1. Introduction

In recent years, power demand has increased substantially while the expansion of power generation and transmission has been severely limited due to limited resources and environmental restrictions. As a consequence, some transmission lines are heavily loaded and the system stability becomes a power transfer-limiting factor. Flexible AC transmission systems (FACTS) controllers have been mainly used for solving various power system steady state control problems[1].

The literature shows an increasing interest in this subject for the last two decades, where FACTS devices are introduced in power systems to increase the transmitting capacity of transmission lines and provide the optimum utilization of the system capability. This is done by pushing the power systems to their limits[2,3]. It is well documented in the literature that the introduction of FACTS devices in a power system has a great influence on its dynamics. As power system dynamics changes, many sub-systems are affected, including the protective systems. The application of SVC was initially for load compensation of fast changing loads such as steel mills and arc furnaces.

Here the objectives are: increase power transfer in long lines, improve stability with fast acting voltage regulationdamping low frequency oscillations due to swing modes and damping sub-synchronous frequency oscillations due to torsional modes and over-voltages dynamic control[3].

Unlike the power system parameters, the controlling parameters of FACTS devices, as well as their installation points could affect the measured impedance[8-13]. This variation has a direct effect on the protective zones settings for distance relays protected this line compensated. In the presence of FACTS devices, the conventional distance characteristic such as MHO and quadrilateral are greatly subjected to mal-operation in the form of over-reaching or under-reaching the fault point. Therefore, the conventional characteristics might not be utilized satisfactorily in the presence of FACTS devices.

May researcher study the impact of series FACTS devices on distance protection, for Static Synchronous Compensator (STATCTOM) in[4] is study on a novel method of distance protection in transmission mine high voltage, and in[5] is study on a novel distance relay setting on for transmission line and in[6] simulation studies on the distance relay performance in the presence phase to earth fault, and in[7] is study the impact of inserting STATCOM on MHO distance relay setting in Algerian transmission line.

For Static Synchronous Series Compensator (SSSC) in[8-10] analysis of measured impedance (Zseen) by Distance Relay in the presence fault on transmission line 400 kV, and in[11] is study the global impact of insertion SSSC on the digital relay.

For Compensator based Thyristor Controlled Series Capacitor (TCSC) in[12] is study this impact on Z_seen by distance relay in double circuit lines considering MOV operation and in[13] is study artificial neuron network (ANN) based protection system on transmission line.

Therefore, it is essential to study the effects of shunt FACTS devices on the protective systems, especially the distance protection, which is the main protective device at HV and EHV levels. In[14] is study the performance comparison of distance protection schemes for shunt FACTS compensated transmission lines, and in[15-19] is study the impact of shunt FACTS based SVC on distance relay tripping characteristic and impedance Z_seen in presence fault, and in[20] is comparative study between SVC devices and STATCOM.

In this paper, the three protection zones setting (Z₁, Z₂ and Z₃) for a MHO distance relay based analytic method on a 400 kV single transmission line installed at eastern Algerian electrical network is considered. The investigation concerns TCR and TSC shunt FACTS devices, for different values of firing angle (α) for injected reactive power ± 60 MVar shunt compensation on midline transmission high voltage.

2. MHO Distance Relay in Transmission Line HV

MHO Distance protection is so called because it is based on an electrical measure of distance along a transmission line to a fault. The distance along the transmission line is directly proportional to the series electrical impedance of the transmission line (Z_L) between busbar A and B as shown in figure 1. The distance protection measures distance to a fault by means of a measured voltage to measured current ratio computation[21]. The philosophy of setting relay at Sonelgaz Group[22,23] is three forward zones (Z₁, Z₂ and Z₃) for protection the line HV between busbar A and B with total impedance Z_AB.

Figure 1. Setting zones for distance relay

The setting zones for protected transmission line without shunt FACTS devices are expressed by[22,23]:

(1)

(2)

(3)

The total impedance (Z_AB) of transmission line AB measured by distance relay is given by:

(4)

Where K_VT and K_CT are ratio of voltage to current respectively installed at busbar A. The characteristic curves X = f(R)[7,10,23] for MHO distance relay are represented in figure 2.

Figure 2. Characteristic curves of MHO distance relay

The presence of SVC devices has a direct influence on the total impedance of the protected line (Z_AB).

Connected at midline point of the line, the SVC is considered as a reactor (X_SVC =1/B_SVC) and lead to the new setting zones which can be expressed by:

(5)

(6)

(7)

3. Principal and Modelling of Static VAR Compensator (SVC) Devices

Figure 3. Structures of SVC devices

The SVC controls voltage where it is connected by adjusting its susceptance in order to supply or absorb the required reactive power (Q_SVC)[25]. SVC consists of TCR and a set of TSC in parallel, and an associated controlling system. The controlling system operates to regulate the voltage at its connecting point, according to its controlling strategy within its operational limits.

