Australasian Universities Power Engineering Conference- AUPEC2016
DG Integration Issues in Unbalanced Multi-Phase Distribution Networks N. K. Roy1, H. R. Pota2 and M. A. Mahmud3 1
Department of EEE, Khulna University of Engineering & Technology, Khulna 9203, Bangladesh 2 School of EIT, The University of New South Wales, Canberra, ACT 2610, Australia 3 School of Engineering, Deakin University, VIC 3220, Australia Email:
[email protected],
[email protected], and
[email protected]
Abstract—This paper analyzes positive and negative impacts of the integration of distributed generation (DG) in unbalanced multi-phase distribution networks. Several aspects, for example, the static voltage stability, power loss, short-circuit and voltage unbalance studies are conducted on an IEEE 13 node unbalanced distribution feeder. Simulation platform used here is DIgSILENT PowerFactory. In this paper, synchronous generators and solar photovoltaic (PV) units are introduced as distributed generators. It is found that, although a DG unit has a positive impact on the voltage stability and power loss of the system, it increases the short-circuit current at all nodes of the feeder. In certain cases, it can impact both positively and negatively on the voltage imbalance phenomenon of a distribution network depending on the single-/three-phase connection of PV units to it. Keywords—Distributed generation; Power loss; Photovoltaic; Voltage stability; Voltage unbalance.
I. INTRODUCTION With the increasing penetration of distributed generation (DG) and expansion of system, power system engineers are facing several technical problems [1],[2]. Distribution networks exhibit a large number of complexities due to their different load characteristics, network structures and circuit connections [3]. Several studies show that the integration of DG improves the efficiency and voltage profile of a system [4]-[6]. However, it is also investigated that, in some cases, the presence of DG can deteriorate voltage stability and power losses [7]. As modern distribution systems are operated under greatly stressed conditions with reduced stability margins, the voltage stability analysis of a distribution system with DG is essential for its proper planning and control. High capacity DG resources may change the direction and phase angle of the fault current and cause unwanted operations of the protection relays. To calculate the phase shifts of transformers in sequence networks and examine the behavior of the fault currents of distribution systems, a short-circuit analysis algorithm is developed in [8]. As it is both uneconomical and technically challenging to replace the original protection system in a distribution network, an optimal DG placement method for maximizing the penetration level of DG in it without changing its original relay protection scheme is given in [9].
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The impact of DG on an existing network’s short-circuit level is investigated in [10] which concludes that it is increased by the system’s impedance parameters. As a shortcircuit analysis is different for different networks, it must be conducted before any DG is connected to the relevant utility network. A voltage profile and short-circuit analysis of distribution systems with DG is carried out in [11] considering a balanced network. However, as the nature and connection of loads in a distribution network makes it unbalanced, analyzing a multi-phase unbalanced distribution network is essential. The effects of voltage imbalances on power distribution systems may be serious [12], [13]. A severe one can cause an additional power loss, relay malfunction, failure of the motor, power electronics converter and transformer, and inaccurate measurements by many meters. The level of DG penetration can also negatively affect the voltage unbalance phenomenon. The objective of this paper is to determine the effects of DG on voltage stability, power loss, short-circuit currents and the voltage unbalance of a multi-phase unbalanced distribution system. This paper is arranged as follows. Section II describes the test system components; Section III presents the impact of DG on voltage stability, power loss, short-circuit current and the voltage unbalance phenomenon of a distribution network; and Section IV discusses the conclusions. II. SYSTEM DESCRIPTION The IEEE 13 node test feeder is a distribution network feeder with single-, two- and unsymmetrical three-phase elements, as shown in Fig. 1, with its components presented in Table I. In this paper, voltage-dependent load models, which consist of a combination of constant current, constant power and constant impedance loads, are used. The distributed load is modeled by five load elements with equal distances between them along the distribution line [15]. The test system is modified by connecting a synchronous generator (SG) and two PV units at buses 633 and 634, respectively. In-built models of the generators available in DIgSILENT PowerFactory are used to perform simulations, with the power factor of DG units considered unity and their actual output powers given in Table II.
7.27 MW (Fig. 2) to 9.57 MW. As DG installed near load centers results in reduced voltage drops, the voltage stability margin of the system is improved. 1.1 1.0
Voltage (pu)
0.9 611 632 633 634 645 646 652 671 675 680 684 692 RG60
0.8 0.7 0.6
Fig. 1. IEEE 13-node unbalanced distribution test feeder [14].
