Investigation into the Implementation of Auto Reclosing Scheme in Distribution Networks with High Penetration of DGs S.F. Tan Student Member IEEE
[email protected]
Abstract- Renewable energy driven distributed generators (DGs) are mostly connected to radial distribution network with overhead feeders. It is estimated that 80% of network faults are temporary single phase to ground faults and therefore automatic restoration is required. A combination of automatic re-closer devices, circuit breaker, sectionalizer and fuse is applied to clear the temporary fault and isolate the permanent fault in order to optimise the service continuity to customers. Investigation into the sustainability of different DG technologies during temporary faults on overhead network should be carried out to (i) assess the possibility of preserving DG in services as far as possible to optimise the electric supply continuity and (ii) identify the requirements of automatic restoration in distribution networks with high penetration of DGs. On the other hand, it has been reported that future distribution networks are very likely to be operated in ring mode in order to maximise the installed capacity of distributed generation. The ring network operation mode will need special considerations in network feeder protection compared to radial network operation mode in the context of DG. Thus the implementation of auto reclosing in ring network with DG will also need further investigation. This paper reports an investigation into the impact of different wind power based distributed generations on auto reclosing schemes in MV overhead distribution networks that are operated in radial and ring modes. The paper is divided into two parts; the first part present literature survey on the development of system protection schemes in power distribution network with high penetration of DG. The second part present results of dynamic simulation studies on the operation of auto reclosing scheme in network with DGs when operated in different operating modes. A particular emphasis is placed on examining the application of single pole auto resoling scheme in the distribution network.
I. INTRODUCTION In the UK, a typical rural distribution network has open ring network topology. It is designed to operate in radial operation mode during normal network condition but switch to ring operation mode in the event of fault condition to improve the system reliability [1]. It is estimated that 80% of network faults are temporary single phase to ground fault and therefore automatic restoration is required. A combination of automatic recloser, circuit breaker, sectionalizer and fuse is applied to clear the temporary fault and isolate the permanent
S.K.Salman Senior Member IEEE
[email protected]
fault in order to optimise the service continuity to customers. Three phase auto reclosing scheme had been widely used in MV distribution network to clear the network fault. The drawback of three phase reclosing is the temporary disconnection of the all three phase of the fault feeder from the rest of network during all fault condition regardless type of fault. Thus the application of single phase reclosing scheme (SPAR) has been investigated in MV distribution network to maintain the service continuity to customer under temporary single phase fault condition. Under this scheme only the faulty phase is tripped while the healthy lines remain connected to the network. Continued increase of distributed generation (DG) penetration into existing distribution networks in recent years has resulted in bi-directional power flow and changes in network voltage profile. These in turn caused important impact on the operation of conventional auto reclosing scheme in distribution network [2-5]. It has been reported that the interconnection of DG interrupts the switching operation of auto reclosing and prevent temporary network faults from being cleared [3]. Currently, operation of reclosing may cause damage to distributed synchronous generator due to out of phase reclosing during power restoration process [3-4]. Nowadays, generation of electricity using wind power is increasingly based on DFIG and it appears that no study has been conducted so far to investigate the impact of DFIG on the application of auto reclosing schemes in existing distribution network. On the other hand, it has been reported that in order to maximise the installed capacity of distributed generation, future distribution networks are very likely will be operated in ring mode [6]. Recent investigation [7] showed that ring network operation mode will need special considerations in network feeder protection compared to radial network operation mode in the context of DG. Thus the implementation of auto reclosing in ring network with DG will also need further investigation. This paper reports an investigation into the impact of different wind power based distributed generations on auto reclosing schemes in MV overhead distribution networks that operated in radial and ring modes. This research work is carried out using PSCAD/EMTDC power system software.
