Continuing Education Course #042
"Basic Tools for Electrical Protection and Short
Circuit"
Test Worksheet
6. The zero sequence vector is normally expressed by:
a. Va = 1/3(Va + aVb + a2Vc)
b. Vb = 1/3(Va + Vb + Vc)
c. none of the above
1. Overcurrent is________.
7. The negative sequence components system. Vector
a. the amount of current that flowing in appropriately Va cuts the axis ____ on rotation, then aVc ____
designed equipment will result in loss, damage or both degrees later and then a2Vb ____ degrees later.
b. to be in excess of the equipment design and of the
a. 120, first, 240
correct design value
b. first, 249, 129
c. Both of the above
c. first, 120, 240
d. None of the above
8. For transformers, the impedances are the zero,
2. A symmetrical AC is described by a system
positive and negative but the ____ sequences are equal.
a. with three voltage waveforms of equal magnitude.
a. positive and negative
b. separated by 90 degrees.
b. reactance
c. separated by 120 degrees.
c. resistance
d. a and b
e. a and c
9. Three phase core type transformers exhibit zero
f. None of the above
sequence reactances that are equivalent to a closed
delta winding due to its magnetic design are present in
3. The basic principle of symmetrical components
three phase_____.
method is applied to
a. core type transformers
a. unbalanced system represented by three phase
b. Shell type transformers
circuit vectors.
c. Saturable reactors
b. balanced system of vectors separated by 120
degrees.
10. Normally synchronous rotating equipment, such as
c. None of the above
generators and synchronous motors specify ______
reactances.
4. The vector “a” can be expressed in the following
a. Zero
manner:
b. Three
a. 1|120.
c. Positive sequence
j90
b. ε .
c. 1+j45.
11. ____ allows highest initial fault current value.
a. High speed relay
5. We can represent any three vector system by three
b. Low speed relay
vectors systems that are _______in magnitude and
c. Subtransient reactance
separated by either 120° or Zero.
a. zero
12. For a synchronous generator the ____ is the
b. equal
reactance value after about 0.1 s of the start of a fault.
c. different
a. transient reactance
b. subtransient reactance
c. synchronous reactance
13. The _____ relay provides protection for large
generators as well as smaller generators in back up
units, such as in peaking plants.
a. Positive sequence
b. Zero sequence
c. Negative-sequence
20. If the Base KVA of a system is 4000 KVA, the
base voltage is 100 V and the impedance of the
equivalent impedance at the short circuit point is 5.0
ohm at an angle of 36.8699 degrees, the p.u. value of
its reactance is _______.
a. 0.0001
b. 0.0012
14. _____ will permit the protection engineer to set the c. 0.0005
protective devices as well as evaluate the proper
settings of electrical equipment.
21. The most common per unit values chosen as base
a. ANSI Standard C38.010-1979
are the _________.
b. IEEE 323
a. base Voltage and the base KVA
c. Short circuit calculations
b. base impedance and the current
c. none of the above
15. For high voltage (above 600V) breakers, the
ratings use _______ current values based on ANSI
22. In a 1000 KVA transformer with an impedance of
Standard C37.5-1979.
8.5% it is to be converted to a 20000 KVA base, the
a. Asymmetrical
per unit value of the reactance is (assuming zero
b. Symmetrical
resistance):
c. Zero sequence
a. 0.044
b. 0.17
16. The per unit value of a voltage in a system is the
c. 0.055
ratio of the voltage to the ____ voltage.
a. Design
23. A 500 HP induction motor has an approximate
b. Equivalent
KVA of:
c. Base
a. 450
b. 475
17. The value used for impedance in the sources is the c. 500
_____ Reactance "X".
a. Subtransient
24. In order to perform coordination between
b. Transient
protection devices, the curve of the device that trips
c. Zero
first must be located _____ the curve used by the next
tripping device.
18. If the base voltage for a system is 69000 V and the a. to the Left of
voltage at a motor in the HV MCC is 13.8 kV. The p.u. b. to the Right of
value of the voltage at the motor is_____.
c. Intercepting
a. 0.100
b. 0.300
25. The main purpose of coordination is to ______ the
c. 0.200
fault area without interruption of service or power to
the rest of the components of the system.
19. The resistance in a transformer can be normally
a. isolate
_____in short-circuit calculations.
b. modify
a. neglected
c. coordinate
b. added to the reactance
c. set to j0.45