172 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 32, NO. 1, FEBRUARY 2017 A Non-unit Protection Scheme for DC Microgrid Based on Local Measurements A. Meghwani, S. C. Srivastava, Senior Member, IEEE, and S. Chakrabarti, Senior Member, IEEE Abstract—This paper proposes a nonunit protection scheme for DC microgrids (DC MGs) utilizing only local measurements. This scheme is developed based on the natural characteristics of DC current and its first and second derivatives under fault transients. Since it is based on local measurements, the problems associated with the communication delay are avoided. The selected protection scheme detects and discriminates the faults within a few microseconds of its inception. In this paper, a method to calculate the thresholds, used for the protection scheme, is also discussed. The proposed scheme is validated on a ring-type DC MG architecture under different fault scenarios and tested through MATLAB/Simulink simulations. Index Terms—Current derivatives, DC microgrid, local measurements, nonunit protection. I. INTRODUCTION N the recent past, significant research and development efforts have been made to integrate renewable energy resources, such as wind turbines, and photovoltaic systems into the power distribution networks [1], [2] forming an important part of smart grid. The energy policy in many countries across the world envisages increased penetration of the renewable energy resources and Distributed Generations (DGs). For instance, India is trying to increase the usage of renewable generation up to 30% by 2030 [3]. A low or medium voltage electrical network, consisting of distributed resources, especially renewable sources of energy, storage devices, and loads, is known as a Microgrid [4]. The electrical network can be AC, DC, or a mix of the two, and may or may not be connected to the main AC grid. During normal operating conditions, the microgrid is connected to the AC grid at the point of common coupling. The loads are supplied from the local sources and, if necessary, also from the AC grid. If the power consumed by the loads is less than the power produced by the local sources, the excess power can be exported to the AC grid. A DC MG is more suitable where the majority of the loads contains sensitive electronic equipments. The advantage of the DC MG compared to an AC microgrid is that loads, sources, I Manuscript received December 21, 2015; revised March 10, 2016; accepted April 5, 2016. Date of publication April 21, 2016; date of current version January 20, 2017. This work was supported by the Department of Science and Technology, New Delhi, India, under project no. DST/EE/20100258. Paper no. TPWRD-01843-2015. The authors are with the Department of Electrical Engineering, Indian Institute of Technology, Kanpur 208016, India (e-mail: anjum@iitk.ac.in; scs@iitk.ac.in; saikatc@iitk.ac.in). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2016.2555844 and energy storage can be connected through simpler and more efficient power-electronic interfaces [5]. Other advantages are, high efficiency [6], ease of paralleling of sources on the DC bus [7], and more power transfer capacity [8]. One of the main challenges in adopting the DC distribution system is the lack of effective protection to the fault in DC network. The faults in the DC MG can be categorized as: 1) AC side faults: Voltage Source Converters (VSCs) are widely used as AC-DC or DC-AC converters for electrical power conversion. The protection against AC faults on grid connected converters forms a part of AC system protection design [9]. 2) Internal faults: VSC internal faults include failure of power electronics device such as Insulated-Gate Bipolar Transistor (IGBT). In terms of fault tolerant VSCs, the research efforts have been to protect the system from possible faults by providing a backup converter or redundant devices [10]. 3) DC network faults: A DC network fault, with parallel connected VSCs, is the most severe fault in DC MG. It causes over current and under voltage due to the presence of a large filter capacitor of the VSC and low impedance offered by the cable. The freewheel diodes of VSCs are subject to over currents and are unable to provide protection against DC side faults, such as DC link short circuit, DC cable short circuit, and DC cable ground faults. These conditions need to be analysed in detail in order to design an effective protection system. A line to ground fault is the most common type of fault in the DC network [11], [12]. The causes can be the physical damage, environmental stresses, electrical stresses, and cable ageing. A scheme based on differential current protection