Adaptive Distance Protection for Transmission Lines Incorporating SSSC With Energy Storage Device

A Static Synchronous Series Compensator incorporating energy storage device (SSSC-ES) at the DC bus enables the exchange of true power with the system in addition to line reactance compensation. This facilitates improved control of real and reactive power in a transmission system. However, the four-quadrant operation of SSSC presents additional problems for the accurate operation of conventional transmission line protection. In this paper, an in-depth analysis of the effect of emulated reactance and resistance by SSSC-ES installed at the center of a hybrid series compensated line on the performance of distance relay is carried out. An investigation is performed with all possible operating modes and control strategies of SSSC-ES, and the possibility of mal-operation of distance protection are explored. A novel adaptive setting based on constant reactance and constant resistance emulation control strategy of SSSC is proposed for quadrilateral distance relay to prevent the mal-operation. MATLAB / SIMULINK is used for the simulation study.


I. INTRODUCTION
FACTS controllers make the AC transmission system flexible by enhancing system controllability and power transfer capability [1]- [3]. Series FACTS controllers are mainly used to compensate for the line reactance and hence to control power transfer in long transmission lines. Series FACTS controllers also enhance the transient stability of a mixed AC/DC system [4]. For economic reasons, it is preferable to use a combination of a fixed series capacitor, and a voltage source controller (VSC) based SSSC [5]. An added advantage of SSSC is the provision for integrating an energy storage device (battery, fuel cell, or Super Conducting Magnetic Energy Storage) for real power supply/absorption. It results in ''enhancement of power transfer control, transient stability improvement, and power oscillation damping'' [6].
Non-availability of the main transmission line will result in loss of power to a large section of the system and economic loss to both the supplier and the customer. Therefore, protection scheme for lines should ensure reliable and fast operation to localize the damage and maintain transient stability.
The associate editor coordinating the review of this manuscript and approving it for publication was Dongbo Zhao.
Distance protection is the most favored scheme for primary transmission lines, and the current practice is to adopt digital protection techniques [7], [8]. During system disturbance, compensation of transmission lines by series or shunt connected FACTS controllers results in modification of primary power system parameters due to control actions. Conventional distance relay performance is expected to be impacted by the operation and control of FACTS controllers. Few researchers have investigated the impact of FACTS controllers on the accuracy of distance relay operation. Efficacy of digital distance relay due to the operation of series FACTS controllers is studied in [9]- [11]. References [12]- [14] discusses the impact of shunt connected FACTS controllers and factors affecting distance protection of UPFC compensated lines is investigated in [15]. Adaptive distance protection for lines integrating FACTS controllers is described in [16]- [19]. Work reported considers a limited range twoquadrant operation of SSSC and STATCOM without energy storage device.
SSSC without an energy storage device can operate only in a limited reactance compensation range. It injects a reactive voltage and negligibly small real voltage to meet the converter and capacitor losses. The angle of injected voltage Limited work is reported on the analysis of the impact of FACTS controllers incorporating energy storage devices on power system operation and control. In [20], change in ''steady-state characteristics of SSSC equipped with energy storage'' is highlighted. Transient stability enhancement with a unified STATCOM-fuel cell is discussed in [21]. However, the effect of an energy storage device connected at the DC bus of FACTS controllers on distance relay operation is not investigated. Initial attempts have thrown some light on the effect of integrating energy storage device (SMES) with SSSC and UPFC on the performance of transmission line protection [22], [23]. However, the effect of emulated reactance and resistance by SSSC and control strategy for SSSC on distance relay performance is not analyzed thoroughly. Emulated reactance and resistance by SSSC with an energy storage device is presented in [24]. However, the impact on distance relay performance during fault is not investigated. SSSC without energy storage emulates positive or negative reactance and negligible positive resistance to meet the losses of SSSC. Hence only reactance seen by distance relay is expected to get modified. However, SSSC with energy storage emulates large positive or negative resistance. Thus both the reactance and resistance seen by distance relay get modified substantially. The impact of control strategy adapted for SSSC-ES on the impedance measured by distance relay, during fault is not thoroughly investigated. There is no reported work on adaptive distance relay setting for lines compensated with SSSC-ES based on emulated reactance and resistance. The foremost aim of this work is to thoroughly study and analyze the impact of emulated reactance and resistance by fourquadrant operation of SSSC and various control strategies for SSSC on the performance of conventional distance relay. A new methodology for adaptive relay setting based on the computation of emulated resistance and reactance of SSSC to prevent the relay mal-operation is presented.

