A modified bridge-type nonsuperconducting fault current limiter for distribution network application

Received Mar 11, 2021 Revised Apr 29, 2021 Accepted Jul 12, 2021 The electrical distribution network is undergoing tremendous modifications with the introduction of distributed generation technologies which have led to an increase in fault current levels in the distribution network. Fault current limiters have been developed as a promising technology to limit fault current levels in power systems. Though, quite a number of fault current limiters have been developed; the most common are the superconducting fault current limiters, solid-state fault current limiters, and saturated core fault current limiters. These fault current limiters present potential fault current limiting solutions in power systems. Nevertheless, they encounter various challenges hindering their deployment and commercialization. This research aimed at designing a bridge-type nonsuperconducting fault current limiter with a novel topology for distribution network applications. The proposed bridge-type nonsuperconducting fault current limiter was designed and simulated using PSCAD/EMTDC. Simulation results showed the effectiveness of the proposed design in fault current limiting, voltage sag compensation during fault conditions, and its ability not to affect the load voltage and current during normal conditions as well as in suppressing the source powers during fault conditions. Simulation results also showed very minimal power loss by the fault current limiter during normal conditions.


INTRODUCTION
In recent years, the electrical distribution network has grown in complexity with the introduction of distributed generation (DG). These additional technologies have led to issues of increased power losses, voltage sags/swells, and increased fault currents [1]. Research shows that high penetration of photovoltaic (PV) systems can lead to an increase in fault current magnitude in the order of 7% [2]. These fault currents are higher in locations closer to PV generations [3]. An increase in PV penetration leads to an increase in fault currents which also leads to an increase in protective relays fault currents, and these relays fault currents depend on the locations of the PV systems [4]. A sensitivity analysis on the impact of rooftop PV systems on the distribution network showed that the presence of PV systems on a low voltage feeder increased short circuit fault levels by 10% [5]. A single PV system will have very minimal contribution to fault current but when considering the collective contribution from all PV systems installed across the network, the fault

Faults and fault current calculation
A fault is an abnormal condition in the electrical network that comes as a result of the failure of operating equipment. Two categories of faults can occur [29], a) The open-circuit fault that results in the seizure of current flow in the circuit, and b) the short-circuit fault that is as a result of insulation failure due to overloading and overstressing of feeders or degradation of feeder's insulation which leads to high current flow in the circuit.
Various methods are used for short-circuit fault current calculations, amongst them is the sequence method. The sequence method of fault calculation involves building the impedance matrix of the circuit and calculating the fault current. For an electrical circuit with a sending end voltage , a line impedance and a load node j, the voltage at node j before the occurrence of a ground fault at that node is given by; After the fault occurrence, the voltage is zero giving a change in voltage of − . As a result, the current flow, from node j into the circuit is; Where is the current from node j due to the fault and the total impedance due to the fault given by; Where is the line impedance and is the fault impedance. Since before the fault, no current was flowing into the circuit from node j, the fault current, from the circuit into node j is then calculated as; The various types of short-circuit faults which are three-phase fault, single line-to-ground fault, double lineto-ground fault, and line-to-line fault differ in their calculations by their expressions for .

Proposed modified bridge-type nonsuperconducting fault current limiter
The topology of the proposed bridge-type NSFCL is shown in Figure 2 (a). The NSFCL is made up of 3 main parts; a bridge rectifier, DC reactors, and a semiconductor switch. An insulated gate bipolar transistor (IGBT) is used as the semiconductor switch. Two DC reactors; one of smaller value placed in series with the IGBT and one of larger value placed in parallel with the IGBT. The IGBT is controlled by a command circuit that turns it ON during normal conditions and OFF during fault conditions. The series reactor is aimed at limiting the abrupt change in the current flow through the IGBT during a fault condition. The DC reactors are modelled each with a reactance and a parasitic resistance. The proposed modified bridge-type NSFCL operates as follows; a. During normal conditions (no fault), the IGBT is turned ON and the parallel branch (R p , L p ) shortcircuited. The series reactor (L s , r s ) is fully charged to the maximum current supplied by the source and therefore acts like a short-circuit. This makes it invisible to the network during normal conditions. The IGBT is kept ON by a command circuit that monitors the series reactor current and compares it with a predefined threshold value so that inasmuch as the series reactor current is lesser than the threshold current, the IGBT remains ON. b. During a fault condition, the IGBT is turned OFF because the series DC reactor current, I d becomes greater than the threshold current. The parallel reactor (L p , R p ) is automatically and quickly inserted into the circuit, thereby limiting the fault current. The IGBT will continuously switch ON/OFF during fault conditions until the fault is cleared; leading to a distorted supplied current waveform during a fault condition. To solve this, an appropriate switching time is chosen for effective fault current limiting capabilities and a relatively smooth limited current during fault conditions.

