lass="lazy 100" data-original="//html.scirp.org/file/27-7600988x40.png" />).

It is noted that, the phasor variables are chosen based on the type of fault occurring in the power transmission line. For single phase-to-ground fault at phase “A”; these variables are

(5)

(6)

(7)

(8)

where

: phase “A” voltage at the relay location,

: phase “A” current the relay location,

: zero sequence current at the relay location,

: zero sequence compensation factor of the line,

: positive sequence impedance of the line,

: zero sequence impedance of the line,

: STATCOM phase “A” current,

: STATCOM zero sequence current.

Similarly, for single phase-to-ground faults in phases “B” and “C”, the appropriate phase quantities have been used. For line-to-line fault between phase “A” and “B”; these variables are

(9)

(10)

(11)

where

: phase “B” voltage at the relay location,

: phase “B” current at the relay location,

: STATCOM phase “B” current.

Similarly, for line-to-line faults between phases “B” and “C” and phases “A” and “C”, the appropriate phase quantities have to be used.

4. Proposed Distance Protection Method

The flowchart of the proposed distance location model is depicted in Figure 3. Instrument transformers are connected to measure the voltage and current signals at the relay point. An analog anti aliasing filter filters the fault transients before sampling, to avoid aliasing of the input waveforms. A data acquisition system with 100 kHz sampling frequency calculates the signals as a string of samples. The Discrete Fourier Transform (DFT) block, transfers the sampled current and voltage signals to phasor quantities. The STATCOM injected/absorbed current is made available at the relay impedance measurement units with the help of SCADA to calculate the correct value of the apparent impedance. The symmetrical component block produces three symmetrical components from the phase currents. Six impedance measurement equations perform the impedance estimation. Among these, three equations detect faults which involve more than one phase and the remaining three equations detect phase-to-ground faults. Each equation estimates the apparent impedance separately and identifies the fault location. If the apparent impedance estimated by any relay equations is within the characteristic, then a trip signal is issued to the circuit breaker. The relay logic uses the Mho type-operating characteristic.

The equations to estimate the apparent impedance for various fault conditions are listed in Table 1. For single phase-to-ground fault at phase “A”, the apparent impedance () is measured by the equations (). Similarly, for single phase-to-ground faults in phase “B” and “C”, the appropriate measurement equations have been used. For line-to-line fault between phase “A” and “B”, the apparent impedance () measured by the equations ().

Figure 3. Flowchart of the proposed distance relay model.

Table 1. Apparent impedance equations for various fault conditions.

Similarly, for line-to-line faults between phases “B” and “C” and phases “A” and “C”, the appropriate equations have to be used. For three phase fault (line-to-line-to-line-ground) any one of the line-to-line fault equations can be used. The STATCOM injected/absorbed current is made available at the relay impedance measurement units to calculate the correct value of the apparent impedance. The SCADA [19] , with dedicated communication links provides the information about the STATCOM injected/absorbed current at the relay point.

5. Simulation Results and Discussion

Figure 4 shows the MATLAB/ SIMULINK model of the 400 kV 50 Hz, 300 km length test transmission system with 100 MVA STATCOM to evaluate the performance of the developed fault location algorithm.

The performance of the proposed method, have been evaluated for various types of faults on the transmission system at various locations. The results i.e. three phase voltages and currents obtained at the relay point have been exported as the input to the distance relay model. The Mho relay characteristics at a sampling rate of 6 kHz (120 samples per cycle) have been used to detect the faults. The relay is set to protect 80% (240 km) of the transmission line. Even though several cases are involved in the analysis, only two cases of impedance trajectories i.e. “A” phase-to-ground fault and “A” phase-to-”B” phase-to-ground fault with zero fault resistance are discussed below. In addition, the effect of the fault occurring before the STATCOM location and after the STATCOM location is also discussed.

Figure 4. The Simulink model used to simulate the proposed algorithm.

5.1. Impedance Trajectories for Single Phase Fault

Apparent impedance trajectories of the conventional distance relay and the proposed distance relay with Mho characteristic for “A” phase-to-ground fault occurring at the fault distance of 240 km is shown in Figure 5. It is evident that the apparent impedance seen in the conventional distance is higher (80.98 ohms) than that of the apparent impedance seen by the proposed distance relay (77.05 ohms), which is outside the Mho characteristic, as during the fault, the reactive power injected by the STATCOM increases the voltage at the connecting point which, in turn increases the apparent impedance seen by the conventional distance relay.

Therefore, the conventional distance relay under reaches and hence it does not give the trip signal. But the apparent impedance seen by the proposed distance relay is almost same as the impedance seen by the conventional distance relay without STATCOM which settles inside the Mho characteristic and issues the trip signal correctly.

