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Short-Circuit Calculations for a Transmission Line in the Algerian Power Network Compensated by Thyristor Controlled Voltage Regulator ZELLAGUI Mohamed 1, HASSAN Heba 2,3 and CHAGHI Abdelaziz 4 1 University of Batna, Algeria Department of Electrical Engineering, Faculty of Technology, Campus CUB, Med El Hadi Boukhlouf Street, 05000, Batna, Algeria, E-Mail:
[email protected] 2 Dhofar University, Sultanate of Oman Department of Electrical and Computer Engineering, College of Engineering, Salalah, P.O. Box: 2509, Post Code: 211, Dhofar, Sultanate of Oman, E-Mail:
[email protected] 3
Cairo University, Egypt Electrical Power and Machines Department, Faculty of Engineering, Giza, Egypt, E-Mail:
[email protected] 4
University of Batna, Algeria Department of Electrical Engineering, Faculty of Technology, Campus CUB, Med El Hadi Boukhlouf Street, 05000, Batna, Algeria, E-Mail:
[email protected]
Abstract – In this paper, the impact of using the Thyristor Controlled Voltage Regulator (TCVR) on short-circuit calculations is studied for a high voltage transmission line in case of a phase to earth fault in the presence of a fault resistance (RF). A 400 kV single transmission line, in the Algerian power network, is used for this study where the line is compensated by TCVR FACTS device to control the voltage and active power of the line. The impact of the fault resistance is studied to reflect its effect on short-circuit calculations including symmetrical current components, transmission line currents, voltage symmetrical components, and transmission line voltages, without and with TCVR. The obtained simulation results match the presented theoretical analysis. Keywords: Power Systems; Transmission Line; TCVR; Short-Circuit Calculations; Earth Fault; Fault Resistance. I. INTRODUCTION Electrical power industry constitutes a high percentage of the world economy. Power systems have to be planned, projected, constructed, commissioned and operated in a way that enables a safe, reliable and economic supply of loads. Accurate forecasting of the power system load demand is highly essential. Despite careful planning, design and maintenance as well as thorough operation, short-circuits could happen in power systems due to internal or external factors [1, 2]. Shortcircuit calculation techniques are formulations which are normally used to analyze faults in power systems that estimate the during-fault system state. Short-circuit
currents are responsible for several types of disturbances in power systems which cause severe effects on equipments and power lines, such as thermal and mechanical stresses [3]. Power system faults can be symmetric or asymmetric. Symmetric faults cause large fault currents. Asymmetric faults are more common and cause unbalanced fault currents. Asymmetric faults are classified as: phase to earth, phase to phase, and phase to phase to earth. In the Algerian Company of Electrical and Gas, more than 83 % of the occurring faults on 220 and 400 kV power systems overhead transmission networks are single phase to earth type and 11 % are phase to phase fault, and 6 % are three phase fault [4]. Short-circuit analysis methods are an important tool used by protection engineers to estimate power system currents and voltages during disturbances. It provides information about settings and coordination as well as efficiency analysis studies. Today, three approaches are used in the industry for such analysis: classical symmetrical components, phase variable approach and complete time-domain simulations [5]. Classical shortcircuit analysis of unbalanced power systems is based on symmetrical components approach [6]. However, in untransposed feeders with single-phase or double phase laterals, the symmetrical component methods do not consider accurately these specific characteristics [7]. Hence, symmetrical components based techniques may not provide accurate results for power distribution systems, which are normally characterized by those asymmetries [8]. Many researchers have focused on the impact of fault resistance on power systems; error in the fault distance estimation in the presence of earth faults [9],
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fault location in distribution power system [10], fault location for transmission power system using sparse measurement method [11], fault location for hybrid transmission lines [12], distance protection performance based MHO and polygonal characteristics [13], adaptive digital distance relaying scheme for double transmission line [14], adaptive digital relaying scheme to tackle recloser-fuse mis-coordination for Distributed Generation (DG) [15], adaptive reach setting of distance relay in presence of UPFC [16], bus-bar protection [17], and fault current including arc fault in the presence of DG in distribution systems [18, 19]. This paper shows the impact of fault resistance in the presence of phase (A) to earth fault at the end of a 400 kV transmission line in the Algerian transmission network. The line which connects two 400/200 kV substations, namely Salah Bey (Sétif) and Bir Ghbalou (Bouira), is compensated by a TCVR installed at the mid of the line. This research study investigates the impact of fault resistance that varies between 0 to 50 Ω, on short-circuit parameters, which are symmetrical current components (I1, I2 and I0), transmission line currents (IA, IB and IC), voltage symmetrical components (V1, V2 and V0), and transmission line voltages (VA, VB and VC). II. THYRISTOR CONTROLLED VOLTAGE REGULATOR (TCVR)
V1 and V2. TCVR operates by inserting an in-phase voltage to the main bus voltage to change its magnitude. To model TCVR, an ideal tap changer transformer can be used without series impedance as in Fig. 2. The turns ratio is given by the ratio of the additional transformation relative to the nominal transformation and its value ranges from 0.9 to 1.1 where 1.0 corresponds to no additional transformation [20].
