a new digital relaying scheme for parallel

International Journal of EmergingElectric Power Systems

Volume 10, Issue 3 2009 Article 3

A New Digital Relaying Scheme for ParallelTransmission Line

Bhavesh Bhalja∗ Rudra Prakash Maheshwari†

Urmil B. Parikh‡

∗ADIT, [email protected]†Indian Institute of Technology, Roorkee, [email protected]‡Indian Corporate Research Centre, ABB Ltd, Vadodara, urmilparikh [email protected]

Copyright c©2009 The Berkeley Electronic Press. All rights reserved.

A New Digital Relaying Scheme for ParallelTransmission Line∗

Bhavesh Bhalja, Rudra Prakash Maheshwari, and Urmil B. Parikh

Abstract

This paper presents a scheme for the protection of parallel transmission line having two dif-ferent configurations (geographical locations). In the proposed scheme, the six line currents at acommon end of a parallel line or three line currents at a separate end of a parallel line are de-composed using wavelet packet transform (WPT) in order to derive the operating quantities forrelay operation. The proposed scheme provides stability against close-in faults, more sensitivitytowards high resistance faults and reliability for discriminating in-zone and out-zones faults duringcomplete loss of generation at one of the buses. When the conventional relay, without an intuitivedirectional element, fails to trip for a remote end fault located in the vicinity of remote bus, theproposed method solves the said problem and also avoids the voltage signal used in conventionalrelays. Furthermore, the suggested scheme completely avoids the requirement of a distance algo-rithm as a backup protection in case of the disconnection of one line (due to maintenance or fault).Moreover, the proposed scheme analyzes solves the problem of simultaneous same phase fault onparallel lines. To validate the proposed scheme, numerous computer simulations have been carriedout on realistic data of part of the Indian 400 kV power transmission system network.

KEYWORDS: fault detection, parallel transmission line, wavelet packet transform

∗Bhavesh Bhalja is with the Department of Electrical Engineering, ADIT – 388121, India; e-mail: [email protected]. R. P. Maheshwari is with Department of Electrical Engi-neering, IIT Roorkee – 247 667, India; e-mail: [email protected]. Urmil B. Parikhis with Indian Corporate Research Centre, ABB Ltd, Vadodara – 390015, India; e-mail: urmil-parikh [email protected].

1. Introduction

Different types of protective schemes can be used to protect parallel transmission lines such as current, distance and pilot. Current based scheme fails in many situations and do not provide instantaneous protection to the entire line. The apparent impedance estimated by a distance relay is influenced by the combined reactance effect of fault resistance, shunt capacitance and the load current as well as the mutual coupling effect caused by the zero-sequence current of the adjacent parallel circuit [1]. Hence, settings for conventional distance relays must be selected to avoid overreach/underreach operation under the worst case scenarios. This results in suboptimal performance of a relay under other operating conditions [2]. All the distance relaying algorithms available consider the lines as two separate circuits [3], [4]. Many transmission line relaying schemes have been proposed to solve the problems associated with parallel lines [5]-[9].

Current differential protection schemes are also used for the protection of parallel lines [10]. But it requires separate communication channel, which increases cost and complexity of the protective system. Moreover, reliability of the protective system heavily depends on reliability of the communication channels. Bo [11],[12] presented two non-communication protection schemes namely current based delayed operation scheme and voltage based instant operation scheme. From stability considerations, some system configuration and fault conditions do not allow the application of the delayed operation scheme alone, in particular, for system voltage at and above EHV level. On the other hand, the instant operation scheme, although able to isolate the fault instantaneously, could involve unnecessary operation of circuit breakers for faults outside the protected zone. Osman et al. [13] presented wavelet based technique for the protection of parallel transmission lines. Thereafter, Bhalja et al. [14] proposed an adaptive scheme for double-circuit lines using radial basis function neural network.

