A Study on the Agent (Protective Device)-based Fault Determination and Separation Methodology for Smart Grid Distribution System

Abstract

This paper proposes a new fault isolation methodology for a smart protective device which plays an agent role on the smart grid distribution system with the distributed generation. It, by itself, determines accurately whether its protection zone is fault or not, identifies the fault zone and separates the fault zone through the exchange of fault information such as the current information and the voltage information with other protective devices using bi-directional communication capabilities on the smart grid distribution system. The heuristic rules are obtained from the structure and electrical characteristics determined according to the location of the fault and DG (Distributed Generation) when faults such as single-phase ground fault, phase-to-phase short fault and three-phase short fault occur on the smart grid distribution system with DG.

1. Introduction

Recently, from the perspective of global warming, a smart grid distribution system is receiving great interest which has the distributed power sources and the bicommunication capability between distribution facilities that are required to enhance the rate of green energy generation and improve the efficiency of the energy use. The existing central-based distribution automation systems have significant contribution in improving the power supply reliability for distribution system with radial structure [1-2]. However, because it monitors system status, collects operation data and executes fault processing procedure when any fault occur, based on polling method under oneway communication, it can not be applied directly to a smart grid distribution system with bi-directional communication capability due to the reasons described below. i) Because of a time delay caused by the overloaded server computer and communication traffic to perform the complicated and diversity functions of the smart grid distribution system, power supply reliability may be considerably deteriorated. ii) Because its feeder automation solutions and protective devices were developed properly to distribution system with radial structure, it can not be applied directly to a smart grid distribution system which is a mixed of the tree feeders and loop feeders caused by distributed power generations.

Until now, the various methodologies have been presented to support distribution automation system. References [3-4], under normal condition, propose feeder reconfiguration methodologies to reduce the feeder loss of the distribution system, and references [5-6] to obtain the load balancing. On the other hand, references [7-8], under emergency condition, present methodologies to identify fault location, references [9-11] to obtain service restoration and references [12-14] to determine high impedance fault. However, most of these studies propose feeder reconfiguration strategies for central-based distribution automation system based on one-way communication. Although references [15-17] present methodologies using bi-direction communication and in particular, in order to solve the problem i), reference [17] propose heuristic rule-based self-fault zone isolation methodology and self-high impedance fault zone isolation methodology using bi-directional communication separately, because these studies handle only distribution system with radial structure, the proposed methodologies can not be applied directly to smart grid distribution system with a mixed structure of the tree structure and loop structure. Thus, in order to enhance the smart grid distribution system’s operating efficiency and reliability, a smart protective device is needed which can overcome the problems i) and ii) by identifying and isolating by itself the fault using its bidirection communication capability when any fault occurs on the smart grid distribution system with a mixed structure of the tree structure and loop structure caused by distributed generations.

Accordingly, this paper proposes a new fault isolation methodology for the smart grid distribution system with the distributed distribution. It, by itself, determines accurately whether its protection zone is fault or not, identifies the fault zone and separates the fault zone through the exchange of fault information such as the current information, the voltage information with other protective devices using bi-directional communication capabilities on the smart grid distribution system. The heuristic rules are obtained from the structure and electrical characteristics determined according to the location of the fault and DG (Distributed Generation) when faults such as single-phase ground fault, phase-to-phase short fault and three-phase short fault occur on the smart grid distribution system with DG

 

2. Design of Smart Protective Device

A smart protective device is a heuristic rule-based solution that it, by-itself, can identify and isolate the fault zone by exchanging fault information with adjacent devices through two-way communication capability.

2.1 Heuristic rules

The heuristic rules (HRs) are obtained from the structure and electrical characteristics determined according to the location of the fault and DG when faults such as threephase short fault, phase-to-phase short fault and singlephase ground fault occur on the smart grid distribution system with DG

HR 1] If any smart protective device experience the fault current, its WX is equal to WR and its protection zone is dead end zone, its protection zone is fault zone. This rule can be applied when any fault occurs at downstream position from the DG and at dead end zone.

