CT’s, PT’s and PROTECTION
• What is the inrush current peak of the transformer?
6 to 8 time that of full load current.
• Why REF is now is used for HV side also in GT/SUT?
The E/F setting of differential is poor.
• Why IDMT over current relay is always used as backup?
Because setting has to be 200% to emergency loading and TMS be large to grade
with feeder. Therefore very slow for internal faults/terminal faults/uncleared LV
faults.
• Purpose of standby E/F protection in SUT/UT?
Back up for LV winding, LV neutral CT- CDG 12 – resistance earthing – relay set
high time delay to discriminate with LV feeder and trip transformer if sustained E/F,
also protects neutral earthing resistor.
• Why do we use O/C & E/F protection on both sides of transformer?
Power in feed exists on both ends.
• Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?
Delta of aux. Transformer prevents E/F currents from grid into DG neutral.
• Why 100% winding protection is felt essential for main generator stator E/F
protection? (Used in NAPS onwards?)
At MAPS 4% of winding is not protected. Earlier felt that the Electro magnetic stress
due high external fault currents near 4% of neutral may not be high to cause E/F
here. But now felt that the mechanical stress can leads to E/F.
• How 100% winding protection is given there?
a) Inject sub harmonic AC current into generator neutral. Monitor its amplitude. E/F
impedance reduces so current drawn increases and trips (Not used).
b) 3rd harmonic voltage monitored on neutral, fault near neutral upto 25% winding.
3rd harmonic voltage reduces to zero. Above this 3rd harmonic voltage increases,
so combined both 3rd harmonic and zero sequence relays for 100% covering, no
blind zone.
• What is the basic purpose of class-B protection?
Class-B avoids load rejection. For modern machines, the inertia is less and easily
gets damaged on overload. Therefore trip only for internal faults.
Low forward interlock prevents the risk of run away if a CIES valve fails to close.
• What are the effects of GT over fluxing?
a. Eddy current circulation.
b. Magnetising current increases
c. Winding temp increases
d. Transformer noise/vibration increases.
e. Over heating of non laminated metal parts (affected by stray fluxes)
• Why stabilising resistor in REF or residual E/F scheme?
Required against CT saturation under heavy through fault currents.
• Why in transformer the LV CB also be tripped along with HV CB for a primary side
fault?
Auxiliary transformer 415v delta star transformer, if HV CB alone tripped then back
feeding from LV side (say DG runs parallel with transformer)—arcing voltage at the
fault on primary—fault fed for more time – more damage.
• Why high impedance circulating current differential?
Under through faults, CT’s of different phases saturates differently. Net spill current
will operate low impedance CAG relay, so high impedance scheme with CAG
relay and stabilising resistor used.
• How to reduce the CT error?
Error reduces if load increases.
• What is the advantage of housing CT’s with in bushings?
Bushing acts as a primary insulator for the CT.
• Why the earthing transformer primary voltage is 16.5 kV rated in main generator
even though actual voltage during the E/F is root 3 times less?
The transformer should not saturate during E/F otherwise it will cause
ferroresonance with the GT winding capacitance. Dangerous O/V and neutral
shifting will occur. During loss of load or field forcing conditions, the transformer
voltage increases to cause saturation. Saturation can also occur due to point on wave
of application causing flux doubling.
• Where are the following relays used?
a) Very inverse b) extremely inverse relays c) definite time O/C Relay d)
instantaneous O/C Relay.
a) Very Inverse – Used where inverse protection reduces substantially as distance from
source increases, operating time doubles for a fault current reduction from 7 in to 4 in,
used where the short ckt current is independent of generating conditions.
b) Extremely inverse – Used for feeders subjected to peak making currents. Grade with
HRC fuses, e.g. Refrigerator, pumps.
c) Definite time O/C Relay – Where neutral is resistance earthed- fixed ground current.
d) Instantaneous O/C Relay – Used along with inverse O/C relay – to get higher grading
margin. Disadvantage – Under minimum generation it may not operate.
• Why delta – delta CT’s are used for star – star transformer differential protection?
Say primary neutral is not solidly earthed. Then for any earthfault on secondary
terminal, the primary current distribution is so for external fault, the differential is
likely to operate if sequence current from flowing into relay. The 2:1:1 distribution is
possibly only for core type or delta tertiary.
• Show the CT characteristics.
Knee point region (Protection characteristics)
Saturation region
Peak flux density
Linear region
Ankle point
(Measuring CT characteristics)
RMS amp turns
• What is knee point?
Knee point is the region, where 10% increase in flux causes 50% increase in exciting
ampere-turns.
• When will you say that the CT is saturated?
When checking the CT with the secondary injection method a 10% increases in the
voltage causes a 50% increase in the current the CT is said to be saturated.
• What is the problem anticipated due to CT saturation?
