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EE 4210 – 4210 – Power Power System Protection
Study of Numerical Over Current Relay
INSTRUCTED BY: Mr. Lathika NAME
:H.M.J.N. Herath
INDEX NO.
: 070170G
GROUP
: 04
DATE OF EXP
: /12/2010
DATE OF SUB
: 18/02/2011
OBSERVATION SHEET NAME
: H. M. J. N. Herath
INDEX NO
: 070170G
DATE OF PERFORMED : INSTRUCTED BY
: MR. LATHIKA
Current sensitivity for I >
Phase
Pickup Value
Drop off Value
A
1.44A
930 mA
B
1.11A
980 mA
C
1.04A
950 mA
Time characteristics for t > Curve Description
Rated Operating Time (s)
Observed Time
DT
0.02-0.04
999.4ms
SI30XDT
9.52-10.53
10.12s
I30XDT
14.3-15.81
15.1s
VI30XDT
12.82-14.18
13.83s
EI20XDT
24.66-28.67
28.35s
STI30XDT
1.68-1.87
1.872s
LTI30XDT
114-126
128s
Observed values characteristic curves for KCGG relay
Current(A)
Operating time (S)
Operating time(S)
TMS=1
EMS=.5
2
10.38
5.709
4
5.04
2.569
6
3.840
1.919
8
3.258
1.629
10
2.979
1.471
12
2.736
1.352
14
2.570
1.289
16
2.429
1.212
18
2.33
1.156
20
2.27
1.131
Standard inverse characteristics of the KCGG relay 12
10
8
) S ( e 6 m i T
4
TMS 1 2
TMS 0.5
0 0
5
10
15
currrent (A)
20
25
DISCUSSION
The major objective of relay is to detect abnormal conditions of power system and produce a signal to recover this abnormal condition. For this function relay should properly produce signal to isolate only faulty area. This process is known as discriminative. Reason is we should supply power to customer with minimumdisturbance. So to achieve this discriminative process grading between the relays is important. There are mainly two types of over current rela ys. They are, 1. Definite time relay: This type of relay is operated after a definite time period regardless of the magnitude of the current. 2. Inverse Definite Minimum Time (IDMT): Here the operating time is inversely proportional to
the current
Discuss about the three stages Over Current Characteristics of the KCGG Relay
There are three over current stages can be found in the KCCG relay. Settings of each stage are completely independent of each other. Stage 1 provides any of seven inverse time or definite time or definite time characteristics and an adjustable reset timer for the detection of intermittent faults while Stage 2 & 3 provide definite time setting for instantaneous operation. Three stages of this over current characteristic of KCCG relay give us to special option to change the over current characte ristics according to our requirement. What are the advantages of having various Time/Current characteristics?
Advantage of having various Time/Current characteristics is, providing the facility to coordinate and grad with other protection systems in an easy way. And also it facilitates to coordinate any numerical relay with much kind of protection systems easily.
Discuss the operation of IDMT Relay coordination system
IDMT is the short term of Inverse Definite Minimum Time. There are mainly two types of over current relays used in power system. They are mentioned previously. The IDMT Inverse definite minimum time relay is one such type of relay where the delay time of the relay is inversely proportional to the magnitude of the fault current. For example If the fault current is
of the magnitude of 8KA the delay taken by the relay to trip the circuit breaker may be 3 sec likewise if a fault current of 16KA persists in the System the IDMT delay time will automatically reduce to 1.5 sec and so on. It is used in applications of overcurrent relays and earth fault rela ys. Between these two IDMT is widely used. Reason is it is most secure one. IDMT has four types of characteristic depending on the application. a) Standard Inverse (Normal Inverse) – This is the widely used one and it is used when fault levels of near and far end of the system does not vary considerably. b) Very Inverse – This is used when there is considerable fault level reduction with the distance c) Extremely Inverse – Here operating time is proportional to the square of the current and therefore most suitable for grading with fuses and feeders which are subjected peak currents when switching. d) Long Time – This is used for protection of neutral earthing resistors and fault protection for transformer
IDMT relay work according to the induction principle. It consists of Al or cu disc. It is rotated between electromagnet and damping magnet. The fluxes induce eddy currents in the disc and they interact and produce rotational torque. According to the plug setting no of turns is change in exciting coil. The winding of the coil is provided with seven taps, which are brought to the front panel and the required tap is selected by a push-in plug. The time multiplier setting of the relay adjusts the “backstop” of the rotating disc. The time of operation is proportional to the distance through which the disc must rotate in order to operate the contact The torque of this relay is proportional to I 2 below saturation. If the core is made to saturate at very early stages such that with increase of I, K decreases so that the time of operation remains the same over the working range. If the core is made to saturate at a later stage, the characteristic obtained is known as IDMT. The time-current characteristic is inverse, over some range and then after saturation assumes the definite time form. In order to ensure selectivity, it is essential that the time of operation of the relays should be dependent on the severity of the fault in such a way that more severe the fault, the less is the time to operate, this being called the inverse-time characteristic.
Advantages of numeric Over-current relays compared with theElectro-mechanical over- current relays
Power consumption of numerical over current relay is very low compare with electro mechanical over current relay. Because mechanical over current relay need considerable power to start the rotation of the disc to operate. But for numerical relay very low power is required.
Relay operation time is less compare with mechanical over current relay: Considerable time will be consumed for the disc to start the movement.
Accuracy is high: due to some factors such as dust, friction is cause to low accuracy in mechanical over current relay. So may be cause low response to the fault current.
More flexibility: when use mechanical over current relay we can use only IDMT characteristics. But if we use numerical over current relay we can use any characteristics (also new ones can be defined). So flexibility is very high.