Here, WX is the direction of the measured current at selfposition of any smart protective device, WR is the direction of the current IT flowing from substation transformer of tree-structure feeder without DG. On the other hand, WT and WD, as shown in Figs. 1-2, represent the direction of the fault current IT flowing from substation transformer and the direction of the fault current ID flowing from distributed generation when a fault occurs at the location f on the feeder with DG separately. And T represents a substation transformer (MTr), D the distributed generation, and bus Bi,j the location of jth protective device of ith feeder. And the zone protected by the protective device on B1,5 is dead-end zone.

Fig. 1.Case a fault occurs at downstream position of DG

Fig. 2.A case with DG at the end point of the feeder

HR 2] If any smart protective device experience the fault current, and all the downstream smart protective devices on its protection zone which is based on the direction of the fault current does not experience the fault current (more precisely, is less than the fault current threshold value, ISET), its protection zone is fault zone.

This rule can be applied when single-line ground fault in which only one-side end of broken conductor is grounded occurs on tree or loop structure feeder. Fig. 1 shows when the fault occurs on tree structure feeder. As shown in Fig. 1, when any fault occurs at downstream position from the DG, i.e., when any fault occurs at the position f on the zone surrounded by the protective devices Sp and Sq, the protective device Sp experiences the fault current.

However, the downstream smart protective device Sq of its protection zone {Sp, Sq} does not experience the fault current. Keep in mind that in the case of a short fault between two phases or a ground fault with ground resister, the current and voltage measured by Sq does not necessarily become zero.

Fig. 2 shows when the fault occurs on loop structure feeder. As shown in Fig. 2, when any fault occurs at downstream position from the DG, i.e., when any fault occurs at the position f on the zone surrounded by the smart protective devices Sp and Sq, one of Sp and Sq can experience the fault current. Accordingly, Each of Sp and Sq must identify whether its protection zone is fault zone or not separately. If up-side end of broken conductor is grounded, the upstream smart protective device Sp of its protection zone {Sp, Sq} does not experience the fault current, On the other hand, if down-side end of broken conductor is grounded, the downstream smart protective device Sq does not experience the fault current.

HR 3] If any smart protective device experience the fault current, anyone of the downstream smart protective devices on its protection zone which is based on the direction of the fault current experience the fault current, its WX is equal to WR and WX of one of downstream protective devices on its protection zone is not equal to WR, its protection zone is fault zone.

This rule can be applied when a single-line ground fault with the unbroken conductor, a short fault between two phases or a three phase short fault occurs on loop structure feeder. When any fault occurs at upstream position from the DG, i.e., when any fault occurs at the position f on the zone surrounded by the protective devices Sp and Sq as shown in Fig. 2, both of Sp and Sq experience fault current. At this time, if two protective devices all try to identify whether their protections are fault zone or not, it is not efficient. In this case, a smart protective deice Sp in which its WX is equal to WR will be enough for fault zone identification to continue with the fault zone search process. Here, the direction of current measured by the smart protective device Sp is WR, but the direction of current measured by the downstream smart protective device Sq of its protection zone is reverse to WR.

Fig. 3 shows a distribution system introduced to explain the direction of fault current when a single-line ground fault with the unbroken conductor occurs on loop structure feeder.

Fig. 3.Example system to show direction of fault current

In order to simply notation, and show clearly the changes of the voltage and current before and after the fault at the positions of smart protective devices Sp and Sq, the single phase circuit was adopted, and all impedances and section loads on the circuit loads were represented as the resistance component values. In Fig. 3, the voltage, the frequency and the source impedance of power source (PS) are 13.2 kV, 60Hz and 3Ω separately, and the voltage and internal impedance of the distributed generation (D) is 13.2 kV and 20Ω. Section impedance ZR is 0.5Ω, Zp 0.25Ω, Zq 0.25Ω, the ground resistance 1Ω, and section load (SL) 100Ω.

The simulation results obtained from EMTP/ATPDraw shows in Fig 4. In Fig. 4, the fault triggering time is 33.3 [ms], and the fault clearing time is 116.7[ms]. Before the fault, the voltage values at Sp and Sq are almost the same, and phase differences between voltage and current at Sp and Sq are zero. On the other hand, after the fault, the voltage values at Sp and Sq decrease greatly, and the current phase at Sq show the difference of 180 degree as compared with that at Sp. it means that the direction of current after the fault at Sq is reverse direction as compared with that at Sp.

