Abstract— This paper proposes a computationally efficient method for supervisory control of Volt/Var control devices- voltage regulators and capacitor banks –on ...
Supervisory Control for Coordinating Volt/Var Control Devices on a Distribution System Gulcihan Ozdemir, Selcuk Emiroglu, Mesut Baran Department of Electrical and Computer Engineering North Carolina State University, Raleigh, NC 27695 Abstract— This paper proposes a computationally efficient method for supervisory control of Volt/Var control devicesvoltage regulators and capacitor banks –on a distribution system. The method searches for the best control settings for these devices as the operating conditions change on the system in real-time. The objective of the supervisory control is that of a volt/var optimization – minimize the power losses on the distribution circuit while keeping the node voltages within the acceptable range. The method considers multiple voltage regulators and capacitor banks on a distribution feeder. The test results show the effectiveness of method in terms of providing the global optimal solution with fast computational speed. Index Terms— Distribution systems, Power loss reduction, Volt/Var control.
I.
INTRODUCTION
Volt-VAR control (VVC) in distribution systems is one of the main real-time functions. The main objective in VVC is to keep the node voltages within the acceptable range. Adoption of optimization methods allows extending the control to include minimization of power losses [1-3]. VVC has become more challenging with the integration of distributed energy resources (DER). Currently, VVC devices, Load Tap Changers (LTC), Voltage Regulators (VRs) and Capacitor Banks (CAPs), on a distribution system are controlled by their local controllers. The settings/rules for these controllers are static, and therefore, the control scheme becomes ineffective when the system operating point becomes more dynamic due to DERs [4]. Various approaches have been proposed for volt/var control in distribution systems. In [5], a dynamic programming based method together with fuzzy logic control approach is proposed for VVC. The problem is divided in two sub problems to reduce the computational burden. Sub problem one, which is at substation level, is handled using dynamic programming approach. Sub problem two, which is at feeder level, is handled using fuzzy logic control approach. The coordination between two sub problems is provided by power flow calculations. In [6], a method is proposed in the existence of DG together with other voltage control devices. Steady-state
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voltage control with actively participating DG and other control devises LTC and CAPs at substation and at feeder level are studied. A coordinated control algorithm using dynamic programming is developed to reduce the switching operations of the control devices based on load forecast for one day ahead. In [7] volt/var problem is solved by a fuzzy optimization approach to reflect the uncertainties at forecasted load data and wind speed. In [8], supervisory control schemes for integrated volt/var control are proposed. A rule-based controller is used to switch ON and OFF of CAP banks for the next day to maximize active power loss reduction. In [9], multiple voltage control devices, LTC, VRs, and DGs, are used for online voltage control in distribution system. Main purposes are reducing tap operation and giving priority to DG while providing the voltage support along the feeders in the system. Just voltage control issue is considered. Another supervisory type control scheme is proposed in [10] for both voltage regulator and capacitor control used at the substations for volt/var control. The voltage at the end of the line is estimated to keep the voltages within limits on conventional radial feeders. In [11], efficient integer optimization algorithms for optimal coordination of capacitors and regulators are proposed. Two algorithms, one is randomized and the other deterministic, are introduced for conventional radial feeders. The randomized algorithm runs fast but does not guarantee of optimality. Run time of the deterministic algorithm is polynomial bounded on problem size. It approximates the losses in the system as a convex function. Some of the VVC schemes do not consider coordination of control devices at substation level and feeder level. Existence of DGs on a system can also make some VVC schemes ineffective if the method uses static settings/rules for control. On the other hand, utilities has recently started extending their monitoring to feeder level, allowing the real-time monitoring and control capability in a faster time frame, 5-15 minute [12]. This paper makes use this new capability and proposes a method that can be implemented for real-time VVC. The paper proposes to add a supervisory control layer that can adjust the set points of the Volt-Var devices as the
operating conditions change in real-time at rate of 5-15 minute. The method considers coordinated control of all the VVC devices, LTCs, VRs, and Caps that exists on the substation as well as on the feeder. The method employs a computationally efficient search by making use of the operational characteristics of VVC devices. Hence, the proposed method provides the global optimal solution, as opposed to the other heuristics based schemes. Thus, the proposed scheme can be easily integrated with the emerging feeder SCADA and energy management schemes [13]. II.