was proposed in [13], [14] to detect the cable fault in DC network. It requires a reliable communication channel for instantaneous data transfer between the terminals of the protected element. Because of the chances of possible communication failure or data loss, differential protection will require a separate backup protection scheme. This may increase the total cost and size of the protection system, and limits its application in microgrids. Along with poor reliability, usage of communication in protection, also introduces delays, which increases the response time of the protection scheme. Researchers in [15] proposed a nonunit protection scheme, which is highly reliable and fast in operation. In this scheme, as the fault is detected, all the VSCs are disconnected from the AC side. The capacitors on the DC side support the load for short duration. The only limitation of 0885-8977 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. MEGHWANI et al.: NONUNIT PROTECTION SCHEME FOR DC MICROGRID BASED ON LOCAL MEASUREMENTS 173 equivalent circuit shown in Fig. 1(b). The RLC circuit response in the frequency domain during this period can be expressed as [19], i (s) = Fig. 1. (a) Faulted network, (b) Its RLC equivalent circuit, and (c) Equivalent circuit for stage 2. this method is that it needs a complete shutdown of the system following the fault. Authors in [16], [17] investigated a range of unit and nonunit protection solutions for DC distribution systems. It is found that differential protection is the most suitable for such networks due to its high selectivity and speed of operation. The most stringent requirement for a DC protection system is its speed of operation. This is due to the presence of the high rate of rise of fault current and its large steady state value. The fault must be cleared in a timely manner to avoid damage to the sensitive power electronic devices and also to keep the fault current below the interruptible current limit of the breakers. The goal of the proposed scheme is to detect the fault in a DC cable and quickly isolate the faulted section. The scheme is based on locally measured system parameters that do not require any communication channel or system shutdown to clear the fault. This cause almost no time delay, as the measurements are local, except for the processing time. The proposed protection scheme is simple and fast to protect the DC MG from the faults, without interrupting the load. The proposed fault detection algorithm utilizes fault current, and its first and second order derivative to discriminate the fault. Based on these parameters, a protection system design framework is developed. The proposed scheme is tested on a typical DC MG architecture. II. DC-CABLE GROUND FAULT ANALYSIS A DC short-circuit fault is the most severe condition for VSCs [18]. The IGBTs can be blocked for self-protection during faults, leaving reverse diodes exposed to over currents. Irrespective of the location of a short circuit on the DC cable, it can be expressed by an equivalent circuit shown in Fig. 1(a). A short circuit in the DC cable may result into high fault current due to the presence of charged capacitors and low impedance offered by the cable. To find the complete response of the nonlinear circuit, the fault is analysed in two different stages. Stage one is the natural response of the RLC circuit and stage two starts when fault current reaches to peak and capacitor voltage becomes less than the peak of the input voltage. Further analyses of both the stages are discussed in the following sections. A. Stage 1: Capacitor Discharge Immediately after a line to ground fault, the capacitor starts discharging through a cable impedance, as depicted in the vC (0) /L + iL (0) s 1 s2 + R L s + LC (1) where, iL (0) and vC (0) are the current through the inductor and voltage across the capacitor, respectively, just before the occurrence of the fault. L and r are the equivalent series inductance and resistance of the cable up to the fault point. R is the sum of fault resistance RF and r. In this expression, the converter current contribution to the fault is assumed to be negligible because of the slow response of the converter controllers [20]. In the time domain, the fault current i (t) can be written as, i (t) = vC (0) −p 1 t e − e−p 2 t L (p2 − p1 ) + iL (0) −p1 e−p 1 t + p2 e−p 2 t p2 − p1 where, p1 and p2 are the poles of (1), and are given by, 2 R R 1 ± − p1 , p2 = 2L 2L LC p1 , p2 = α ± α2 − ω02 (2) (3) (4) Depending upon whether 1/LC is greater than, less than, or equal to (R/2L)2 , the values of p1 and p2 will be real or complex. As a result, the fault current can be under damped, over