Fig. 4.The voltage and current before and after the fault

Fig. 5 shows the change of the real power before and after the fault at Sp and Sq. The real power before and after the fault at Sp is great than zero (Positive), it means the current direction before and after the fault is the same. On the other hand, the real power after the fault at Sq is less than zero (Negative), it means the current direction before the fault is reverse as compared with that before fault.

Fig. 5.The real power before and after the fault

2.2 The modeling for Two-way communication

The proposed smart protective device identifies and isolates the fault zone before it is opened as lock-out status based on the heuristic rules HR 1-3 abovementioned after exchanging the fault information such fault current value, current direction with adjacent protective devices on the feeder. To apply the proposed method to smart grid distribution system, it is needed for any smart protective device to identify the electrical connectivity with other smart protective devices on the feeder. Therefore, a source zone and a sink zone for each smart protective device are defined which is enclosed by protective devices and based on the current direction WR [17].

Fig. 6 shows the source zone and sink zone of the distribution system.

Fig. 6.The zone modeling for two-way communication

For example, a smart protective device S5 has the zone {S2, S3, S4, S5} as source zone and the zone {S5, S6} as sink zone. Here, the upstream smart protective device set (USPDS) is {S2, S3, S4} and the downstream smart protective device set (DSPDS) {S6}, and the sink zone becomes the protection zone. Also, a smart protective device S4 has zone {S2, S3, S4, S5} as source zone, the zone {S4, S7, S8} sink zone and DSPDS {S7, S8}.

2.3 The inference procedure

Fig. 7 shows inference procedure of fault zone of smart protective device. In Fig. 7, IA, IB, IC, IO, VA, VB and VC represent A-phase current, B-phase current, C-phase current, zero phase current, A-phase voltage, B-phase voltage and C-phase voltage measured at self-position of each protective device.

Fig. 7.The inference procedure of fault zone

In Ix, X means fault phase among three phases. IXk and WXk represent the current value and direction of kth element PDk of downstream smart protective device set (DSPDS). ISET represents threshold value which determines whether the current measured at self-position is fault current or not. And N is the number of downstream smart protective devices of the protection zone.

At first, each smart protective device monitors A-phase, B-phase, C-phase and O-phase current at the self-position. If any phase current of any smart protective device is greater than ISET, the phase is defined as fault phase X and the phase current as fault current IX, and then DSPDS is determined from its protection zone information shown in Fig. 6, and WX and WR for the fault phase are identified. Next, it, when k is 1, obtains the fault information IXk and WXk though communication with element (PDk) of DSPDS and identifies the fault zone by driving the heuristics rules HR 1-3 based on the collected fault information.

 

3. Simulation Results

A smart grid distribution system with 4 feeders is presented to prove the effectiveness of the proposed methodology. Each feeder consists of five line sections and a DG. Fig. 8 shows the configuration of the smart grid distribution system designed using EMTP/ATPDraw.

Fig. 8.Test smart grid distribution system

In Fig. 8, the symbol CAP represents a capacitor installed for reactive power compensation of MTr load, the symbol DGi ith distributed generation Di, the symbol PQ the RL load/capacitor of line section, Lij jth line section of ith feeder, the symbol P/P the phase-to-phase short (or three-phase short) fault generator and the symbol P/G the single-phase ground fault generator.

Table 1 shows the modeling constant for test smart grid system shown in Fig. 8. This modeling data is based on the reference [18]. In Table 1, Di(WT) represents ith distributed generation is wind turbine generator.

Table 1.Modeling constant

To prove the effective of the proposed methodology, total 16 cases shown Table 2 are simulated using EMTP / ATPDraw, In Table 2, # represents the fault case number. FT, k and FS field of Fault Part represent the fault type, the fault feeder number and the fault zone (line section) separately. In particular, in the fault type, ‘1’ means the single-phase ground fault at A-phase, ‘2’ the phase-tophase short fault between A-phase and B-phase and ‘3’ the three-phase short fault. Also, Lk,j means jth zone of kth feeder Fk. In the Fault Current Part, IT the flowing current from substation transformer current, IDi the flowing current from Di, ICB the current at the position of circuit breaker on fault feeder Fk, Ifp the incoming current into FS, Ifq the outgoing current from FS, If the fault current at FS and WX the current direction of Ifq. In particular, all field values of this Fault Current Part are A-phases values except the phase-to-phase short fault cases (case 2, 6, 10 and 14). The phase-to-phase short faults abovementioned are obtained by (A-phase + B-phase)/2. In the IR Part, the filed FZ shows the fault zone obtained by applying the proposed inference procedure to each case.