The paper is divided into two parts. The first part deals with assessing the dynamic behaviour of two different type of DG technologies DFIG and FSIG during temporary fault in radial and ring operated distribution network. The work is intended to study the suitability of application of auto reclosing scheme in future distribution networks which are very likely to be operated in ring mode. The second part deals with assessing the performance of single pole auto reclosing scheme (SPAR) in a network with different type of DG technologies in order to study the suitability of application of SPAR in a network with high penetration of DG. II. DELOPEMENT OF SYSTEM PROTECTION IN DISTIRBUTION NETWORK WITH DG Integration of DG into existing distribution network may change the direction of power flow. This in turn has an important impact on the operation of conventional feeder protection schemes that designed based on the assumption of simple radial network operation. In case of distribution networks with DGs the total fault current contribution from many small DG units can be large enough to alter the network short circuit level and consequently cause the malfunction of protective devices. For instance this may include miscoordination between fuse-fuse or fuse-recloser [2]. In reference [8], the issue of miscoordination between a fuse and a recloser in distribution network with high penetration of DG has been discussed. A procedure to achieve coordination between fuse-recloser had been suggested based on the application of microprocessorbased recloser. Adaptive feeder protection scheme for MV distribution network with high penetration of DG was introduced in [9]. Under this protection scheme, the network is divided into different protection zones with reasonable balance of load and DG. Each protection zone is separated by circuit breaker which is equipped with commucation channel, check synchronization and auto reclosing function. A computer based-relay is installed at the main power distribution substation to monitor power and current flow and control operation of circuit breaker at each network protection zone. Implementation of the proposed scheme will be costly because the installation cost of communication link between DG and distribution substation is expensive. Conventional circuit breaker and recloser need to be upgrade to microprocessor based-type to allow the intensive signal communication process. Fault contribution from embedded generation also affects conventional feeder protection by causing unnecessary tripping of healthy feeder and reducing the detection sensitivity of overcurrent relay [10-11]. This is because nondirectional overcurrent relays are mostly used in feeder protection and the presence of DG(s) may decrease the fault current seen by the overcurrent relay causing it not operate. A high speed fault transient based-directional overcurrent protection method has been suggested in [12]. Under this protection scheme, fault current transient directional signal is detected by each overcurrent relay in the network in the event of fault. Signals are then exchanged via the communication
link between each of these relays to confirm the exact fault location and isolate the faulty section from the rest of the network. According to [13] the operating time setting of time graded overcurrent relay is affected by the increased integration of embedded generators into distribution network. In order to avoid the embedded generation from tripping unnecessary, the maximum clearing time for network fault need to be reduced. The speed of fault clearance in existing distribution network is not an important factor as it is in transmission network where the system stability involved. Future distribution network is expected to accommodate significant proportion of embedded generations, unnecessary tripping of these power plants will affect system stability and reliability. Consequently further investigation into fault clearance speed in distribution network with integrated DGS is needed. In [14] it is proposed replacing conventional overcurrent relay with high speed distance protection schemes. Upgrading of existing ring main unit (RMU) in substation and replacing existing relay with new microprocessor based-relay are needed to implement the proposed protection scheme. Unintentional islanding is another major issue in addition to the feeder protection issues stated above. Unintentional islanding refers to a condition where part of public load becomes islanded with wind power plant caused by fault clearance at the upstream of the distribution feeder. Unintentional islanding is undesired and its impact on islanded load, personnel safety, islanded DG is discussed in [2]. Loss of main (LOM) protection refers to the protection relay which automatically detects such condition and disconnects the DG from the grid. Contrast to LOM protection schemes, it is proposed in [15] to maintain the operation of DG in both islanded and grid condition with an adaptive generator control scheme added to DG. It can be concluded from the literature survay that to optimise the integration of DG into existing distribution network a continual evaluation of existing solutions and development of new protection algorithm for distribution network protection is necessary. In this context, the application of intensive communication system in distribution network protection is unavoidable. III. MODELLING OF POWER SYSTEM USING PSCAD/EMTDC In this work, simulation model of two types of wind generator including (a) fixed speed induction generator (FISG) and (b) doubly fed induction generator (DFIG) has been developed using PSCAD/EMTDC. The dynamic behaviour of wind generator is simulated using shaft torsional model in the component library. Power factor correction (PFC) capacitor is included in the developed FSIG model. Each of stators and rotor side converter together with DC link component in the power electronic converter of DFIG is modelled appropriately. The control strategy of DFIG is that at steady state condition, the generator absorbs modest reactive power from the grid while the generator terminal voltage is kept close to 1.0pu.