Name
Number
Node
13
Unbalanced loads
9
Overhead lines & underground cables Capacitor bank
10
Transformer (including substation transformer) Switch
2
Voltage regulator
1 (per-phase voltage regulator)
TABLE II. Connecting node 633 634 634
0.4
COMPONENTS OF 13 NODE TEST SYSTEM
4
Output Power (kW)
SG PV unit 1 PV unit 2
1113.75 58.00 20.30
7
1.1 1.0 0.9
1
Type
6
Fig. 2. P-V curve without DG (base case).
2
OUTPUT POWER OF DG UNITS
5
Power (MW)
Voltage (pu)
TABLE I.
0.5
0.8 0.7 0.6 0.5 0.4
611 632 633 634 645 646 652 671 675 680 684 692 RG60
4
5
6
7
8
9
10
Power (MW)
III. IMPACT OF DG ON DISTRIBUTION NETWORK The impact of the integration of DG on various factors of a distribution network is presented below. A. Static Voltage Stability Generally, to analyze the static voltage stability, powervoltage (P-V) curves, which determine the maximum loading limit of a system, are used [16]. In this paper, this curve is produced by performing a series of unbalanced power flow solutions using the simulation software for different load levels. For PV analysis with DG, the sources given in Table II are considered together. The results obtained for all nodes without and with DG are plotted in Fig. 2 and Fig. 3, respectively, in which, it can be seen that, in both cases, node 611 has the lowest voltage magnitude. In Fig. 3, it can be visualized that the integration of DG units improves the voltage stability margin by increasing the loading margin from
Fig. 3. P-V curve with DG.
B. Power Loss Reducing the power loss is an important objective for power system engineers to improve the overall efficiency of a system. The real power losses and reactive power consumptions of the system for different connections of DG are summarized in Table III. In these scenarios, both singleand three-phase connections of PV units are considered keeping the same SG. It can be seen that, compared with the base case, the integration of DG reduces the power loss of the system. However, this depends on the connection (single/three-phase) of DG to the network. If three-phase PV units are connected to a three- phase bus, the power loss of the system is reduced. On the other hand, if the penetration of single-phase PV units on a three-phase bus is increased, the real power loss is increased.
As DG interconnection results in a reduced current flow from the substation to distribution network, power losses are reduced. Also, it is known that a voltage imbalance always causes additional power loss in a system [10]. As the connection of a single-phase DG unit to a three-phase node causes a voltage unbalance, more real power is lost. After integrating DG, a reduction in the reactive power consumption (I2X) is observed due to the change in the power flow pattern. TABLE III. Type
POWER LOSS OF TEST SYSTEM Real power (kW)
Without DG
146.87
Reactive power (kVar) 595.79
SG
141.00
573.77
SG+1PV [3-phase]
139.15
566.07
SG+2PV [3-phase]
138.66
563.56
SG+1PV [1-phase-a]
140.10
565.03
SG+2PV [1-phase-a]
141.02
562.97
The increase in the fault current (IF) for solar PV penetration is calculated as follows.
% I F ,increase =
I F ,with PV − I F ,without PV I F ,without PV
× 100
(1)
. The percentage increases in the fault current at different nodes with the connection of single and double PV units at node 634 are shown in Fig. 4 in which it can be observed that an increasing penetration of DG increases the fault current. As the fault current increases with the interconnection of DG in the existing system, it is necessary to augment a system’s circuit breaker’s capacity for its reliable operation. D. Voltage Unbalance Factor The voltage unbalance factor (VUF) can be defined as [17],
C. Fault Current A short-circuit fault results in a very high current and it needs to be interrupted to ensure the safety of the system’s components. As a three-phase short-circuit fault has the most severe impact, an analysis is conducted considering this fault at various nodes of the test system. The peak fault currents are calculated according to the IEC60909 standard using the simulation program.
%VUF =
VN × 100 VP
(2)
1 1 1 Va V0 V = 1 1 a a 2 V b P 3 2 1 a a Vc VN
(3)
where, VP, VN, and V0 are the positive, negative and zero sequence voltages, repectively, and Va, Vb, and Vc the line to
10
%Fault current increase
9
Single PV unit Double PV unit
8 7 6 5 4 3 2 1 0 632
633
634
671
675
680
692
2
line voltages, a = 1∠120 , a = 1∠240 . According to the IEEE Recommended Practice for Monitoring Electric Power Quality [18], the steady-state limit for voltage imbalance is 2%.