Table 2.Simulation result

CASE 1) Single-phase ground fault at the zone L1,1

This case simulates a single ground fault occurred at A-phase on the zone L1,1 as shown in the case 1 of Table 2. The simulation results of the case 1 show in Table 2. In particular, Fig. 9 shows incoming current Ifp_A into the fault zone and the outgoing current Ifq_A from the fault zone. In this case, because a fault occurred at upstream position from distributed generation, the direction of Ifq_A is reverse direction to Ifp_A, Accordingly, the zone L1,1 can be inferred as the fault zone by applying HR 3. This inferred fault zone (FZ) is exactly the same as the simulated fault zone (FS).

Fig. 9.Simulation result for case 1

CASE 6) Phase-to-phase short fault at the zone L2,3

This case simulates a phase-to-phase short fault occurred at A-phase and B-phase on the zone L2,3 as shown in the case 6 of Table 2 and 3. All simulation results of the case 6 show in Table 2.

Fig. 10 shows the incoming A-phase and B-phase currents Ifp_A and Ifp_B into the fault zone, and the outgoing Aphase and B-phase currents Ifq_A and Ifq_B from the fault zone.

Fig. 10.Simulation result for case 6

In this case, because a fault occurred at downstream position from distributed generation, Ifq_A is close to zero. Accordingly, the zone L2,3 can be inferred as the fault zone by applying HR 2. This inferred fault zone (FZ) is exactly the same as the simulated fault zone (FS). In case of the phase-to-phase short fault, the phase difference between Ifp_A and Ifq_A is exactly 180 degree as shown in Fig. 10.

CASE 12) Single-phase ground fault at L3,5

This case simulates a single ground fault occurred at Aphase on the zone L3,5 as shown in the case 12 of Table 2. All simulation results show in Table 2.

Fig. 11 shows the incoming A-phase current Ifp_A into fault zone and the outgoing A-phase current Ifq_A from the fault zone. In this case, because a fault occurred at upstream position from distributed generation, Ifq_A is reverse direction to Ifp_A, Accordingly, the zone L3,5 can be inferred as the fault zone by applying HR 3. This inferred fault zone (FZ) is exactly the same as the simulated fault zone (FS).

Fig. 11.Simulation result for case 12

CASE 15) Three-phase short fault at the zone L4,4

This fault case simulates a three-phase short fault occurred on the zone L4,4 as shown in case 15 of Table 2. All simulation results shows in Table 2 and 3. In particular, Fig. 12 shows the incoming A-phase current Ifp_A into the fault zone and the outgoing A-phase current Ifq_A from the fault zone. In this case, because a fault occurred at downstream position from distributed generation, Ifq_A is close to zero as shown Fig. 12. Accordingly, the zone L4,4 can be inferred as the fault zone by applying HR 2. This inferred fault zone (FZ) is exactly the same as the simulated fault zone (FS).

Fig. 12.Simulation result for case 15

As explained in the cases 1, 6, 12 and 15, the inference results (IR) for all cases shown in Table 2 is exactly same as the simulated fault zones. Therefore, it can be identified that the proposed methodology is very effective to the smart grid distribution system with distributed generations

 

4. Conclusions

In this paper, a heuristic based methodology for smart protective device was designed, which, by itself, identifies and isolates a fault zone after exchanging of fault information such as fault information with other protective devices using bi-directional communication capabilities on the smart grid distribution system. The heuristic rules were extracted from the electrical and structural defined specifically according to the arrangement of the fault and DG when the faults of the diverse type occur at the diverse locations on smart grid distribution with DGs. The methodology proposed was proved for typical test model, and its effectiveness was proved by showing the exact inference results of fault zone for the given fault tests.