II. SYSTEM MODEL
FIGURE 1 shows the test system adapted from IEEE first benchmark model [25].
Here 'R t ' and 'X t ' are the resistance and reactance of transformer, 'X C ' is the capacitive reactance of passive series capacitor, and 'X sys ' is the system reactance on the infinite bus side. 'E b ' is the infinite bus voltage. The relay shown in FIGURE 1 (Z <) is a distance relay at the generator end of the line, which operates when the measured impedance is less than the set impedance. SSSC-ES is installed at the midpoint of the 200Km line. The transmission line is divided into four PI sections to simulate fault at any desired line length, before or after the location of SSSC.

A. SYSTEM EQUATIONS
A 3-level, 12-pulse SSSC is used to reduce harmonics [5]. A DQ model of the system is used for analysis.
Injected voltage by SSSC-ES, where K m = K * ρ se * cosβ; K = 2 √ 6 π for a 12 pulse converter, 'ρ se is the turns ratio of the transformer and From the control point of view, real (in-phase) and reactive (quadrature) voltages are defined for SSSC as A positive value of V P implies SSSC-ES absorbing real power, and negative V P implies SSSC-ES supplying real power. In capacitive mode, V R is negative and is positive in inductive mode. Angle of SSSC injected voltage, Deadangle, β Line current, I (11) where, R l = R t + R line and X l = X t + X line + X sys R line and X line are the resistance and inductive reactance of the transmission line.
Magnitude of generator voltage and the power delivered by the generator are given by Reactive voltage injected by SSSC-ES, Real power drawn by SSSC-ES,

B. CONTROL STRATEGIES
When SSSC is integrated with energy storage, type 1 controller is commonly employed to control both V i and γ . To decide real voltage injection, V P can be held constant or constant R SSSC control or constant P control can be employed. Similarly, to decide reactive voltage injection, V R can be maintained constant or constant X SSSC control can be employed, as shown in FIGURE 2. The values of K P and K i for all the PI controllers are obtained by root locus design and transient analysis [26].

III. METHODOLOGY
Power delivered by the generator (P g. ) is fixed at 0.9 pu. DC voltage of SSSC is assumed to be maintained constant at 0.7 pu by the energy source.
In the system considered for analysis, the rating of the SSSC is 150 MVA. With base MVA of 892.4 MVA and line current of 1 pu, V i max is found to be 0.16809 pu. The system has a fixed series capacitor with a capacitive reactance of 0.45 pu. Considering a series reactance compensation range of 30% to 60%, range of X SSSC is taken as from -0.15 pu (capacitive) to 0.15 pu (inductive). In order to fix the range of P ref , V i ' is determined for various operating modes, and the values obtained are tabulated in TABLE 1 (all values are in pu). VOLUME 8, 2020 The following equations are used to determine the impedance contribution by SSSC-ES.
|I| and X SSSC = V R |I| With reactance compensation by SSSC-ES of ± 0.15pu, the range of P ref is from −0.08pu to 0.08pu. Whereas without reactance compensation by SSSC (Xsssc = 0), P refmax is 0.15pu (absorbing or supplying).

A. EFFECT OF DC OFFSET ON COMPUTATION OF IMPEDANCE AND RELAY OPERATION
It is a well-known fact that a short circuit fault current contains two components, steady-state fault current (AC component) and decaying DC component. RMS value of current with DC offset will be more than the RMS value of the AC component. Hence, the distance relay may mal-operate i.e., it may over reach. Also, the presence of the DC component leads to inaccurate results from the DFT algorithm to compute the fundamental component. Thus it is necessary to get rid of the DC offset from current and voltage signals. A digital mimic impedance based filter is used to eliminate the DC offset [27]. The time constant of the digital mimic filter is selected as the time constant L R of the transmission line. After removing the DC offset component, the DFT algorithm is used to obtain the fundamental component of voltage and current. General DFT expression for the determination of m th harmonic component is given by where m = 1 for fundamental component, 'n' is the current sample and 'N' represents the samples taken during one cycle of power frequency. Full cycle DFT algorithm is used because of its superior performance in filtering harmonics and noise.
Using the above expression, the fundamental components of voltage, current are determined and Z relay is computed.