All the above mentioned papers published so far for the protection of parallel line have used a very simple simulation model. It has been found that many early line protection algorithms worked perfectly on systems as simple as that taken in the referred papers. However, they were found to be lacking in real world applications. Some system structure is needed to produce realistic test data. The two parallel lines should be embedded in a larger power system not just stuck between two sources as done in the said papers. Moreover, when only one line is in service due to maintenance or fault, none of the techniques are able to detect the fault and distance algorithm is used as a backup protection. Likewise, in case of a very special type of fault i.e. cross-country fault; the techniques discussed in the above papers will not notice the fault and once again distance algorithm (as a backup protection) is required. Furthermore, the responses of the relay at common

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Bhalja et al.: Digital Relaying Scheme for Parallel Transmission Line

Published by The Berkeley Electronic Press, 2009

end & at separate end are not symmetrical for the transmission line connected at two different geographical locations. The relay at common end overreaches whereas, the relay at separate end underreaches. However, these errors depend greatly on the infeed of the unfaulted line that is dependant on the relative strength of the sources involved in the fault [15]. Fig. 1 shows two different configurations of parallel transmission line considered in this paper.

Fig. 1: Existing parallel transmission line with two different configurations 2. Proposed Methodology As wavelet transform is better suited for the analysis of certain types of transient waveforms, it has received great attention in power community [16], [17]. Fernandez et al. [18] presented an overview of the wavelet transform applications in a power system, which indicates that the use of wavelet transform in power system protection is increasing day-by-day.

2.1 Fault Detection Proposed technique uses SigD1 component for fault detection. Moreover, db1 was selected as mother wavelet because it clearly detects abrupt changes and transients in fault signals [19]. A variety of different wavelet families have been proposed in the literature such as Daubechies, Biorthogonal, Coiflets and Symlets. Each family has its feasibility depending on the application requirements. Daubechies family is one of the most suitable wavelet families in analyzing power system transients [20]. Although there are no definite criteria for the selection of wavelets, the best choice is a wavelet that most strikingly exhibits the phenomena to be studied. In the present work, the db1 mother wavelet has been used for fault detection as it closely matches the signal to be processed which is of utmost importance in wavelet applications.

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International Journal of Emerging Electric Power Systems, Vol. 10 [2009], Iss. 3, Art. 3

http://www.bepress.com/ijeeps/vol10/iss3/art3DOI: 10.2202/1553-779X.2089

Extensive computer simulation has been carried out to select the best mother wavelet. Different Daubechies families of wavelet namely db1, db2, db4 and db8 have been analyzed. A case study with a single line-to-ground fault at 10% from bus-SSNNL on line-G is examined and Fig. 2 shows simulation result. It can be observed that the db1 mother wavelet gives the best result. This fact is applicable for other fault locations also.

Fig. 2: Comparison of different Daubechies families of wavelet

Considering a sampling frequency of 4 kHz, the measured signals (all line currents) at each end are decomposed (up to level one) into approximation (A1) and detail coefficient (D1) using db1 mother wavelet. In the proposed scheme, fault detection is carried out using one cycle summation of the absolute value of D1 (SigD1) component [21]. If the SigD1 for each line current is less than a certain threshold (α), a healthy state is assumed and output is taken as zero. This means that the fault is either on external line or error due to un-identical CT saturation characteristics and a certain inequality in the two lines. The value of the threshold is chosen, based on extensive simulations on two different configurations of transmission line as shown in Fig. 1, considering different types of faults and loading conditions. It has been found that under all loading conditions, a value of threshold is less than 2.0. Hence, to increase detection sensitivity a threshold value of 2.5 is selected. Fig. 3 shows the process of fault

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detection using SigD1 for line to line fault occurring at 20% on line-G from bus-SSNNL. It has been observed that both phase ‘a’ and phase ‘b’ value of line-G for a-b fault exceed the threshold condition.

2.2 Proposed Digital Protection Scheme The proposed scheme is based on two functions which have been derived from the wavelet coefficients of the decomposed signal and the same are proportional to the fault current.

Fig. 3: SigD1 of all line currents for a-b fault on line-G at 20% from bus-SSNNL

Wavelet packet transform has been applied to each corresponding phase of both the lines, at each end. After third level decomposition of the measured signal, using db4 mother wavelet, the summation of the absolute values of the expansion coefficients known as I SUM (G/H) is derived (equation 1). Here, the db4 mother wavelet gives the best result with respect to different Daubechies families of wavelet.

