% = 0.12

Zone 3 : Z3 = 0.1 x 250% = 0.25

Because of systems symmetry, all six sets of relays have identical settings.

(b) Recall that

The equivalent instrument transformer’s secondary quantities are :

Ω

(c ) Locate point X on the diagram in Figure . We comment on all line breaker operations:

B31 fault is in zone 1 , instantaneous operation ,

B32 directional unit blocks operation.

B23 fault is in zone 2 , delayed operation. B31 should trip first , preventing from

tripping .

B21 fault duty is light. Fault in zone 3 if detected.

B12 directional unit blocks operation.

B13 fault is in zone 2 (just outside of zone 1) , delayed operation.