damped or critically damped. The time taken for the current to reach peak magnitude for under damped (tu dp ) and over damped (todp ) circuit conditions can be written as [13], ω 1 0 tu dp = tan−1 (5) ω0 α todp = ln (p2 /p1 ) p1 − p2 (6) By substituting typical values of the cable parameters and filter capacitor (r, L, and C; refer to Table A1) the time to reach the peak current, during the fault has been calculated for the two extreme cases of faults, i.e., for direct short circuit (RF = 0 Ω) and for high impedance fault (RF = 0.6 Ω). It is found that the processing time allowed to detect the fault current before the load voltage collapses in the DC system is much shorter as compared to the AC system. B. Stage 2: Diode Freewheeling As the fault occurs, the capacitor starts discharging with high fault transients and its rate of rise is limited by the cable impedance. The second stage appears when the voltage of the DC bus drops to zero and becomes negative, i.e., after todp or tu dp . This has the effect on reversing the voltage at the converter terminals and allows the freewheel diodes to conduct, as shown in Fig. 1(c). In this figure, D represents the equivalent of the, diodes conducting in any converter leg [21]. This provides an alternative current path irrespective of the IGBT conducting state 174 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 32, NO. 1, FEBRUARY 2017 and changes the current response. In this phase, the freewheel diodes experience almost ten times the nominal current value which rises rapidly. This work tries to detect the fault on the DC network before the system enters into the second stage, so that the sensitive power electronic converters and its components can be protected from the high fault current. III. DC FAULT CURRENT CHARACTERISTICS In (2), the fault current i (t) depends upon vC (0) which is fixed for a given DC MG and the line current (iL ) just before the fault. The time derivative of (2) can be written as: di (t) vC (0) = −p1 e−p 1 t + p2 e−p 2 t dt L (p2 − p1 ) + iL (0) 2 −p 1 t p1 e − p22 e−p 2 t p2 − p1 (7) Immediately after the fault, at t = 0+ , the magnitude of the current derivative is given by, vC (0) di (t) = − iL (0) (p1 + p2 ) (8) dt L Substituting the values of p1 and p2 from (3), (8) can be written as, vC (0) di (t) R = − iL (0) dt L L (9) For RF = 0, R is equal to the equivalent cable resistance up to the fault point, denoted by r. For direct short circuit, the contribution of the iL (0) component, compared to vC (0) to the di/dt magnitude will be very small and is neglected. Hence, only vC (0) component may be considered in deciding the primary threshold for the protection. The second order derivative of the line current can be obtained by differentiating (7) with respect to time, and can be written as, d2 i (t) vC (0) 2 −p 1 t p1 e = − p22 e−p 2 t 2 dt L (p2 − p1 ) + iL (0) 3 −p 1 t −p1 e + p32 e−p 2 t p2 − p1 (10) Immediately after the fault at t = 0+ , the magnitude of the second derivative can be given by the following equation. d2 i (t) vC (0) (p1 + p2 ) + iL (0) p21 + p22 + p1 p2 =− 2 dt L (11) By substituting the values of p1 and p2 in terms of circuit parameters, (11) can be written as, 2 R d2 i (t) vC (0) R 1 = − + i (0) − (12) L dt2 L2 L2 LC The second derivative magnitude depends upon the equivalent resistance R = RF + r. The initial line current contribution to d2 i/dt2 will be zero for R = Rc , where Rc = L/C. For small range of RF , i.e., 0 < RF < Rc , the equivalent circuit of the network during the fault will be under damped. This results in Fig. 2. Fault current, and its first and second derivative profile for different values of R F . high magnitude of the first order current derivative, which is much larger than its second order derivative. The test system in Fig. 1(a) is simulated for different values of RF to observe the current i, di/dt, and d2 i/dt2 profile for under and over damped circuit conditions. In this work, the fault is created at t = 0.2 s, and di/dt, d2 i/dt2 are calculated at t = 0.2001 s, as shown in Fig. 2. It is observed that both the derivatives are equal in magnitude because of the insufficient number of data samples to calculate the second order derivative at the time of fault. This is further explained in Section IV.B. The d2 i/dt2 is calculated at time t = 0.2002 s, and found that, as RF increases, d2 i/dt2 magnitude approaches to di/dt value. The initial line current may have more impact on di/dt and d2 i/dt2 in the presence of RF . During normal operation, transients such as step load change and operating mode change, the calculated first and second order derivatives should not cross the threshold. Line