SUPERVISORY VOLT-VAR CONTROL
A simplified diagram showing the devices employed for VVC on a distribution system is shown in Fig.1. Conventionally, these devices are controlled by local controllers and hence there is no coordination between especially the voltage control devices, such as LTC and VR and the capacitors, CAPs. Hence, the goal in the proposed scheme is to provide coordination between the VRs and CAPs so that the optimal VVC performance can be maintained even on systems with rapidly changing operating conditions, such as the systems with DGs.
Since the control U in this case take discrete values, this is a nonlinear integer programming problem. On a typical system, with substation LTC, and one/two VRs and 3-4 switched CAPs, this problem becomes computationally challenging, especially for implementation at a micro SCADA level at the substation. III.
PROPOSED SCEME
The proposed scheme makes use of the weak coupling between the voltage correction by VR and the reactive power compensation by CAPs [14]. This allows us to develop a computationally effective search that can find the optimal set points for VRs and CAP on/off status for a given operating point. This search, thus, provides a global optimal solution for the overall VVC problem. Figure 2 shows the flow diagram of the proposed coordination scheme. As the figures shows, the method solves the VR and CAP problems separately at each iteration, and the solutions are updated at each iteration until the controls reach to their optimal points.
Figure 1: Volt/Var devices on a distribution system [15]. In VVC, the goal is to keep the node voltages on a feeder within the allowable limits according to the ANSI standard C84.1 while keeping the power loss minimum. Hence, the basic VVC problem can be formulated as an optimization problem:
min Ploss ( x) gi ( x, u ) 0
s.t.
i 1...n
Vi min Vi Vi max
Figure 2. Flow diagram of proposed coordination scheme.
16 utap 16 where Ploss is the power loss on the feeder, x represents the node voltages, and u represents the control variables, i.e, the tap positions of LTC/VRs and switched CAP on/off status. To be effective, this VVC problem for each feeder needs to be solved at each control update interval, (target is 5 -15 minutes), and the new settings for the devices needs to be sent to their local controllers.
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A. Volt Problem The voltages on a distribution system are kept within the required limits mainly by volt devices – LTC at the substation and VRs on the feeder. Hence, the goal in the Volt problem is to determine the proper settings for these devices at each
control period as the system operating point changes in realtime. Note that in real-time application, current setting of volt devices is known, and hence, this provides a good initial point to determine the new settings, as these devices are designed to act slowly. Thus, in the proposed scheme, tap positions of each LTC and VR are varied to determine the settings that keep the voltages within the limits. Then the search is narrowed further to determine the settings that results in reduced power loss on the system. LTCs and VRs have usually 33 taps (including the neutral), and each tap corresponds to 0.00625 V in pu. Hence, a VR at tap position utap provides voltage boost/buck of 0.00625*tap V in p.u. Therefore, for each volt device, the tap setting as a discrete variable, has 33 possible values:
utap [16, 15, 14,..., 0,...,14,15,16]
B. Var Problem Capacitors are used to maintain the power factor on the feeders and at the substation. This in turn helps to minimize power loss on the feeder. Switched capacitors are employed to allow for flexibility in adjusting the power factor as the operating conditions change on the system by switching them on/off. Switched capacitors are arranged in banks that can be switched on/off. In this paper, it is assumed that CAP banks are single phase and switched on/off individually. Hence, the goal in this sub-problem is to find the on/off status of each CAP bank such that it provides the best power loss reduction. Since the number of switched CAP banks on a feeder is usually limited, up to 3 or 4 locations on a typical feeder. The possible switching combinations in such cases are not too high, and thus we propose to adopt a full search in this case to determine the best switching scheme for the CAPs.
(3) IV.
Hence, possible combination of tap settings will be quite large, if there are a few volt devices on the system. For example, even with one 3-phase LTC and three 1-phase VR, total possible combinations are 1,185,921. Hence, the goal here is to reduce the search in determining the proper settings. This is achieved as follows: a) Start from the initial tap position. b) Move two taps up and down during the first pass for each device for each phase:
utap [2, 1,0,1,2]
TEST RESULTS
To test the performance of the proposed scheme, the modified IEEE 13 node test system is used [16]. Figure 3 shows the system. The system has one 3 phase LTC at the substation and one VR at node 671 (which is assumed to be controlled on a phase basis), and two switched CAPs banks – one single-phase CAP at node 611 and one three-single phase CAP at node 675. To test the effectiveness of the proposed scheme on this system, a typical daily load profile shown in Fig 4 is used. The load profile has 15-minute sampling.