11
RG60
Node
Fig. 4. Increase in three-phase short-circuit current at different nodes.
To investigate the impact of DG on the voltage unbalance of the system, the VUF percentages are calculated for both single- and three-phase connections of PV units to perform a comparative analysis. Table IV shows these percentages for different connections of DG units. It is found that the system has greater voltage imbalances than the statutory limits in some nodes. Although the connection of DG reduces the voltage imbalance of the system compared with that of the base case, as expected, a single-phase DG unit connected at three-phase node increases it. Also, if the number of singlephase PV units is increased, the %VUF increases. However, this depends on the selection of phases for the single-phase DG’s connection to the network which is illustrated in the next section.
TABLE IV.
VUF (%) AT VARIOUS NODES
Node
2.5
VUF (%)
0.97008
SG +1PV3-ph 0.96301
SG +1PV1-ph 1.00135
SG +2PV3-ph 0.96055
SG +2PV1-ph 1.01588
0.88731 1.05521
0.88019 1.04622
0.90947 1.05468
0.87772 1.04312
0.92282 1.0923
2.84999
2.28255
2.27506
2.29863
2.27246
2.30685
2.84999
2.28255
2.27506
2.29863
2.27246
2.30685
2.97791 2.84999
2.41546 2.28255
2.40791 2.27506
2.42783 2.29863
2.40529 2.27246
2.4348 2.30685
Base case
SG
632
1.56408
633 634
1.57182 1.73288
671 692 675 680
Phase a Phase b Phase c
2.0
%VUF
2.0
%VUF
Without Q generation capability With Q generation capability
2.5
1.5
Statutory Limit
1.5 1.0
1.0
0.5 0.0
0.5 632
633
634
671
675
680
632
692
Node
Fig. 5. VUFs for connection of PV in different phases
Variations in the VUF for the connection of a single-phase PV unit (unit 1) in different phases of node 634 are calculated and presented in Fig. 5 in which it can be seen that this unit can reduce the VUF if it is connected in the proper phase which is phase-c in this case. Therefore, when it is necessary to connect a single-phase PV unit to a three-phase node, it is essential to select the appropriate phase for PV connection. E. Reduction of Voltage Unbalance Reactive power/voltage of DG units can be controlled using their local controllers. Figure 6 shows the different modes of operation, that is, with and without reactive power (Q) generation capability of the generator’s controller. Here, the reference currents of the controllers are obtained based on the desired active and reactive powers and measurements of terminal voltage [19]. In this analysis, all the considered DG units are connected to the network. The simulation results indicate that DG units with their Q generation capabilities reduce the VUF significantly compared with that of their operation without Q generation capability. Therefore, operating DG units with their Q generation capability has a positive impact on the VUF of the system.
633
634
671
675
680
692
Node Fig. 6. VUFs with and without Q generation capability of local controllers of DG units
IV. CONCLUSION In this paper, various issues concerning distribution networks with DG are investigated. Based on the results obtained from the analyses, the following conclusions are drawn. •
• •
•
The integration of DG reduces the power loss of the system. However, the connection of a single-phase rather than three-phase DG unit to a three-phase node increases the real power loss. The static voltage stability margin of the system can be increased by increasing the penetration of DG. The fault current of the system is increased at all nodes due to the integration of DG. Also, as the PV penetration level increases, the short-circuit current of the system increases. The integration of DG impacts the voltage imbalance phenomenon of the system. If a three-phase DG is connected at node 634 of the test feeder, it improves the VUF compared with that of the base case. However, a single-phase connection of DG can increase or decrease the VUF of the system
•
depending on the selection of the phase in which it is connected. The effectiveness of DG units heavily relies on their local control structure. A DG unit with its Q generation capability reduces %VUF of the system.
Finally, it can be concluded that there is a significant protection concern associated with the voltage imbalance and rise in the fault current due to the integration of DG. To investigate the impact of DG on a system’s stability, a detailed dynamic analysis should be performed which is the future aim of this work. ACKNOWLEDGMENT This work is partially supported by a University Grants Commission funded aid from the Committee for Advanced Studies & Research of KUET, Bangladesh. REFERENCES [1]
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