A. STEADY STATE CONDITION
Initially, analysis is carried out without any system disturbance (No fault). Directional quadrilateral distance relay having −15 • directional angle setting and 115 • negative restrain angle setting [28] is considered. Impedance of the line is Z = (0.04 + j1) pu. Zone 1 reach of the distance relay is assumed to be 80% of the protected line. Hence reactance setting for the distance relay is, Xset = 0.8 pu. A fault resistance of 15 (0.0455 pu) is considered to select a resistance setting for the relay. Hence resistance setting for the relay is Rset = 0.032+0.045 = 0.077 pu. These values are used as the zone 1 relay settings for the relay at the generator end in the various case studies considered for the analysis.
Analytically impedance values can be obtained using the following equations.
Impedance offered by SSSC, Apparent impedance seen by relay, As discussed in section III, keeping the reactance compensation by series capacitor fixed at 45%, net reactance compensation can be varied from 30% to 60%. Impedance values obtained for 60% and 30% net compensation levels are presented in TABLE 2 and the plot of Z relay on R -X plane is shown in FIGURE 3.  Change in the operating condition of SSSC-ES will result in a change in line current as well (see TABLE 1). Hence, as per equation (18), both resistance and reactance seen by the relay will be different for various operating modes of SSSC-ES. However, from FIGURE 3, it is clear that the relay does not operate since Z relay is outside the relay characteristics.  Distance relay will not operate, since Z relay measured is outside the set impedance.

B. THREE PHASE FAULT ON THE LINE SSSC NOT IN FAULT LOOP
The impact of operation of SSSC-ES on the relay efficacy, during fault, is studied by considering a three-phase fault on the line. Constant 'P' and constant V R control strategy is used for SSSC in case of case studies 6 to 14. The effect of other control strategies and unsymmetrical faults is investigated in sections IV. C and D.
In case of symmetrical fault at a distance 'x' pu on the line, with zero fault impedance, following expressions are used to determine analytically the impedance measured by the relay. For faults up to 50%-line length (SSSC outside fault loop), For faults between 50% and 100%-line length, SSSC is in the fault loop and hence, (20) Analytical and measured values of Z relay during various operating modes of SSSC-ES for a three-phase fault at 25% is shown in TABLE 4 and it almost matches with values obtained analytically using equation (19). The plot of Z relay is shown in FIGURE 5. SSSC is installed at the midpoint of the line. Hence for a fault at 25% of line length, SSSC is outside the fault loop and thus Z relay is not impacted by the operating mode of SSSC-ES. Hence SSSC will not affect distance relay operation for any fault on the line till 50%-line length.

1) SSSC IN FAULT LOOP, REACTANCE COMPENSATION
For a three-phase fault at 75%-line length, with reactive voltage injection only (both inductive and capacitive modes of operation), obtained values of Z relay is tabulated in TABLE 5. FIGURE 6 gives the obtained impedance locus.
It is evident from TABLE 5 that measured, and analytical values of Z relay during fault for a given operating mode of SSSC are different since constant reactive voltage control is used for SSSC-ES. During a fault, line current increases, and hence with the injected voltage of SSSC held constant, emulated reactance by SSSC decreases. Further, it is noted from FIGURE 6 that zone 1 relay unit under reaches for a VOLUME 8, 2020  Similarly, in the capacitive mode of operation (case 10), Z relay during fault has decreased. This is likely to make the relay over reach.

2) RESISTANCE COMPENSATION
To analyze the effect of real power exchange by SSSC-ES on the distance relay operation, the following operating modes are considered for a three-phase fault at 75%. Reactive voltage injection is zero.  TABLE 6, and the impedance locus is plotted in FIGURE 7. When SSSC-ES supplies power, Z relay decreases due to negative resistance emulation, and when SSSC-ES draws power, apparent resistance increases due to positive resistance emulation.
It can be further observed that with increase in positive resistance emulation, measured impedance moves towards the boundary of relay characteristics. This can lead to the relay under reach, if SSSC-ES absorbs more power (emulates a higher value of positive resistance) as evident from FIGURE 7 (case 14, with Pref = 0.15)).