current i12 and their derivatives are monitored to see the effect of step load transient and mode switching transient, as shown in Fig. 3. It is observed that the calculated derivatives are much smaller than their respective thresholds, and hence, unwanted tripping is avoided. IV. PROTECTION SYSTEM COMPONENTS The protection scheme for DC MG is analytically developed in previous sections. The protection system is designed and developed for DC MG of Fig. A1 (refer to the Appendix) using various components. Detailed description of these components and their functionality are provided in the following sections. A. Protective Device (PD) A PD, considered in this paper, consists of a digital relay, a solid state circuit breaker, and a current transducer, as shown MEGHWANI et al.: NONUNIT PROTECTION SCHEME FOR DC MICROGRID BASED ON LOCAL MEASUREMENTS 175 TABLE I TIME TO REACH PEAK CURRENT (tu d p ) SEEN BY PDS FOR DIFFERENT FAULTS IN ms Fault F1 F2 F3 F4 F5 tP D 1 . 2 tP D 2 . 2 tP D 3 . 2 tP D 4 . 2 tP D 5 . 2 1.7 2.8 3.1 4.4 4.0 2.2 1.1 2.2 3.3 4.2 3 2.3 1.3 3.4 3.5 4.1 3.0 3.3 3.5 4.0 5 4.5 4.8 3.3 2.4 B. Parameter Determination To protect the system from line to ground fault, the protection algorithm is implemented in all P Ds. The current derivative and its rate of change are numerically calculated as discussed in the following sections. 1) di/dt Calculation: di/dt is calculated using backward finite difference approximation [25], and can be written as, Δi i (t0 + Δt) − i (t0 ) di = lim = Δ t→0 dt Δt Δt Fig. 3. Line current i1 2 , and its first and second derivative profile under normal transients. Fig. 4. Protective Device and its components. in Fig. 4. DC current can be measured by a current transducer such as Hall effect devices. The sensor output is a voltage signal which facilitates an easier integration with the digital signal processing devices. Digital relays are equipped with microcontroller for setting the thresholds, and analog to digital converters to transform the measurements into digital form. The current is sampled and its rate of change is calculated and compared with the threshold setting to generate a trip signal. The DC cables are protected by PDs connected on both the ends as shown in Fig. A1. The faulted network in the DC system should be isolated, as fast as possible, to avoid shutdown. Reference [19] compared different DC CB technologies to find their suitability in the DC shipboard. The authors compared solid state CB (SSCB), Hybrid CB, and electromagnetic CB [22]–[24] on the basis of their operating time for unmanned aerial vehicle application. Comparison between the circuit breaker operating times suggests that SSCBs are the most suited for use within DC networks. Therefore, in this paper 50 μs operating time (typical for SSCB) of circuit breaker is considered. There is no additional delay in the operation because of utilization of the local measurements. (13) and Δi can be written as, Δi = ik − ik −1 (14) where, k is the sampling instant, and ik , and ik −1 are the present and the the previous sampled line current. Since the sampling time is same for all the measurements, henceforth only current difference is measured at consecutive sampling instants to calculate the time derivative. 2) d2 i/dt2 Calculation: d2 i/dt2 is calculated for each line using the present and the previous calculated value of di/dt at time interval dt, and can be written as: d2 i d di d Δi Δ2 i = (15) = = 2 dt dt dt dt Δt (Δt)2 where, Δ2 i can be written as, Δ2 i = Δik − Δik −1 2 Δ i = ik − 2ik −1 + ik −2 (16) (17) The fault occurs at k = 0 and k is increased by one at every instant of sampling. At k = 1, Δi = Δ2 i because i0 = i−1 = iL (0). As a result, the actual value of Δ2 i is calculated at k = 2 and utilized for tripping decision. The calculated Δ2 i is compared with the respective threshold setting. Once the calculated value exceeds the threshold level, PD will issue a trip signal. 3) Sampling Frequency: The first and second order current differences are likely to be maximum for under damped circuit condition. To capture the current profile accurately under this condition, the time to reach the peak current is calculated for different fault locations. Based on the calculated time the sampling frequency is decided. The current transient during fault depends upon the network parameter and fault location. The time required to reach the current at peak value is analytically calculated for faults at different locations and listed in Table I. 176 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 32, NO. 1, FEBRUARY 2017 From Table I, the minimum time to reach the peak of current is observed as 1.1 ms