Fig.1 Schematic diagram of investigated system with 2.0MW wind generator.
machine speed(p.u)
ring netw ork
1.06 1.05 1.04 1.03 1.02 1.01
31.2
22
30
1
tim e(s )
(a) 1.2 1 0.8 0.6 0.4
radial netw ork
0.2
ring netw ork
31.4
0 30.3
In order to examine the transient performance of both FSIG and DFIG under temporary fault condition, three phase to ground fault ground is applied at F1 at feeder 1 once when it is operated at radial and then in ring mode. Fig. 2 and Fig. 3 show the variation of generator terminal voltage and machine speed of FSIG and DFIG during fault condition. It was found in both of Fig. 2 and Fig. 3 that these generator parameters have regained their pre-fault values after the clearance of the fault at F1. This result shows that the transient stability of wind generator is maintained which means the critical clearing time (CCT) of generator has not been exceeded. This investigation has shown that FSIG can not retain its pre-fault
radial netw ork
1.07
22
IV. TRASIENT PERFORMANCE OF FSIG AND DFIG UNDER NETWORK FAULT CONDITION
1.08
terminal voltage(p.u)
Fig.1 shows the power system under consideration. The system is modelled based on 11kV rural overhead distribution network in the UK [16]. A 2.0MW, 0.69kV wind generator is connected at the end of feeder 1 though a 2.5MVA, 0.69/11kV step up transformer with an impendence of 6%. The grid is represented by a voltage source with short circuit level of 100MVA and a distribution substation represented by a 10MVA, 33/11kV transformer with an impendence of 6%. Each of the 0.5MVA loads supplied by feeders 1 and 2 is connected to their corresponding feeders though 0.5 MVA, 0.415/11kV step up transformer with an impendence of 6%. Other loads connected to other feeders are lumped together and represented by a single load of 2.5MVA. The latter is assumed connected at the end of a feeder though 3.0MVA, 0.415/11kV step up transformer with an impendence of 6%. B1 to B5 are circuit breakers that located every 5km along feeder 1 and feeder 2. The radial operation mode is implemented by opening the B6 that located at the end of feeder 2, whereas the ring operation mode is implemented by closing it.
1.09
28.9
wind gen erator 2M VA
29.1
G
27.7
0.5M VA
27.9
0.5M VA
5km
5km
26.6
B3
B2
26.7
F1
25.4
5km
24.3
B1
25.5
0.5M VA
0.5M VA
grid
B6 normally open
5km
5km
23.1
feeder2
3.0M VA B5
B4
24.4
5km
23.2
B7
condition when the fault duration is greater than 408ms in ring network and 225ms in radial network. Meanwhile, DFIG can not retain its pre-fault condition when the fault duration is greater than 800ms in ring network and 519ms in radial network. The values are therefore considered as generator’s CCT for their respective network of operation. The investigation shows the generator’s CCT in radial network is much lower compared to the ring network. Therefore to preserve the wind generator in operation (at the faulty feeder) during fault condition, a shorter tripping time of circuit breaker is need in radial network compared to ring network. As shown in Figs. 2(a) and (b), the transient performance of FSIG in both radial and ring network has no significant difference. In the contrast, it shown in Figs. 3(a) and (b) the transient performance of DFIG in a ring network is better compared with that in a radial network. The magnitude of generator terminal voltage drop is much lower in ring network compared to radial network. The generator speed has more oscillation when it is connected to radial network compared to ring network.
tim e(s )
(b) Fig.2 Variation of (a)machine speed and (b) terminal voltage of FSIG following a three phase to ground fault at F1 in radial and ring network with duration of 205ms and 408ms.
machine speed(p.u)
1.05 1.04
radial netw ork
1.03
ring netw ork
1.02 1.01 1 0.99 0.98 0.97 37.3
35.6
33.9
32.2
30.5
28.8
27.1
25.4
23.7
22
0.96
tim e(s)
The fault is applied 25s into simulation and cleared by opening both B1 and B5 200ms after fault inception. Three different recloser opening time (Tr) have been considered in this investigation including 0.5s, 1.0s and 2.0s. According to reference [3], in practical power distribution network, Tr varies between 0.2 second to several seconds. Fig.4 and Fig. 5 show the variation of machine speed for FSIG and DFIG respectively in radial network. Fig. 6 and Fig. 7 show the variation of machine speed for FSIG and DFIG respectively in ring network. It was found from Fig.4 and Fig.6 that the variation of machine speed of FSIG at the considered three Tr are significantly different in radial network compared to ring network. In contrast, Fig.5 and Fig.7 show the variation of machine speed of DFIG at the three different Tr in radial and ring network are similar to each other. This indicates that the effect of Tr setting in of SPAR scheme in ring network with integrated DG is negligible. The investigation also shows that, SPAR scheme do not affect the transient performance of both FSIG and DFIG.