Analytical analysis
The proposed NSFCL inserted into the test network is shown in Figure 2 (b) and analyzed as follows;  Figure 3. During normal conditions, the reactor charges during the positive cycle of the line current and discharges during the negative cycle. During charging, current flows from the source through D 1 , L s , r s , IGBT, and D 3 to the load through the transmission line. The voltage equation, in this case, is given by; where: and tanθ = Lω R making i (t) the subject of the (9), we obtain

Figure 3. Line current and reactor current waveforms during normal operation
During the negative sequence of the line current, the series DC reactor is in the discharging mode which begins at t 2 as shown in Figure 3. In this mode, all the diodes are turned ON and the series DC reactor is short-circuited. Hence do not interfere in the normal operation of the network, implying.
the supplied line current in this mode can be obtained from; where: therefore, from (17), line the current is obtained to be, where: Z = √R 2 + (Lω) 2 , θ = tan −1 Lω R and i 2 = i 2(t) The discharging of the series DC reactor is a result of its parasitic resistance. At t = t 3 , the series DC reactor current again equalizes the line current. In the discharging mode; from t 2 to t 3 , the DC reactor has no effect on the network because it is not being charged. Similarly, the effect the series DC reactor has on the network in the charging mode is very negligible because the current it carries is almost equal to that of the line current. The charging and discharging currents of the series DC reactor are shown in equations (12) and (18). From these equations, it is seen that both charging and discharging currents consist of ripple and DC components. It is important to minimize the ripple component as much as possible because it is responsible for the voltage drop across the series DC reactor's inductance, L s during normal operation [30]. The series DC reactor current is given by where i max is the reactor's maximum current and i rd,p−p is the peak to peak value of the reactor AC current. From Figure 3, integrating the discharging equation (18), we obtain where: T = (t 3 − t 0 ) = 10ms for 50Hz networks [30]. From (19) and (20).
for r s = 0 from (23) and (24), it is seen that increasing L s increases I DC and reduces the ripple component.
b. During fault conditions During fault conditions, the IGBT is turned OFF and the parallel path is automatically and instantly inserted into the network. In the charging mode in fault conditions, the source voltage is given by where: making i (t) the subject of (25), we obtain L d = L s + L p (34) t 9 is the start of discharging during fault condition. The supplied current (inrush current) in this mode is obtained from where: therefore, the inrush current could be obtained from (35) It is seen that the power loss as a result of the introduction of the fault current limiter is very small and negligible compared to the overall feeder losses.

d. Voltage drop and power loss compensation
The voltage drop in the proposed modified bridge-type NSFCL is across the power electronic switch (IGBT) and the series DC reactor during normal operation. This voltage drop can be resolved by appropriately sizing a DC power source or rectifier circuit and placing it in series with the series reactor as shown in Figure 4. The DC voltage source will aid in smoothening the DC reactor current during normal operation. Hence eliminating the ripple component of the DC current and thereby reducing the power loss in the NSFCL. The voltage of the DC source is calculated as: where: U bat is the DC source voltage, U d is the voltage drop across a single diode, U sw is the voltage drop across the IGBT, r s is the series DC reactor resistance and I d is the reactor current.