for fault F2 . To capture the high di/dt current profile accurately, approximately 1/10th of the time is taken as sampling period [26]. V. THRESHOLD SETTING CALCULATIONS The line current is sampled using high bandwidth current transducers, and converts it into digital form using analog to digital converters embedded into the digital relay. Other mathematical processing, such as Δi and Δ2 i pickup threshold settings, their comparison with the pre-set values, are implemented in the digital relay. Δi and Δ2 i threshold setting calculations are, further, discussed in the following sections. A. Threshold Settings for Δi The Δi settings are, further, classified as fixed threshold and adaptive threshold. Fixed pickup threshold, denoted by Δim ax , is calculated once for a given grid configuration. The adaptive threshold (Δim in ) depends upon the operating condition of the microgrid. 1) Fixed Threshold: The magnitude of Δi depends upon the fault location and its impedance. The threshold of Δi, will be calculated for the direct short circuit fault that occurs at the end of the line. The equivalent circuit considered in this case will be under damped for 0 ≤ RF ≤ Rc . During a fault, the expression for Δi for the time interval Δt can be derived from (9) and written as, Δim ax = vC (0) Δt Leq (18) where, Leq is the equivalent inductance up to the fault F1 (refer Fig. A1 in the Appendix). As a fault occurs, the filter capacitor connected to the VSC starts discharging with di/dt limited by Leq , and is given by, Leq = Lc1 + (Lc5 + Lx ) Lx Lc5 + 2Lx (19) where, Lc5 and Lx are the inductance of the cable from Bus 1 to Bus 5 and VSC to Bus 1, respectively, as shown in Fig. 5(a). The fixed threshold settings are calculated based on the equivalent circuit considered from the P D1.2 up to the fault point. In order to validate the equivalent model and its parameters in terms of the frequency response and rise time, the Bode plot of impedance is drawn for the complete system and its equivalent model as shown in Fig. 5(b). The fixed pickup threshold, Δim ax , for P D1.2 is determined by considering the fault F1 , close to P D2.1 , i.e., at 100% of the line inductance (Lc1 ). If fault occurs close to the P D1.2 , Δi magnitude will be maximum. On the similar lines, the fixed thresholds are calculated for P Dx.2 and P Dx.1 , where x {1, 2, ..5} and tabulated in Table II. These settings depend upon the cable parameter and the DC MG voltage, and are independent of the operating condition of the microgrid [27]. 2) Adaptive Threshold: For high impedance fault, the line loading also plays an important role in deciding the Δi magnitude. To calculate the threshold Δim in for a fault, the worst Fig. 5. plots. (a) Faulted network, its RLC equivalent circuit and, (b) their Bode TABLE II FIXED THRESHOLD (Δim a x ) SETTINGS FOR ALL PDS Bus No. x 1 2 3 4 5 Cable Length P D x . 2 and P D ( x + 1 ) . 1 (m) Δ i m a x Fixed Threshold (Amp) for P D x . 1 Δ i m a x Fixed Threshold (Amp) for P D x . 2 1000 500 200 300 1000 1100 2000 4000 3000 1000 -2000 -4000 -3000 -1100 -1000 case fault impedance of 0.6 Ω is considered. The value of Δi is calculated for different line loading using (9). Since, the fault resistance, RF r, the value of Δim in can be calculated as, Δim in = vC (0) 0.6 − iL (0) Δt Leq Leq (20) The equivalent inductance up to the fault point is calculated using (20). The value of Δim in depends upon the line current iL (0) at the time of the fault. The threshold setting for Δi keeps on varying with the operating condition of the grid. Hence, the setting for Δim in is self adjusted with different modes of operation of the microgrid. B. Issues With the Δi Protection Scheme For a given DC MG architecture and rating, the change in fault current Δi depends upon the following parameters: 1) Cable Length: For a direct line to ground fault, the change in fault current, i.e., Δi, immediately after the fault, depends upon the cable inductance. High current transients are observed if the fault occurs near the source, i.e., with small cable length, as shown in Fig. 6(a). MEGHWANI et al.: NONUNIT PROTECTION SCHEME FOR DC MICROGRID BASED ON LOCAL MEASUREMENTS 177 Fig. 7. Δi vs Δ 2 i for fault F1 at different locations and resistances 0 < R F < 0.6 Ω. Fig. 6. Δi1 2 for fault F1 (a) at distance d from P D 1 . 2 (b) for different line current flows and (c) for different fault resistance R F . 2) Line Loading: Change in the line current also depends upon the initial value of the line current before the occurrence of a fault. For heavily loaded lines, change in the fault current will be less as compared to the lightly loaded lines, as shown in Fig. 6(b). 