Reduction in operating time is calculated for four different transmission line protection relays using mathematical set of equations and developed simulation models (described in section 2). Calculated and simulation outcomes are discussed below.
3.1. Differential relay presence effect on power transmission line fault
Firstly, the operating time of the relay in single line to ground fault condition is calculated.For differential relay maximum driving voltage at load side is, Vd=30000V. At over-loading state in relay,the assumed current exceeds by 25%.The reduction in operating time delay is calculated using equation (1):
$${t_{R.B}}=\frac{{0.14\left( {tsm} \right)}}{{\left( {ps{m^{0.02}}} \right) - 1}}$$ 1
Where
\(psm=\frac{{Vrelay}}{{ps}}\)
and\(Vrelay=300V,ps=1V,Tcb=0.5s,V\hbox{max} fault=300V\)
\(Ttotal=tcb+tcb\)
\(Ttotal=0.5011s\)
Thus operating time of differential relay for LG fault is ̴ 0.5s. Similarly, operating time of 0.5s have been calculated for LLLG and LLG respectively as shown in Table 1.
Figure 9 represents circuit breaker opreating time in seconds when LG, LLG and LLLG fault are generated respectively in the system. In all faults the realy triped as soon as fault is generated at 0.5s and value of voltages are droped to zero.
3.2. Distance/ impedance relay presence effect on power transmission line fault
In order to calculate operating time, let the value of the nominal current exceed up to 25% from rated value.By using equation (1), the reduction in operating timeis calculated for distance relay by changing certain parameters as Irelay=15A;ps=1A; IOC=150A instead of using Vrelay= 300V; and ps=1V, and total calculated operating time of distance/impedance relay is 0.525s.Here, MATLAB is also used for calculation of operating time of distance relay when LG, LLG and LLG faults are generated, as shown in figure 10.
From figure 10, In case of LLLG fault distance relay the relay operates exactly at 0.5sof the fault conditions while in case of LLG faultthe circuit breaker did not operates exactly at 0.5s but it takes a delay and operates at 0.5125sWhich means at this condition distancerelay do not operates instantly to remove the faul. In case of LG fault the CB operated at 0.52s in this case it took longer time to operate than two other faults when fault occurred at 0.5s. As the result types changes CB behaved differently for different faults and CB operating time changes.All the results are reported in table 1 for comparison.
3.3. Over and under voltage relay presence effect on effect on power transmission line fault
In case of manual calculation for over voltage condition for LLG fault, which most commonly occurs. Maximum driving voltage at load side=4000V.At overloading in relay let assume current exceed 25% of nominal value.By using equation (1) calculate the reduction in operating time for over and under voltage relay by changing certain parameters as Vrelay=9.37V and ps=0.8V and total calculated operating time of over and under voltage relay 0.515s in case of LG fault.
MATLAB is also used to calculate operating time when LG, LLG and LLG faults are generated at 0.5s, as presented in figure 11.In figure 11 as the LLLG fault occurred, the relay trips and CB operate at 0.5s and protect the system without any delay in the system,in this case of LLG fault, the relay operates at 0.525s with a delay time of 0.025s and in case of LG fault the circuit breaker operates at 0.515s, with the delay of 0.015s.
In both cases when the relay operated as under voltage, it trips when the value of voltage gets down from rates value and in case of over voltage condition the relay trips when the value of voltage gets more than that of rated value of voltage.
3.4. Over-current relay presence effect on effect on power transmission line fault
Calculating operating time for of over-current relay in case of single line-to-ground (LG) fault. Maximum driving current at load side in system=80A. At overloading condition in the relay assume that the current value exceed by 25%.By using equation (1) calculate the reduction in operating time for distance relay by changing certain parameters such asIrelay=30A,ps=1A,tsm=0.01s instead of Irelay=15A ps=1A IOC=150A and total calculated operating time of over and under current relay is 0.519s. Relay operating time of overcurrent relay in the case of LG fault= 0.519s
The results have been drawn with three phase faults (LLLG) and double phase to ground fault (LLG) in the system and checking the behavior with and without relay operations and operating time of 0.519s dedcued respectively. Here MATLAB is also used to calculate operating time of CB when LG, LLG, LLLG faults occurred at 0.5s, as presented in figure 12.
From figure 12, for LG, LLG and LLLG faultsCB operated at 0.519s, which the delay of 0.019s which means the overcurrent relay never operates instantly at fault condition (as fault condition is 0.5s). It will always take time delay to operate and clear the fault in all LLLG, LLG, and LG condition. This means overcurrent relay is less effective in clearing fault as soon.
3.5. Comparative analysis of four protection relay model
The operating time of four different relays are calculated and deducted by using the MATLAB models for all three Three-phase (LLLG), double line to ground (LLG) and single line to ground (LG) faults. Looking at a comparative prospective the differential relay operated at 0.5s for all fault conditions while the Distance/Impedance relay operated at different time for all three fault conditions with an average operating time of 0.5125s which is more than differential relay so differential relay is better than Distance/Impedance relay, 97.56% in operating time similarly over and under voltage relay operated at different operating time for all three faults and on an average it operates at 0.513s which is still higher than differential relay operating time and differential relay is 97.46% better in its operation from over and under voltage relay. At the end overcurrent relay which operated at similar delayed value for all three types of faults that is 0.519s which means overcurrent relay never operates instantly in any type of fault condition so differential relay is 96.34% better than overcurrent relay in its operating time. All the operating times of above mentioned four relays for all three types of fault (LLLG, LLG and LG) are given below in Table 1.
Table No. 1 Operating time of all relays
Relay protection Schemes
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Circuit Breaker operating time for LLLG fault
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Circuit Breaker operating time for LLG fault
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Circuit Breaker operating time for LG fault
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Differential relay scheme
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0.5s
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0.5s
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0.5s
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Distance or Impedance relay scheme
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0.5s
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0.5125s
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0.525s
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Over and under voltage relay scheme
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0.5s
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0.525s
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0.515s
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Over current relay scheme
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0.519s
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0.519s
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0.519s
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