(5)
Note that LTC is usually three phase and all the phases are controlled together, ie. set to the same tap, whereas VRs can be three phase or single phase, and sometimes the phases are controlled separately. Thus, possible number of control adjustments tap is affected greatly by such variations. In this paper, it is assumed the VRs are controlled individually on each phase. In the search, tap setting for LTC and VRs is searched one phase at a time. Furthermore, each VR is adjusted at a time, starting from the substation, as this mimics the time coordination between these devices. While adjusting the taps for a VR, upstream VR tap positions are kept fixed. With this approach computation time is reduced dramatically, so that a full search on the possible combinations can be conducted. This full search identifies the tap settings which results in voltages that are within limits, i.e., the feasible search space. It also helps us identify the settings that yield the minimum loss calculation, Ploss, among the feasible search space. c) If the previous step does not find any feasible solution, then the search is repeated, but the range of tap adjustments in (5) is adjusted by changing the initial tap by one level up and/or down, with five taps at a time. This step is repeated until we find the best solution.
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Figure 3: IEEE 13 node test feeder.
Power flow simulations, using OpenDSS [17-18], have been performed to simulate daily operation with 15 minute updates. Proposed supervisory control scheme is implemented in Matlab, and it is used to determine the optimal settings for LTC, VRs (tap setting) and CAPs (on/off) at each update.
Figure 4 Load Profile
Figure 5 shows the tap control profile for the LTC and VR with the proposed scheme. Figure 6 shows the tap control profile with conventional control. Figure 7 shows the switched CAP on/off control profile. Comparing the LTC and VR control profiles in the figures, it is seen that the two control sequence is rather different, and with the proposed scheme, tap controls are reduced. Figure 8 shows the voltage profile with conventional control. Figure 9 shows the voltage profile resulting from the proposed control sequence. As these figures show, while with conventional control, the voltages stay close to upper portion of the allowable range almost all the time due to high tap settings, voltages stay close to the rated value (1 pu) most of the time with the proposed scheme, as in this case, VR does not raise the voltage higher values than necessary.
Figure 7.Capacitor control with proposed scheme.
Figure 8 Voltage changes according to the changes in load, conventional.
Fig. 5: LTC and VR control with proposed coordination
Figure 9 Voltage changes according to the changes in load, proposed.
Figure 10 shows the power loss profile calculated at each 15 min. control period, and compares it to the one obtained with optimal solution (obtained by exhaustive search). The figure also shows the power loss under conventional scheme. As the figure illustrates, power loss profile under the proposed scheme is the same as the profile under the optimal Fig. 6: LTC and VR conventional control
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solution. Numerical results are also given in Table 1 and they show how much improvement can be reached with the scheme proposed. These results verify that the proposed method provides the optimal solution for this test system.
REFERENCES [1] [2] [3]
[4]
[5] Figure 10. Comparison of conventional and coordinated control for power loss.
[6]
[7] TABLE I.
minimum maximum
POWER LOSS
Ploss (W) conventional
Ploss (W) proposed
3.9568e+05 4.4150e+05
6.1596e+04 1.0689e+05
V.
% improvement
84.43 75.79
CONCLUSIONS
This paper proposes a computationally efficient method for supervisory control of Volt/Var control devices- voltage regulators and capacitor banks –on a distribution system. The method searches for the best control settings for these devices as the operating conditions changes on the system in realtime. The test results show the effectiveness of method in terms of providing the global optimal solution with fast computational speed. Test results show the effectiveness of decoupling volt/var problem to reduce the complexity of the search. Also, adopting the systematic way to figure out taps that provides voltages to be within the feasible range helps further reduce the computation time, and the proposed scheme becomes computationally feasible. Iteration number becomes directly proportional to the size of feasible taps. Further checking the minimum power loss at each part of decoupled problem reduces the computation time dramatically. Test results also show that the coordination provided by the proposed scheme reduces LTC and VR tap adjustments, and provides better voltage regulation by keeping the node voltages on the feeder around rated values.
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