∑ ∑= =

=n

j mmHGSUM jWjI

1

7

03)/( )()( (1)

4



Here, j is the most recent sample and n is the number of samples/cycle (4 kHz). Thereafter, two operating quantities P1 & P2 are derived by taking the

difference and the summation of I SUM (G/H) signal corresponding to each phase at each (common) end. For transmission line connected at two different geographical locations (configuration 2), low bandwidth communication channel is required in order to derive the said operating quantities.

)()(1 )( HSUMGSUM IIjP −= (2)

)()(2 )( HSUMGSUM IIjP += (3)

The final performance of the scheme is given by plotting the trajectory of

P1 against P2 as shown in Fig. 4. To get a final decision, the relay should identify the trajectory of P1 against P2 as being in the forward or reverse zones. The first and the second quadrants determine the operating (forward) zone whereas the third and the fourth quadrants determine the restraining (reverse) zone (fault behind the relay shown in Fig. 1).

If the trajectory of P1 against P2 is greater than a predetermined positive threshold +γ, a trip signal should be sent to the trip coil of the circuit breaker of line-G. Whereas, if the trajectory of P1 against P2 is less than a negative threshold -γ, a trip signal should be sent to the trip coil of the circuit breaker of line-H.

Fig. 4: Trajectory of P1 against P2

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2.3 Selection of Threshold An extensive simulation of parallel transmission line with two different configurations for all probable types of faults with varying load conditions has been carried out. Under normal operating condition, for both the configurations (Configuration 1 & 2), the trajectory of P1 against P2 lies between ± γ. Different types of external faults have been simulated outside the protected zone and the performance of the trajectory of P1 against P2 has been analyzed. Based on computer simulations, it has been observed that the trajectory of P1 against P2 for transmission line between SSNNL-NAGDA bus (Configuration-1) varies between -0.5 to +0.5, in the event of an external short-circuits. Hence, to increase detection sensitivity, a threshold value of ± 0.55 is assigned to ±γ. Similarly, based on the simulations of ASOJ-AMRELI-INDORE line (Configuration-2), threshold value of ± 0.55 has been assigned to bus-A, and ± 1.2 is assigned to both, bus-B & bus-C respectively.

Simulation Model A part of the Indian 400 kV power transmission system, as shown in Fig. 5, has been used to access the problems associated with parallel transmission lines and also to validate the proposed scheme. The line-to-be-protected is SSNNL-NAGDA parallel transmission line which starts and ends at common bus (Configuration 1). For Configuration 2, the line-to-be-protected is ASOJ-INDORE-AMRELI parallel transmission line which emanates from a common end and stops at two different geographical locations. The transmission line parameters and the generating station details are given in Appendix.

Test data for verifying the proposed scheme have been generated by modeling the complete system of Fig. 5 using the PSCAD/EMTDC software package [22]. The transmission line is represented using the Bergeron line model. The other components of the power system, such as generators, generator transformers, inter-connecting transformers (ICTs) etc. are designed according to the collected data and specifications. All the generators at SSNNL and Wanakbori power station are designed and grouped into sub-pages. The relays, shown in Fig. 1, are located at each end of the transmission line for two different configurations. The simulated system was subjected to the various types of possible faults. The performance of the proposed scheme has been evaluated for different types of in-zone and out-zones faults. Relay responses for some special cases such as remote end fault located in the vicinity of the remote bus, high resistance fault, cross-country fault, one line disconnected and loss of generation at one end (bus) were also investigated.

3.

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Fig. 5: The Indian 400 kV power systems containing the line-to-be-protected for

two different configurations 4. Simulation Results

4.1 Configuration-1 (Parallel line connected on the same bus at both ends)

4.1.1 Internal Fault For the power system model shown in Fig. 5, line to line (a-b) fault at 20% from SSNNL-bus on line-H has been analyzed. Fig. 6 shows the relay trajectory in the form of P1 against P2 for the respective phases of the parallel lines. The performance of the technique has been indicated as a trajectory of P1 against P2. It has been observed from Fig. 6 that the proposed technique correctly identifies the faulted phase as the trajectory of phase-a and phase-b exceeds the threshold value. Moreover the relay trajectory of phase-c remains well below the threshold value.