RESULTS AND DISCUSSION
The parameters chosen for the simulation of the proposed bridge-type NSFCL using PSCAD/EMTDC are shown in Table 3. Electromagnetic transient analysis of the test network was done without the proposed modified bridge-type NSFCL, with the NSFCL, and with the NSFCL with battery. The simulation settings were; a) simulation runtime of 0.5s, b) a line-to-ground fault occurred at 0.3s and lasted for 0.05s, and c) the circuit breaker cleared the fault 0.03s after its occurrence (that is at 0.33s) and restored the network 0.07s later (at 0.4s). The simulation results are:

Line current
The line current shoots to more than 30kA during fault conditions when no NSFCL is used as can be seen in Figure 5. With the insertion of the proposed bridged-type NSFCL, the fault current is limited to the desired value (below 0.6kA), thereby protecting the source and the load during fault and enabling the circuit breaker to safely clear the fault as shown in Figures 6 (a) and (b). This also removes any possible stress on the network during fault conditions. The introduction of a DC source in the NSFCL smoothens the DC reactor current during normal conditions (Figure 6 (b)) compared to without the DC source (Figure 6 (a)). Hence reduces power losses in the NSFCL as shown in Table 4. In addition, the designed bridge-type NSFCL does not affect the line current waveform during normal conditions as seen in Figures 7 (a)-(c).   During a fault condition, the sending end voltage experiences a slight drop in magnitude when no NSFCL is used as is seen in Figure 8 (a). This voltage drop is a result of stress on the power source due to the fault. This stress is removed by the proposed NSFCL and therefore no voltage drops during fault condition Figure 8(b). This makes the proposed NSFCL suitable for voltage ride-through applications.

Sending end active and reactive power
The occurrence of the line-to-ground fault leads to an overshoot of the active and reactive powers supplied by the source when no NSFCL is used leading to dangerous stress on the generating units and excess system overload as shown in Figure 9 (a). This situation is adequately solved by the proposed NSFCL which keeps the supplied active and reactive powers within limits during fault conditions until the fault is cleared by the circuit breaker as can be seen in Figure 9

Load (receiving end) voltage and current
When no NSFCL is used in the network, the load continues to receive small voltage and current during fault condition until the fault is cleared as shown in Figures 10 (a), and (b). The insertion of the proposed NSFCL into the network suppresses these ripples during fault conditions as illustrated in Figures 11 (a) and (b). It should also be noted that the proposed modified bridge-type NSFCL does not distort load voltage and current waveforms during normal conditions even without the DC source. Therefore, the proposed design does not introduce total harmonic distortions in the network.

CONCLUSION
In this paper, the necessity of fault current limiters in power systems were examined and the drawbacks of existing NSFCLs were outlined. The aim was to propose an efficient and effective bridge-type nonsuperconducting fault current limiter with a novel topology for distribution network applications. The target was to develop an NSFCL that is almost invisible to the network during normal network operation and therefore leading to very minimal power losses, and on the other hand, adequately limiting the fault current to desired values during fault conditions. The proposed modified bridge-type NSFCL was designed and simulated using PSCAD/EMTDC and results showed outstanding performance of the novel NSFCL in, i) fault current limiting, ii) sending end voltage sag compensation during the fault, iii) suppression to desired values of supplied active and reactive powers during fault conditions, iv) not distorting load voltage and current waveforms, and v) minimal power losses during normal condition.
The proposed modified bridge-type NSFCL proves to be better than existing NSFCLs in terms of the reduced number of components used and the novel series and parallel DC reactors configuration used. The proposed novel NSFCL is a cost-effective and all-in-one efficient solution for distribution network fault current limiting, voltage ride-through capability enhancement, power quality improvement, and voltage sag compensation. These problems are problems that are faced by the distribution network with the increasing number of DGs being integrated into the network. With the proposed bridge-type NSFCL, there will be no need for protective equipment upgrades or replacement. The future of this research work will be the practical implementation of the proposed NSFCL to validate its practical effectiveness as simulation results have demonstrated its effectiveness in distribution network applications.