C. IMPACT OF CONTROL STRATEGY
Various control strategies considered for the analysis are described in section II B. During short circuit fault, the current increases. Hence, from the equation (17), it is clear that if the control strategy is to maintain SSSC injected voltage constant (constant V P and V R ), impedance contribution by SSSC decreases. If the control strategy is to maintain emulated resistance and reactance of SSSC-ES constant (constant Rsssc and constant Xsssc control), the controller will try to maintain the impedance emulated during fault at the corresponding pre-fault value. However, this would call for an increase in SSSC injected voltage (as per equation (17)). Hence limiting the injected voltage by SSSC-ES within the rating of SSSC-ES, impedance offered by SSSC-ES during fault will be slightly different from the corresponding values during the steady-state condition, even with constant R SSSC and constant X SSSC control. The following case studies illustrate the impact of the control scheme for SSSC on the relay operation.
The pre-fault emulated impedance of SSSC-ES is (0.09847-j0. 15). This is due to the fact that SSSC-ES will inject a higher voltage to maintain the emulated impedance constant during a fault condition. However, when constant V P and constant V R control strategy (case 15) is adapted, the emulated R SSSC and X SSSC during fault is substantially decreased in comparison to prefault values. This is not surprising as the SSSC injected voltage is held constant, and an increase in line current during fault leads to decreased emulated impedance. It is seen from FIGURE 8 that the distance relay over reaches and maloperates with constant V P , V R control, whereas the relay does not mal-operate with constant X SSSC , R SSSC control. This clearly demonstrates that apparent impedance measured by the relay and hence the extent of the relay under reach or over reach depends on the control strategy of SSSC as well.

D. EFFECT OF UNSYMMETRICAL FAULTS
A computational algorithm for the distance relay is implemented in such a way that Z relay is the positive sequence impedance up to the fault, regardless of the fault type (Symmetrical / Unsymmetrical) [29].
The following case studies illustrate the impact of SSSC on distance relay operation during unsymmetrical faults.
1) Fault at 75%; No SSSC 2) Fault at 75%, SSSC-ES operating in 4 th quadrant, capacitive mode, absorbing real power (Xsssc = -0.15, Pref = 0.08). It is interesting to note from FIGURE 9 (a) and 9 (b) that for a given fault location and operating mode of SSSC-ES, the apparent impedance seen by the relay is almost independent of fault type. It is further observed from FIGURE 9 (b) that the apparent reactance is decreased due to capacitive reactance compensation by SSSC, and apparent resistance is increased due to positive resistance emulation by SSSC. This results in under reach of distance relay for all fault types.

E. ADAPTIVE RELAY SETTING TO AVOID MAL-OPERATION
The preceding sections clearly show that distance relay will mal-operate (over reach or under reach) depending on the operating mode and control strategy of SSSC-ES. With constant V P and constant V R control strategy, emulated impedance by SSSC-ES during fault can be estimated, and a lookup table can be prepared. This lookup table can be used to modify the relay setting in relation to change in the operating mode of SSSC. However, in constant V P and constant V R control strategy, emulated impedance by SSSC for a given operating mode of SSSC will also depend on the fault location. If the fault is close to SSSC location, emulated impedance by SSSC will decrease substantially due to high fault current. Hence adaptive relay setting to avoid relay maloperation is almost impossible in constant V R , V P control.
In the case of constant X SSSC and constant R SSSC control strategy of SSSC, emulated impedance by SSSC for a given operating point, during fault will almost be the same as the prefault impedance and is independent of the fault location. Hence constant reactance and constant resistance control strategy is best suited for adaptive relay setting in case of lines compensated with SSSC-ES. Emulated reactance and resistance values obtained for several operating modes of SSSC-ES can be utilized to dynamically modify relay settings accordingly and thus prevent the mal-operation of the distance relay. Adjustment of relay setting corresponding to change in the operating mode of SSSC-ES can be easily implemented with numerical quadrilateral distance relay with proper communication to the relaying point. The following two case studies validate this.

1) AVOIDING RELAY OVERREACH WITH ADAPTIVE RELAY SETTING
In section IV B, it is clearly shown that relay will under reach in the inductive mode of operation and the relay may over reach for the capacitive mode of operation. For capacitive mode with X SSSC = −0.15pu, prefault emulated reactance by SSSC-ES is -j0.15 pu. Hence decreasing the set reactance (X set ) by 0.15pu, relay characteristics is modified, as shown in FIGURE 10. It is found that the relay over reaches and operates for a fault beyond the zone reach (fault at 85%) with standard relay characteristics. It is evident from FIGURE 10 that with modified relay characteristics, distance relay will not over reach and mal-operate.