In order to investigate the impact of SVC on power systems, appropriate SVC model is very important. It is connected in shunt with the transmission line through a shunt transformer and thus[26] is represented in figure 3 and figure 4 shows the equivalent circuit at which SVC is modeled.

The overall action of the thyristor controller on the linear reactor of the TCR and capacitor of the TSC is to enable the reactor to act as a controllable susceptance, in the inductive or capacitive mode, which is a function of the firing angle α[1,2,25,26]. The figure 4.a and 4.b represent the substance for TCR and TSC respectively.

Figure 4. Injected susceptance of SVC devices

The current I_TCR directly related to the firing angle of thyristors (α), which was varied between 90° and 180°[1,2,25]. The value of the substance B_TCR will be represented by the equation[2,25-27]:

(8)

Where,

(9)

For the TSC, the expression B_TCS is defined in reference by the equation[1-3,25-27]:

(10)

Where,

(11)

4. Case Study and Simulation Results

The figure below represents the power system studied in this paper and is the 400 kV, 50 Hz eastern Algerian electrical transmission networks at SONELGAZ group[22]. The MHO distance relay is located in the busbar A at Ramdane Djamel substation (Skikda) to protect transmission line between busbar A and busbar B at Oued El Athmanai substation (Mila), the bus bar C at Ain M’lila substation (Oum El Bouaghi). The shunt FACTS study type SVC is installed in the midpoint of the line protected by a MHO distance relay as represented in figure 5.

The parameters of the transmission line and installed SVC device are summarized in the appendix.

Figure 5. Algerian electrical networks study in presence SVC

Figure 6. Characteristic curves of the installed SVC; a). BTCR = f (α), b). BTSC = f (α)

The characteristic curves of the SVC compensator (TCR and TSC) used in this network are indicated by figure 6 .a and figure 6.b respectively.

Figure 7 and 8 represented the impact of the variation angle α in the presence of TCR and TSC on the total reactance X_AB and resistance R_AB respectively measured by distance relay.

Figure 7. Impact of α on XAB measured by distance relay

Figure 8. Impact of α on RAB measured by distance relay

As can be seen the presence of TCR and TSC has a direct influence on the parameters of the protected line X_AB and R_AB. The total impedance measured by the distance relay without SVC devices is equal 4,8407 + j 46,8048 (Ω), the settings zones are summered in table I.

Table 1. Setting Distance Relay without SVC Devices

4.1. Setting Relay on Presence TCR

Figure 9 and 10 represented the impact of the firing angle variation on the settings zones reactance and resistance respectively for the protected transmission line.

Figure 9. Impact of the firing angle α on the setting zones in the presence of TCR

Figure 10. Impact of the firing angle α on the setting zones in the presence of TCR

From figures 9 and 10, the setting of the three zones of protection is dependent on the angle α of the TCR.

4.2. Setting Relay on Presence TSC

Figure 11. Impact of the firing angle α on the setting, zones in the presence of TSC

Figure 12. Impact of the firing angle α on the setting, zones in the presence of TSC

Figure 11 and 12 represented the impact of the firing angle variation on the settings zones reactance and resistance respectively in the presence of the TSC compensator.

As can be seen from figures 11 and 12 the setting of the three protection zones is dependent on the angle α of the TSC.

5. Conclusions

The results presented in this paper show the direct effects of SVC devices i.e. TCR and TSC insertion, on the total impedance of a transmission line protected Z_AB and Z_seen by MHO distance relay based analytical method. The impact is investigated for different values of the thyristors firing angle. As demonstrated these angles injected variable substance (B_TCR or B_TCS) in the protected line which results in direct impact on the total impedance of the protected line.

In fact this effect varies the settings zones by increasing performance of the total system protection and avoiding unwanted tripping of circuit breaker in the presence of SVC compensator on single electrical transmission line 400 kV.

ANNEXES

1). Parameters of Transmission Line:

Un = 400 kV, fn = 50 Hz, ZL = 0, 03293 + j 0, 3184 Ω/km,

lAB = 147 km, and lBC = 87 km.

2). Parameters of SVC Devices:

2.1). TCR:

QTCR = + 60 MVar, L = 6,40 mH, Un = 11 kV.

2.2). TSC:

QTSC = - 60 MVar, C = 1,60 mF, Un = 11 kV.

2.3). Coupling transformer:

Un = 11/400 kV, Y/∆, Sn = 60 MVA, XTR = j 0,0371 Ω.

3). Parameters of Current Transformer:

Ipri = 1000 A, Isec = 5 A, KCT = 200.

4). Parameters of Voltage Transformer:

Vpri = 400000/√3 V, Vsec = 100/√3 V, KVT = 4000.