The test results of the transmission system the conventional distance relay and the proposed distance relay for “A” phase to ground fault are shown in Table 2. It clearly shows that when the fault occurs between the relay point and the STATCOM location (between 10 and 150 kilometers, in this case), there is not much change in the apparent impedance measured by the distance relay i.e. measured impedance is almost the same as that of the system without STATCOM.

For example, when the fault occurs at 120 km, the apparent impedance measured by the conventional distance relay is 38.58 ohms and the proposed distance relay is 38.57 ohms. This is due to the fact that when the STATCOM is not present in the fault loop, then the measured impedance is equal to actual line impedance of the line section between the relay point and the fault point.

When the fault (“A” phase-to-ground fault) occurs beyond the STATCOM location i.e. between 150 and 300 kilometers, the apparent impedance of the system is greater as the STATCOM is involved in the fault loop. It is observed that (refer to Table 2) when the fault occurs at a distance of 240 km, the proposed distance relay measures 77.05 ohms, whereas the conventional distance relay measures the apparent impedance as 80.98 ohms, which is higher than that of the proposed distance relay measurement.

5.2. Impedance Trajectories of Phase-to-Phase Fault

The impedance trajectory of the conventional distance relay and proposed distance relay for “A” phase-to-”B” phase-to-ground fault created at a distance of 240 km is shown in Figure 6. It clearly shows that the protection

Figure 5. The apparent impedance trajectories for single phase fault.

Figure 6. The apparent impedance trajectories for phase-to-phase fault.

Table 2. Variations of the apparent impedance for “A” phase-to-ground fault.

zone of the conventional distance relay under reaches, whereas the proposed distance relay estimates the exact, apparent impedance of the transmission system and operates as anticipated.

The variations of the apparent impedance for “A” phase-to-”B” phase-to-ground fault with different fault locations of the conventional distance relay and the proposed distance relay are shown in Table 3. For a system with STATCOM, it is observed that when the fault occurs before the STATCOM location, the proposed distance relay and conventional distance relay measure almost the same values. But, if the fault occurs after SATACOM location i.e. for 240 km, the proposed distance relay measures 77.03 ohms and conventional distance relay measures 78.58 ohms. It clearly shows that when the fault occurs between the relay location and after the STATCOM location, the conventional distance relay measurement is higher than that of the proposed distance relay measurement.

The results (refer to Table 3) show that the new proposed distance protection method effectively mitigates the impact of STATCOM on the apparent impedance measurement of the distance relay and functions properly without error.

5.3. Estimation of Fault Location

The test results under several fault scenarios with different fault locations are shown in Table 4. The table consists of fault type, fault location, estimated fault location using conventional measurements and the proposed distance relay measurements. A single-phase fault (“A” phase-to-ground fault) condition is an example, where when the fault is at 240 km, the estimated fault location based on conventional measurement is 253 km, but estimated fault location based on the proposed distance relay measurement is 240.7 km.

Similarly, for the phase-to-phase fault (“A” phase-to-”B” phase-to-ground fault) condition, the estimated fault location based on conventional distance relay measurement is 245.5 km, but the estimated fault location based on the proposed distance relay measurement is 240.7 km. In almost all the cases the proposed distance relay measurement error is below 0.3%. The results clearly show that the proposed distance protection method to estimates the fault location correctly and effectively mitigates the impact of STATCOM on the distance relay performance for various fault conditions.

Table 3. Variations of the apparent impedance for “A” phase-to-“B” phase-to-ground fault.

Table 4. Estimation of fault location with types of faults.

6. Conclusion

A fault location method for STATCOM connected transmission line distance protection using Current Compensation Method (CCM) was developed and tested successfully to overcome the problems arising from the conventional methods. The fault loop apparent impedance in the proposed method is corrected based on the STATCOM injected/absorbed current and accordingly the actual distance to the fault is calculated. Comprehensive equations required for new digital distance relay to calculate the accurate fault location are derived. Test results proved that the proposed method yielded accurate estimates of the fault location and effectively mitigated the impact of STATCOM on the distance relay performance. The accuracy of the proposed method is high in almost all the cases and the error is kept below 0.3%. By using the proposed method, it is possible to design an accurate and reliable distance protection scheme for STATCOM compensated transmission lines.

NOTES

*Corresponding author.

Cite this paper
Ilango, R. and Raja, T. (2016) Fault Location Method for STATCOM Connected Transmission Lines Using CCM. Circuits and Systems, 7, 3131-3141. doi: 10.4236/cs.2016.710266.
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