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Fig. 6: Performance of relay-1 at SSNNL-bus for a-b fault on line-H at 20% from

SSNNL-bus 4.1.2 External Fault Studies have been conducted to examine the performance of the proposed scheme at both ends during wide variation of external faults. A single line-to-ground fault has been simulated out side the protected zone on NAGDA-INDORE single-circuit line (Fig. 5) at 15 km from the NAGDA-bus. The simulation results are shown in Fig. 7 and Fig. 8. It has been observed from Fig. 7 and Fig. 8 that the relay trajectories for all the phases are well below threshold conditions at both ends and hence, ensure the stability of the proposed scheme in case of external faults.

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Fig. 7: Performance of relay-1 at SSNNL-bus for a-g fault (External) on

NAGDA- INDORE single-circuit line at15 km from NAGDA-bus

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Fig. 8: Performance of relay-2 at NAGDA-bus for a-g fault (External) on

NAGDA-INDORE single-circuit line at 15 km from NAGDA-bus 4.1.3 High Resistance Fault

Many algorithms related to parallel transmission lines fail to detect fault with a considerable value of fault resistance [13]. To analyze this effect, a case study has been set up and a single line-to-ground fault with a fault resistance equal to 75 Ω at 50% from SSNNL-bus on line-G has been simulated. Fig. 9 shows the simulation result. It has been observed from Fig. 9 that the high resistance fault has no significant effect on the relay performance. It has also been observed that even though it slows down the rise of the fault responses by decreasing the trajectory of P1 against P2, still the proposed technique works properly.

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Fig. 9: Performance of relay-1 at SSNNL-bus for a-g fault on line-G at 50% from

SSNNL-bus with RF = 75 Ω 4.1.4 Close-in Fault When a fault occurs in the vicinity of the remote bus and is associated with a small short-circuit capacity derived at the remote bus then the protective relay, without an intuitive directional element, fails to detect this type of fault. This is due to equal magnitude of current flowing in both the lines at remote bus [23]. To analyze this effect and to review the relay performance, a single line-to-ground fault with a fault resistance equal to 25 Ω at 99% from SSNNL-bus on line-G has been simulated. Fig. 10 shows simulation results.

As indicated in Fig. 10, the fault trajectory of relay-2 at remote bus (NAGDA) crosses the threshold boundary after few samples from the fault inception. Hence, relay-2 would detect the fault, and trips the remote circuit breaker. At the same time it has also been observed by the author that the relay at SSNNL-bus (relay-1) also operates and trips the circuit breaker.

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Fig. 10: Performance of relay-2 at NAGDA-bus for a-g fault on line-G at 99%

from SSNNL-bus with RF = 25 Ω 4.1.5 Influence of Sources

The short-circuit impedances are influenced to a great extend due to the occurrence of large power system contingencies, such as a generator outage in the vicinity of the line-to-be-protected. The extreme cases occur when the total generating capacity at one end of operations is out of order. These cases occur where only one generation unit is connected to the power system by a line to be protected [24]. In this condition, the current at remote end is reversed, and hence, a directional element of the protected zone is reversed, making it difficult to identify the fault being internal or external. Many algorithms are unable to detect the fault in the said situation [13], [14], [25]. This is because the current flowing in both the lines at remote end is almost same, and hence relay would not operate. To analyze the performance of the proposed scheme, a case study has been set up and a double line-to-ground fault with a fault resistance equal to 50 Ω at 15% from SSNNL-bus has been simulated. Here, the generator at Wanakbori Power Station (WPC) has been kept out of operation. Fig. 11 and Fig. 12 show the

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simulation results. It is observed that, the relay-1 at SSNNL-bus and relay-2 at NAGDA-bus operates successfully under the said condition.