2) AVOIDING RELAY UNDER REACH WITH ADAPTIVE RELAY SETTING
From section IV B, it is very clear that the relay will under reach when SSSC-ES absorbs power due to positive resistance emulation.
With SSSC-ES absorbing power (Pref = 0.15), pre-fault emulated R SSSC is 0.184pu. Hence the relay characteristics is modified by increasing the set resistance (Rset) by 0.184pu, as shown in FIGURE 11. The distance relay under reaches for a fault at 75%, with standard relay characteristics. It is encouraging to note that with modified relay characteristics, the distance relay will not under reach and operates correctly.

F. DISCUSSION
Based on the studies carried out, the following inferences are drawn.
• Under the steady-state condition, it is shown that distance relay operation is not affected by SSSC-ES when it is used within the rated operating range.
• When SSSC-ES is not in fault loop, distance relay operation is not affected by SSSC-ES.
• When SSSC-ES is in fault loop, Z relay is dependent on operating mode and the control strategy used for SSSC-ES. Thus the operation of conventional distance relay with standard settings is not reliable.
• In the capacitive mode of operation, the distance relay will over reach due to negative reactance emulation and in inductive mode of operation, the relay will under reach due to positive reactance emulation.
• While absorbing real power, the distance relay will under reach due to positive resistance emulation, and while supplying real power, it may over reach due to negative resistance emulation.
• Extent of over reach and under reach also depends on the control strategy used for SSSC-ES.
• For a given operating mode of SSSC-ES and fault location, Z relay is not impacted by the fault type.
• A numerical quadrilateral distance relay with an adaptive relay setting based on constant reactance and constant resistance control strategy for SSSC-ES will avoid distance relay mal-operation.

V. CONCLUSION
In this paper, the impact of emulated reactance and resistance by four-quadrant operation of SSSC incorporating an energy storage device on the transmission line protection is analyzed. An investigation is carried out with symmetrical and unsymmetrical faults on the line. It is clearly shown that SSSC-ES does not affect distance relay performance when it is outside the fault loop. When SSSC-ES is in the fault loop, R SSSC , X SSSC values and hence Z relay will change significantly, depending on the operating mode of SSSC-ES and control strategy used for SSSC-ES. All these make the distance relay setting a challenging task, and the conventional distance relay with standard settings is shown to mal-operate, i.e. under reach or over reach. A novel adaptive setting for numerical quadrilateral distance relay based on constant reactance and constant resistance control strategy for SSSC-ES is presented to prevent the mal-operation of the distance relay. Highlights and main contributions of this work are • A DQ model for SSSC with an energy storage device and in-depth analysis of the impact of operating mode and control strategy for SSSC-ES on the performance of distance relay in all the four quadrants of operation.
• A novel adaptive relay setting based on emulated R SSSC and X SSSC control to prevent the mal-operation of the quadrilateral numerical distance relay for lines compensated with SSSC-ES.
• A reliable protection of SSSC-ES compensated lines presented in this work will help the transmission systems to employ VSC based series compensation integrating energy storage/source, thus utilizing all the four quadrants of operation. Transmission system data R t = 0.0; X t = 0.14; Z L1 = 0.04 + j1; Z 0 = 0.08 + j2; Xc = 0.45; X SYS = 0.06; V g = V g θ ; E b = 1 0; SSSC data: 150 MVA; Transformer tap = 1/6; V dc = 0.7pu H. V. GURURAJA RAO received the B.E. degree in electrical engineering from Karnatak University, in 1989, the M.Tech. degree in power systems from the University of Mysore, in 1993, and the Ph.D. degree from Visvesvaraya Technological University, in 2020. He was a Faculty Member of electrical and electronics engineering with the KVG College of Engineering, Sullia, from 1993 to 2004. He is currently an Assistant Professor (selection grade) in electrical and electronics engineering with the Manipal Institute of Technology, Manipal. His research interests include power system protection, stability, power electronics application to power systems, and embedded systems. He is a Fellow of the Institution of Engineers, India, a Life Member of the Indian Society for Technical Education, and a member of the Institution of Engineering and Technology. R. C. MALA received the B.E. degree in electrical power engineering and the M.Tech. degree in power systems from the University of Mysore, in 1988 and 1992, respectively, and the Ph.D. degree from Visvesvaraya Technological University, in 2020. She was a Faculty Member of electrical and electronics engineering with the KVG College of Engineering, Sullia, from 1992 to 2004. She is currently an Associate Professor in electrical and electronics engineering with the Manipal Institute of Technology, Manipal. Her research interests include power system dynamics, power electronics application to power systems, and control systems. She is a Fellow of the Institution of Engineers, India, and a Life Member of the Indian Society for Technical Education.