Fig. 2. Model of TCVR [22].
Therefore, TCVR can be modeled as an ideal tap changer transformer without series impedance [20, 22]. The TCVR coefficient KTCVR has the following ranges as given by the following equations: VT C V R
The advantages of using Flexible AC Transmission Systems (FACTS) devices in power systems are numerous. In steady-state, in a meshed network, these devices can permit to operate transmission lines close to their thermal limits and to reduce the loop flows. In this respect, they provide or absorb reactive power, increase or reduce voltage and control series impedance or phaseangle. Moreover, FACTS devices may have benefits in case of short-circuits by limiting the short-circuit current [20]. Furthermore, this technology offers the possibility to move an installation when it becomes not useful anymore. Different types of devices have been developed and there are various ways to classify them in terms of the technology of the used semiconductor, the possible benefits of the controllers, and the type of compensation. Fig. 1 shows the active power flow equation between two buses 1 and 2 and the variables that can be controlled by each FACTS device [21, 22].
(1)
K T C V R .V bus
0.15 d K T C V R d 0.15
(2)
0.15 u V bus d VTC VR d 0.15 u V bus
(3)
III. PHASE TO EARTH FAULT CALCULATIONS IN THE PRESENCE OF TCVR Fig. 3 shows the equivalent circuit of a transmission line compensated by TCVR in case of a phase to earth fault. The fault occurs at phase A at the end of the transmission line, in the presence of a fault resistance denoted by RF.
Fig. 1. Effects of FACTS devices on active power equation [22]. Fig. 3. Earth fault equivalent circuit with TCVR.
V1 and V2 are the voltage magnitudes of buses 1 and 2, X12 is the transmission line reactance and (δ1 - δ2 = δ) is the difference in phase angle between the two phasors
The basic equations for this type of fault [1], [23] are:
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IB
IC
V1 V 2 V 0
VA
(4)
0
RF u I A z 0
The zero component of the voltages is given by:
(5)
V0
The symmetrical components of currents are [24-25]: ª I0 º « » I « 1» «¬ I 2 »¼
ª1 1« 1 3« «¬1
1 º 2 » a » a »¼
1 a 2
a
ªIA º « » I « B» «¬ I C »¼
(6)
V0
M
(V S r V T C V R ) u Z L .0 R F
IA
I0
(7)
3
The symmetrical components of the voltages are: ªV 0 º « » V « 1» «¬V 2 »¼
ª1 1« 1 3« «¬1
1 º 2 » a » a »¼
1 a a
2
(8)
V 0 V 2 R F u I A
V s r VT C V R M 1
1
1
3
> M 0 @ > M 2 @ RF 3
VB u I A (10)
where,
n F .Z L .1 u I 1
M 1 ° ®M 0 ° ¯M 2
IA
V s r VT C V R
3
(11)
n F .Z L .0 u I 0 n F .Z L .2 u I 2
ª¬ n F . Z L .1 Z L .2 Z L .0 º¼ R F u I A (12)
From equations (12), the current at phase (A) in the presence of a TCVR device is given by: IA
3 u V S r VT C V R
n F u Z L .1 Z L .2 Z L .0 3 u R F
(13)
From equations (7) and (13), the symmetrical components of the currents in the presence of a TCVR are: I1
I2
I0
VA
(9)
and,
V S r VT C V R n F u Z L .1 Z L .2 Z L .0 3 u R F
V1
V S
V S r VTC VR M 1
Ab
a 1
(22)
2
3u a 1
(23)
Ad
aa
(24)
Ae
a 1
2
2
V2
V S
M 2
n F u Z L .1 Z L .2 Z . L 0 3 u R F
(26)
VC
V S
3 u R F u V S r VT C V R n F u Z L .1 Z L .2
Z L .0 3 u R F
r V T C V R . > Aa u Z L .2 Ab u Z L .0 Ac u R F ) @ n F u Z L .1 Z L .2 Z L .0 3 u R F
V S
r VT C V R u ª¬ Ad u Z L .2 Ae u Z L .0 A f u R F ) º¼ n F u Z L .1 Z L .2 Z L .0 3 u R F
(27)
(28)
(29)
Hence, the short circuit calculations are only related to the following: TCVR parameters (VTCVR and operation mode), and fault conditions (location (nF) and resistance (RF)). IV. CASE STUDY AND SIMULATION RESULTS The case study of this research work is the 400 kV electrical transmission network, in the north part of the Algerian power network. The TCVR is installed at the mid of the transmission line connecting bus-bar A at Salah Bey (Sétif) substation and bus-bar B at Bir Ghbalou (Bouira) substation. TCVR data, transmission line parameters and fault conditions are given in the Appendix.