Fig. 11: Performance of relay-1 at SSNNL-bus for a-b-g fault on line-H at 15%

from SSNNL-bus through RF = 50 Ω with generator at WPS is out of operation

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Fig. 12: Performance of relay-2 at NAGDA-bus for a-b-g fault on line-H at 15%

from SSNNL-bus through RF = 50 Ω with generator at WPS is out of operation

4.1.6 One Line Disconnected Normally, the two parallel lines are working together with equal loads. However, in case of permanent faults or line maintenance only one line is in service. In this condition, the algorithm based on current magnitude comparison cannot be used as the value of the current samples is absolutely zero [7]. Likewise, the current difference algorithm or distance based algorithm cannot remain connected to energize parallel lines in cases where one of the parallel lines is switched off [23], [14]. Such cases can be easily solved using the proposed technique.

To investigate this fact, a case study has been set up wherein, line-G is not in service and a-g fault occurs on line-H at 40% from SSNNL-bus. It has been observed from Fig. 13 that the proposed technique correctly identifies the faulted phase as the trajectory of the phase-a, exceeds a newly derived threshold value. In a situation when one line is out of service, an old value of threshold is required to be shifted adaptively to a new value so that the stability of the healthy phases is maintained. Here, a new value of threshold (γ) is obtained from extensive

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simulations, and taken as ± 2.5. Shifting a threshold value can be easily done by using the parallel line’s operating status.

Fig. 13: Performance of relay-1 at SSNNL-bus for a-g fault on line-H at 40%

from SSNNL-bus with line-G disconnected 4.1.7 Inter-Circuit Fault Consider the case of a simultaneous fault between phase ‘a’ and ground on line-G and between phase ‘b’ and ground on line-H. This type of fault is known as inter-circuit fault. To analyze this condition, a-g fault on line-G and b-g fault on line-H at 50% from SSNNL-bus has been simulated. Fig. 14 shows the simulation results. It has also been observed from Fig. 14 that the fault is correctly identified by the proposed scheme and a single-pole tripping signal for phase fault in each line will be sent to its circuit breaker.

Moreover, detection of a very special fault case such as simultaneous line-to-ground fault on the same phase of both the lines (a-g fault on line-G and a-g fault on line-H) is very difficult [25]. Normally used techniques would not notice this type of fault. To analyze the above condition, a case study with a

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simultaneous a-g fault on line-G and line-H for different fault locations has been examined. Fig. 15 shows the simulation results.

It has been observed from Fig. 15 that the two operating quantities, P1Aa and P2Aa at local end (SSNNL), are always above a pre-defined threshold (γ). On the other hand, operating quantities, P1Ba and P2Ba at remote end (NAGDA), are also greater than a threshold value except in case of close-in faults. Hence, for a close-in simultaneous same phase faults (up to 30%), remote end relay fails to operate, whereas, local end relay operates successfully. In such a situation, the trip decision is sent to the remote end through a low bandwidth communication link so as to initiate tripping.

Fig. 14: Performance of relay-1 at SSNNL-bus for a-g fault on line-G and b-g

fault on line-H at 50% from SSNNL-bus

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Fig. 15: Behavior of operating quantities at SSNNL and NAGDA end for

simultaneous same phase fault at different fault locations

4.2 Configuration-2 (Parallel line emanated from the same bus at local end but connected at two different geographical locations at the other ends)

For configuration-2, the performance of the proposed scheme for parallel transmission line has been checked for all types of faults (as done for configuration-1). However, as it is not feasible to include all of the simulation results due to space limitations, the relay trajectory for few cases are given.

4.2.1 Internal Fault For the power system model shown in Fig. 5, a single line to ground fault involving phase ‘a’ at 40% from bus-ASOJ on line-G is simulated. Fig. 16 shows the relay trajectory of the respective phases at common end (Asoj) and at two separate ends (Amreli and Indore). It is observed from Fig. 16 that the proposed technique correctly identified the faulted phase of line-G as phase-a trajectory

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exceeds the threshold value. Moreover the relay trajectory for all the phases of line-H is well below the threshold value.