3) Fault Impedance: Fault impedance introduces a damping in the current transients, which reduces its rate of rise and peak value. As a result, Δi reduces as the fault impedance increases, as shown in Fig. 6(c). The threshold of Δi setting is calculated for each cable, considering 100% length and 0.6 Ω fault impedance. For small cable with high impedance fault, i.e., at F1 , the relay P D1.2 issues a trip signal. The same relay may operate for the fault in longer cable with small fault impedance, i.e., at F2 . P D1.2 operate for fault F2 causes an unwanted line outage, which results into poor power quality. This is a selectivity issue and it becomes more pronounced as the cable length reduces. To solve this problem, threshold based on Δ2 i is applied. C. Adaptive Threshold Settings for Δ2 im in Similar to Δi calculation, Δ2 i is also derived analytically. The value of Δ2 im in is calculated for the worst case RF value. The target is to detect and isolate the fault in the grid before initiation of the second stage as explained in Section III.B. Any protection system must locate the fault in DC MG in a timely manner so that the circuit breaker operates before the current reaches its peak value. By substituting R = 0.6 Ω in (12), Δ2 im in can be calculated using: Δ2 im in = − vC (0) 0.6 0.36 + iL (0) 2 Δt2 2 Leq Leq (21) Since, RF Rc , the initial line current effect cannot be neglected. In this mode, the current is over damped because (R/Leq )2 1/Leq C and, hence, iL (0) will also contribute to Δ2 im in . Since the line loading depends upon the operating mode of the DC MG, the calculation of Δ2 im in should be self Fig. 8. Flow chart of the proposed protection scheme. adjustable. For a given operating condition, Δ2 im in has been calculated and shown in Fig. 7. For the under damped circuit conditions, where r < RF < Rc , Δ2 i will be less than Δ2 im in . VI. FAULT DETECTION ALGORITHM The currents are continuously sampled and monitored by their respective PDs connected on both sides of the cable. Under steady state, the calculated value of Δi and Δ2 i will be zero. During fault transients, the calculated current difference will exceed the threshold settings. Fig. 8 shows the flow diagram of the algorithm used in this paper. According to the flowchart, for faults with 0 ≤ RF ≤ Rc , the calculated Δi will be greater than Δim ax . This causes 178 Fig. 9. IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 32, NO. 1, FEBRUARY 2017 Impact of the short circuit at F1 in Fig. A1. Fig. 10. Short circuit at F1 cleared by P D 1 . 2 and P D 2 . 1 . Fig. 11. High impedance fault F1 cleared by P D 1 . 2 and P D 2 . 1 . initiation of trip signal to the breaker, which will isolate the faulted section of the network. In case of line to ground faults with Rc ≤ RF ≤ 0.6 Ω, the calculated Δi will lie within the range of Δim in < Δi < Δim ax . In this case, the decision for tripping will be transferred to Δ2 i based threshold setting. Calculated Δ2 i will be greater than Δ2 im in for high impedance faults, where RF lies between Rc and 0.6 Ω. VII. SIMULATION RESULTS AND OBSERVATIONS To demonstrate the proposed protection scheme, fault F1 shown in Fig. A1 is simulated at t = 1.5 s. High transient currents as shown in Fig. 9, cause a voltage collapse within 5 ms of the fault occurrence. This implies that the protection required for the DC system should be much faster as compared to the AC system [7]. A. Case I: Fault in Grid Connected Mode 1) Direct Short Circuit Fault: The magnitude of Δi12 is continuously calculated from sampled current, and compared with the threshold given in Table II. As the fault F1 occurs, calculated Δi12 exceeds the pre-set value and generates a trip signal for their respective PDs. P D1.2 and P D2.1 to isolate the faulted line, thereby, restoring the system, as shown in Fig. 10. The direct line to ground fault is detected within 100 μs, which ensures that the system does not enter into the second stage of fault. In this short time span, the load bus voltage reduces to 0.95 kV and, hence, VSC will not be affected by the fault. Large filter capacitor discharge through a small RF causes the line current i12 to reach to 5 pu before the activation of the PD. As a result, the converters and other system components are protected. The DC bus voltage momentarily drops to 0.78 pu because of high di/dt during the fault, and restores back to 1 pu as the fault is cleared. To further test the reliability and consistency of the scheme, faults at different locations are simulated, viz., at F3 and F4 , as shown in Figs. 12 and 13, respectively. MEGHWANI et al.: NONUNIT PROTECTION SCHEME FOR DC MICROGRID BASED ON LOCAL MEASUREMENTS Fig. 12. Fault F3 in grid connected mode. Fig. 14. 