(14)
(16)
(17)
r V T C V R u Z L .2
(25)
3u a 1
The inverse component of the voltages is defined by: V2
(21)
2
(15)
r VT C V R u ª¬ Z L .2 Z L .0 2 u Z L .1 3 u R F º¼ n F u Z L .1 Z L .2 Z L .0 3 u R F
(20)
a a
The direct component of the voltages is defined by: V1
From equations (8), (16), (18) and (20), the three phase voltages of the transmission line in the presence of TCVR are given by:
From equations (5) and (8), the direct component of the voltages become: V1
Aa
Af
ªV A º « » V « B» «¬V C »¼
(19)
n F u Z L .1 Z L .2 Z L .0 3 u R F
Ac
I2
RF u I0
The coefficients A1 and A2 are defined as follows:
From equation (4) and matrix (6), the symmetrical components of the currents take the following form: I1
0
(18) Fig. 4. Impact of VTCVR on PL.
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Fig. 4 shows the impact of the voltage controlled by TCVR (capacitive and inductive modes) on the active power variation when the angle δ varies between 0° 180°. Fig. 5.a, b, c represents the variation of the current symmetrical components I1, I2 and I0, respectively and Fig. 6.a, b, c represents the variation of the line currents IA, IB and IC, respectively as a function of the fault resistance with and without using TCVR. Fig. 6.a, b, c, represents the effect of the fault resistance on the transmission line currents.
(a)
(a) (b)
(b) (c) Fig. 6. Impact of RF on the transmission line currents. a). IA = f (RF), b). IB = f (RF), c). IC = f (RF).
(c) Fig. 5. Impact of RF on the current symmetrical components. a). I1 = f (RF), b). I2 = f (RF), c). I0 = f (RF).
From this Fig. 5.a, b, c, it is obvious that the three symmetrical currents are equal, in case of using TCVR or without it, which matches equation (7). It is also clear that increasing the value of RF leads to decreasing the value of the three symmetrical current components. In Fig. 6.a, b, c, it is noticeable that the line currents of phases B and C are always zero which is confirmed by equation (4). However, increasing the value of RF shows a reduction in the line current of the faulty phase (A) whether using TCVR or not. Fig. 7.a, b, c represents the variation of the voltage symmetrical components V1, V2 and V0 , respectively and Fig. 8.a, b, c represents the variation of the line voltages VA, VB and VC , respectively as a function of the fault resistance in case of using TCVR and without it.
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(a)
(b)
(b)
(c) Fig. 8. Impact of RF on the transmission line voltages. a). VA = f (RF), b). VB = f (RF), c) VC = f (RF).
(c) Fig. 7. Impact of RF on the voltage symmetrical components. a). V1 = f (RF), b). V2 = f (RF), c). V0 = f (RF).
Fig. 8.a, b, c, represents the effect of the fault resistance on the transmission line voltages.
(a)
From Fig. 7.a, b, c, it is clear that the increase of RF value leads to an increase in the direct voltage component and a decrease in the inverse and zero voltage components. These remarks are valid in case of using TCVR and without it which are confirmed by equations (16), (18) and (20). In Fig. 8.a, b, c, it is obvious that the increase of RF value leads to an increase in the value of the line voltages of phases A and B, compared with the voltage phase C which is decreased with the increase of fault resistance RF for the studied cases. V. CONCLUSIONS This research work investigated the impact of the fault resistance on short-circuit calculations for a transmission line equipped with TCVR. The case study was for a 400 kV transmission line compensated by TCVR, in the Algerian power network, when subjected to a single phase to earth fault in the presence of fault resistance. The authors concluded that short-circuit calculations are directly related to the parameters of prefault conditions, specially the fault resistance. Simulation results highlighted the effect of fault resistance on short-circuit calculations where these obtained results matched the given theoretical analysis. Hence, the paper recommends changing the settings for different principal protection devices, used for protecting high voltage transmission lines such as
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distance and directional over-current relays, based on the illustrated results. For each different scenario, the proposed method will calculate the new parameters for the specified short-circuit case, as well as the varying fault current between minimum and maximum values. In order to increase the total system protection performance in the presence of TCVR, more care must be taken regarding the indicated variation in the fault current. This is important to avoid the unwanted tripping of circuit breakers, the poor selectivity of protection relays and to improve the overall performance of protection relays. Further research for different case studies, concerning short-circuit in the presence of an inter-phase fault, are currently under preparation for future publications. APPENDIX A. Transmission Line: UL = 400 kV, f = 50 Hz, l = 205 km, Z1 = 0.0291 + j 0.3081 Ω/km, and Z0 = 0.0873 + j 0.9243 Ω/km. B. TCVR: Vmin = - 45 kV, Vmax = + 45 kV. C. Fault Conditions: nF = 100 %, RF = 0 to 50 Ω.
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