The CT will not be able to drive the current through the circuit causing nonoperation
of relays. In some other case when the currents in the two phases are
compared for relay operation the relay may operate due to unbalance.
• How can you de-saturate the CT?
Pass ac current through the primary and vary the current from zero to maximum with
secondary in shorted condition.
Pass dc current in the secondary and vary it from zero to maximum.
• Why CT should not be open circuited?
Very high voltage will be induced in secondary due to less back emf resulting in the
failure of the insulation.
• What precaution should be taken while removing a current operated relay when the
equipment is in service?
Ensure that the CT is not getting opened by shorting the appropriate terminals.
(Eventhough the terminals are automatically shorted once relay is removed the above
point may carried out to ensure the same)
• What do 10p15 mean?
When the current passed through the CT is 15 times the rated current then the
secondary current will have a composite error of 10%
• Where core balance CT’s are used?
In earth fault protection used. It senses the zero sequence current.
• What are the specifications of CT?
Protection CT - Error. Alf. KpV.
Metering CT - Error. Burden.
Differential CT - Class PS.
Core balance or E/F CT - 5-p type.
Primary current -
Rating of CT - 1. 15 ( full load current )
Short time rating - 1 sec.
• Why differential protection for PHT motors?
For more than 2500 kW motors it is required to provide differential protection. It is
biased Relay against internal phase fault or earth fault very fast. Insensitive to
starting current and stalling current.
• What are the errors of the following CT’ s 5p. 10p. 15p. At rated current?
5p - 1 % Ratio error ± 60 min phase error
10p - 3 % Ratio error ± 60 min phase error
15p - 5 % Ratio error ± 60 min phase error
• What is the operating point in the Magnetising characteristic of protection CT &
measuring CT?
Protection CT – Operation at ankle point only.
Measuring CT – Operation from ankle to knee point
• What is over voltage interturn test for CT?
With secondary open, pass rated current in primary for 1 min. Then check secondary
for insulation.
• A CT has 2 – secondary windings. If we use only one secondary winding can we
keep the unused secondary winding short circuited?
No. If it is short-circuited then the ratio will not get correctly. The turns of primary
winding will be shared between 2 secondary windings. So the unused secondary
winding should kept open.
• But is it advisable to keep the secondary of CT in open conditions? Will not induce
very high voltage?
If the CT has only one secondary winding, we should keep it always short cktd for
safety, but if the CT has multiple secondary, then if one secondary voltage is kept
limited by suitable loading, then the other secondary voltage is eventually limited
proportionately.
• Why PT fuse fails protection?
Mho relays will mal-operate if PT voltage is lost to the relay, so tripping blocked by
sensing PT fuse failure.
• What is the 2 stage stalling protection for PHT motor?
Because locked rotors withstand time of motor is less than starting time of motor
under reduced voltage conditions.
Stage 1 = 350% 6 sec for starting at rated voltage
(Because starting time = 6 sec + hot stall time = 7 sec)
Stage 2 = 175% 15 sec to permit 14 seconds starting time under reduced voltage
condition
(Since starting current is less, stage 1 will not operate)
• Purpose of start up protection? Is it always in service?
Trips the generator. If generator is excited with internal fault the over current 50s trip
the generator to prevent major damage. The earth fault relay 64c also. The relays are
polarised dc armature type, sensitive to all frequencies, since the frequency need not
to be 50 Hz initially during start up. Start up protection is cutout as soon as generator
CB is closed.
• What is the standard CT polarity?
Primary current enters at P1 and secondary current leaves at S2.
• Does over load relay give 100% guarantee against the single phasing?
No. It depends on the motor load and the motor winding (star or delta).
• What are the effects of single phasing?
a. Current will increase √3 times.
b. More heat in stator and rotor parts.
c. Insulation failure and short circuit & Ground fault may occur.
• What is the purpose of CT & PT?
For transformation of current, voltage to a lower level for the purpose of
Measurement, Protection and Control.
• Where CT secondary of 1A we are using?
For long distance current transmission, to reduce the IR drop.
• What is the nomenclature of English electric relay?
a) First letter-operating quantity
b) Second letter-movement
c) Third letter-application
d) Fourth letter-special variation.
• Define knee point voltage.
The voltage applied to secondary of CT keeping the primary open at which
10% increase in voltage causes 50% increase in excitation current.
• What is differential protection?
It is the current balance type protection, in which vector difference between current
entering the winding is used for relay operation.
• What are the checks on CT & PT?
a) Polarity checks
b) Insulation checks
c) Ratio checks
d) Knee point voltage (only for PS class CT)- magnetising characteristic test.
• What is Local Breaker Back up protection?
In case of local breaker fails to operate during fault due to mechanical failure this
protection will protect the system from sever damage. It will trip all the other
breakers in that bus after time delay.
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