(a) 1.03
1.2
1.028 1.026 machine speed(p.u)
0.8 0.6 0.4
Tr=0.5s
1.022
Tr=1.0s
1.02
Tr=2.0s
1.018 1.016 1.014
radial netw ork
0.2
1.024
1.012
ring netw ork
31.6
30.9
30.2
29.6
28.9
28.2
27.5
26.8
26.1
25.4
24.7
24.1
22
37.2
35.7
34.2
32.6
31.1
29.6
28.1
26.6
25
23.5
22
23.4
1.01
0
22.7
terminal voltage(p.u)
1
tim e (s )
time(s)
Fig.4 Variation of machine speed of FSIG following a single phase to ground fault at F1 in radial network .
Fig.3 Variation of (a)machine speed and (b) terminal voltage of DFIG following a three phase to ground fault at F1 in radial and ring network with duration of 519ms and 800ms.
1.005
1
V. OPERATION OF SPAR SCHEME IN THE NETWORK WITH WIND GENERATOR machine speed
Tr=1.0s Tr=2.0s
0.99
0.985
32.6
32
31.4
30.9
30.3
29.7
29.2
28.6
28
27.4
26.9
26.3
25.7
25.1
0.98 24
In this investigation, the system under consideration is divided into protection zones such that when a fault occurs at certain protection zone it is cleared by the pair of circuit breakers that are located at the two ends of the line section within the protection zone. For instance, a fault at F1 is isolated by simultaneously tripping circuit breakers B1 and B5. Under this protection scheme, wind generator connected at the end of feeder 1 can be maintained in service during network fault condition. In order to asses the impact of SPAR scheme on the transient performance of FSIG and DFIG, single phase to ground fault is applied at F1 at feeder 1 once when the network is operated at radial and then in ring mode.
Tr=0.5 0.995
24.6
(b)
tim e(s )
Fig.5 Variation of machine speed of DFIG following a single phase to ground fault at F1 in radial network
ACKNOWLEDGMENT
machine speed(p.u)
1.024
The authors would like to thank The Robert Gordon University for providing facilities. S.F. Tan would like to thank Robert Gordon University for providing financial support to undertake this research. Tr=0.5
References
Tr=1.0s
1.016
Tr=2.0s
[1] [2] [3] 31.9
31.2
30.5
29
29.8
28.3
27.6
26.9
26.2
25.5
24.8
24.1
23.4
22
22.7
1.008
tim e(s )
Fig.6 Variation of machine speed of FSIG following a single phase to ground fault at F1 in ring network.
VI. CONCLUSION
[4] [5]
[6]
The literature review covered under sections II shows that optimisation of the integration of DGs into distribution networks requires development of new network protection algorithms. The application of communication technology into the distribution network protection system is unavoidable. The results of the investigation presented in section IV show that time setting of feeder protection devices required to maintain the transient stability of DG, which is determined by the generator’s CCT, is significantly different between ring and radial mode of operations. The results of shown in section V reveal that application of SPAR into power distribution network with integrated DGs does not affect the transient performance of DG.
[7] [8]
[9]
[10] [11] [12]
1.003
[13] 1.001
[14]
machine speed(p.u)
0.999 Tr=0.5s
[15]
Tr=1.0s
0.997
Tr=2.0s
[16]
0.995
0.993
0.991
32.7
32.1
31.4
30.8
30.2
29.6
29
28.3
27.7
27.1
26.5
25.9
25.2
24.6
24
0.989
time(s)
Fig.7 Variation of machine speed of DFIG following a single phase to ground fault at F1 in ring network.
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