179 High impedance R F = 10 Ω fault F1 . detect fault is inversely proportional to the fault current. But in the proposed algorithm, only two operating time are possible: when only Δi is required to detect the fault and, when both Δi and Δ2 i are utilized in detecting the fault. In Fig. 11, the fault F1 is cleared by P D1.2 and P D2.1 . The line current i12 reaches 0.1 kA before the fault clearance due to the low value of the di/dt. In this paper, a typical value of RF = 0.6 Ω is considered and the protection system is designed. The scheme is also simulated for high ground fault resistance RF = 10 Ω, and the result is shown in Fig. 14. B. Case II: Fault in Islanded Mode Fig. 13. Fault F4 in grid connected mode. 2) Fault With Rf : The system is tested for the line to ground fault F1 via fault resistance RF = 0.6 Ω. Other operating conditions and fault location remain same as in the previous case. In this case, Δi12 < Δim ax and, therefore, Δ2 i is calculated and compared with its threshold. The fault is detected within 200 μs because of the requirement of the second calculation and its comparison. In this scheme, the time to detect the fault does not depend on the fault impedance, which is typically present in the traditional AC protection. In AC protection, the time to To further test the robustness of the proposed algorithm, the fault F1 is simulated in islanded mode of operation. In this mode, AC grid is disconnected from the rest of the network and is not able to supply the DC MG load. As a result, the load voltage drops to 1.15 kV, or 0.95 pu. Since the AC grid is not supplying power to the system, current i12 will be same as i23 as shown in Fig. 15. As the fault F1 is cleared, both the line currents i12 and i23 become zero with no support from the grid. A direct short circuit fault causes a high di/dt, which results in the load voltage drop to 1 kV, with the same operating conditions. The impact of the fault F1 with RF = 0.6 Ω, is shown in Fig. 16. C. Case III: Measurement Noise The proposed protection scheme is further tested in the noisy environment. The noise can be introduced in the signal due to measuring device or due to the usage of communication channel. Since, the measurements and their derivatives are processed 180 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 32, NO. 1, FEBRUARY 2017 Fig. 17. Fig. 15. Fault F1 detected under noisy environment. Direct short circuit fault F1 in islanded mode. derivatives are much below their pick-up threshold setting and, hence, cause no unwanted tripping. For more noise in the signal, the de-noising methods can be utilized before evaluating their derivatives [29]. VIII. CONCLUSION An effective protection system scheme for a DC MG has been proposed in this paper. The natural characteristics of the fault current have been divided into under damped and over damped categories, and used to develop the protection algorithm. There is practically no time delay involved, as the measurements utilized are local. In this scheme the first and the second order derivatives of the current have been utilized to detect the low and high impedance faults in the network. The derivatives are applied to discriminate various faults such as faults on the adjacent lines, and faults with different impedance values. The thresholds for the derivatives are analytically calculated and utilized in the algorithm. The proposed protection scheme is simple, fast and suitable for protecting DC MG from line faults, without interrupting the normal operation of the grid. A protection solution for loop type DC MG system is demonstrated, which is easy to extend to other configurations. Fig. 16. High impedance fault F1 in islanded mode. locally, the noise introduced due to the communication link can be neglected. Therefore, only the measurement noise is considered. The power distribution factor of the noise in the measurement can be taken as Gaussian [28]. The system is simulated for fault F1 with the signal to noise ratio of 20 dB, as shown in Fig. 17. It is found that the noisy signal and their APPENDIX SYSTEM CONFIGURATION A loop type DC microgrid system is considered for designing the protection system, as shown in Fig. A1. The DC bus voltage is controlled by grid-VSC in grid connected mode or by battery converter in islanded mode [30]. The component ratings of all the modules are given in Table A1 [31]. MEGHWANI et al.: NONUNIT PROTECTION SCHEME FOR DC MICROGRID BASED ON LOCAL MEASUREMENTS [10] [11] [12] [13] [14] [15] Fig. A1. DC MG Architecture considered in this work. 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