Fig. 16: Performance of relay-1, 2 & 3 at common end & separate end for a-g fault on line-G at 40% from bus-Asoj

4.2.2 High Resistance Fault To analyze this effect, a case study is set up (Fig. 5) and a single line-to-ground fault with a fault resistance equal to 50 Ω at 75% from bus-Asoj on line-H is simulated. Fig. 17 shows simulation result. It is observed that the fault current levels and therefore the magnitudes of the current samples go on progressively smaller as the fault resistance increases. Though, a significantly slower relay performance is achieved the proposed scheme operates successfully.

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Fig. 17: Performance of relay-1, 2 & 3 at common end & separate end for a-g

fault on line-H at 75% from bus-Asoj with RF=50 Ω

4.2.3 Inter Circuit Fault To analyze this effect, a case study with a single line-to-ground fault on line-G and b-g fault on line-H at 50% from ASOJ-bus has been set up. Fig. 18 shows the simulation results. It has been observed from the said figure that the relay trajectory at common end exceed both the positive and the negative threshold values. On the other hand, relay-2 and relay-3 trajectory at separate ends (B & C) exceed at a time only the positive or the negative threshold value.

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Fig. 18: Performance of relay-1, 2 & 3 at common end & separate ends for a-g

fault on line-G & b-g fault on line-H at 50% from ASOJ-bus 5. Conclusion A new WPT based scheme for the protection of parallel transmission line for two different configurations is analyzed and discussed in this paper. Based on operating quantities, relay decides the fault is in forward zone or reverse zone. According to the analysis and results of simulation studies, the following conclusions can be drawn.

The proposed scheme overcomes many protection problems that parallel transmission line faces such as high resistance fault, close in fault, cross-country fault and provides discrimination between out-zone and in-zone faults in case of loss of generation at one end. The test results show that the suggested technique avoids the requirement of distance algorithm as a backup protection in case of disconnection of one line due to maintenance or fault. Moreover, the proposed scheme analyzes & solves the problem of simultaneous same phase fault on parallel transmission lines. The proposed system was tested extensively by using realistic data that was generated by modeling an existing power system using PSCAD/EMTDC package. All the probable types of fault and network variations are investigated.

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APPENDIX The parameters of parallel transmission line used for simulation (refer Fig. 5) are given in Table-I. Table I: Line parameters for a 400 kV parallel transmission Line

The generating station details and lines connected to generating station

used for simulation are (refer Fig. 5) given in Table-II and Table-III.

Table II: Generating station details used for simulation Name of Generating

tation

Units Generated Reactance p.u.)

Generator Transformer

ICTs (400/220 KV)

No. XMVA)

Get. Voltage (kV)

X2 X0 No X MVA

Voltage Ratio (kV)

% Z No X MVA

% Z

WANAKBORI X 247 15.75 kV .25 .15 3 X 250 15.75 / 220 14.41 1 X 315 12.0 4 x 250 15.75 / 420 14.59

SP SSNNL)

X 222 13.8 kV .24 .14 6 X 250 13.8 / 420 14.5 2 X 315 12.0 X 55.56 11 kV .21 .15 5 X 63 11 / 220 12.5

Type of Configuration

Line-to-be-protected

Length(km)

Z1 (Ω/km)

Z0 (Ω/km)

Z0m (Ω/km)

Configuration 1 SSNNL-NAGDA

275 0.0297+j*0.332 0.162+j*1.24

0.0486+j*0.372

Configuration 2 ASOJ-INDORE-AMRELI

290 0.0297+j*0.332 0.162+j*1.24

0.0486+j*0.372

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Table III: 400 kV lines connected to generating station

Name of Generating Station

400 kV LINES Name of Line S/C or

D/CLoad (MW)

Length (km)

WANAKBORI (TPS)

1. ASOJ S/C 100 76 2. SOJA S/C 400 96 3. DEHGAM S/C 400 67

SSP(SSNNL) (HYDRO- POWER STATION)

1. KASOR S/C 450 103 2. ASOJ S/C 350 83 3. DHULE – 1 D/C 75 236 4. DHULE – 2 75 5. NAGDA – 1 D/C 100 275 6. NAGDA – 2 100

S/C: Single Circuit Line D/C: Double Circuit Line

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a new digital relaying scheme for parallel

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