IEEE INDICON 2015 1570164563 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 60 61 62 63 64 65
Optimal Voltage Regulation of a Distribution Network by Output Power Management of DGs Akanksha Shukla
Kusum Verma
Department of Electrical Engineering Malaviya National Institute of Technology Jaipur, India
[email protected]
Department of Electrical Engineering Malaviya National Institute of Technology Jaipur, India kverma.ee@ mnit.ac.in
Shahbaz A. Siddiqui Department of Electrical Engineering Manipal University Jaipur, India
[email protected] iv) Type 4: DG injecting real power P but consuming reactive power Q.
Abstract—The integration of Distributed Generation (DG) into distribution networks is associated with large practical problems. With the large variation in load demand and the high penetration level of these distributed energy resources, the voltage stability issue has become a prime concern for the system operator. This paper proposes an optimal voltage control method for regulating the voltage of LV distribution network with DGs whenever the voltage at any bus exceeds normal operating limits. The algorithm is based on the voltage sensitivities to active and reactive power. The voltage regulation is achieved by power output management of DGs with re-dispatching their power outputs using Nonlinear Programming (NLP) optimization technique. The algorithm is tested on standard IEEE-33 bus radial distribution system and the result obtained shows the effectiveness of the proposed algorithm
Normally the voltage regulation at distribution network is provided by the On-Load Tap Changer (OLTC) Transformers connected at the substation complemented by voltage regulators. However, during emergency conditions these control actions may fail in presence of DG as their response time is slow [4]. In [5], a two way communication technology is utilized for distributed control for voltage regulation in smart distribution feeders. The maximum amount of active power supplied by DG into each system bus without causing voltage violations is determined in [6]. In [7], the coordination of multiple reactive power devices located on the distribution system near the end user to control the voltages is investigated. The local voltage variations are controlled by the two proposed methods in [8]. Optimal voltage regulation algorithm is developed and solved using linear programming to control the voltage by adjusting multi-DG output in [9]. In [10], two coordinated voltage control algorithm were developed for distribution system in presence of DGs. A voltage control method with a single DG was presented in [11]; it also proposed a method for coordinating DGs and traditional voltage control devices using a centralized system to minimize system losses. A combined control of substation voltage and reactive power of distributed generations has been reported in [12]. Majority of these methods proposed the control of voltage with single DG by adjusting its generation. In [10] and [12], communication system is required to connect to all the nodes of the system whether the DG is connected to that node or not. The need for global communication limits the benefits of coordinated voltage control methods.
Keywords— Active power control; distributed generation; optimal voltage control; nonlinear programming; voltage sensitivity.
I. INTRODUCTION The interconnection of Distributed Generation (DG) sources such as photovoltaic, fuel cells, and micro turbines in distribution system networks has increased considerably in the recent years. The CIGRE defines DG as the generation, which is not centrally planned and not centrally dispatched. They are connected to the distribution network and are smaller than 50– 100 MW [1]. Although DG has positive impact on the distribution systems in terms of system reliability and power loss but sometimes it may interfere with voltage control processes and may lead to the stability problems [2]. DGs are normally classified into four major types. This classification is based on real and reactive power delivering capability of DG as follows [3]:
In most of the existing methods proposed in the literature, the DGs are operated either in PFC (Power Factor Control) mode or in UPC (Unity Power Control) mode. In this paper, an optimal voltage regulation method suitable for both the PFC proposed. This is achieved by regulating the generation of all
i) Type 1: DG injecting real power P only. ii) Type 2: DG injecting reactive power Q only. iii) Type 3: DG injecting both real power P and reactive power Q.
978-1-4673-6540-6/15/$31.00 ©2015 IEEE
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the DGs while maintaining the bus voltages within the specified threshold limits. For the DGs operating in UPC mode, the active power output of each connected DG is regulated and for DGs operating in PFC mode, the active power output as well as the reactive output is regulated without changing their power factors. Nonlinear Programming (NLP) optimization technique is used to obtain the re-dispatch the power output of DGs. The proposed method is investigated on IEEE-33 radial bus test system. The test results of the proposed algorithm shows that the proposed method is able to control the voltage within the threshold limits under different operating conditions by re-dispatching the active/reactive power output of the DGs.
where Q0 and Q pre are reactive power output of DG after and before voltage regulation respectively. The amount of active power output of DG to be regulated is given as
ΔP = P0 − Ppre
(6)
where P0 and Ppre are the active power output of DG after and before control respectively. The control of voltage from initial value V0 to reference value V for a given bus can be expressed analytically as
V = V0 + JVP ΔP + JVQ ΔQ
(7)
Equation (7) can be rearranged as II. PROPOSED VOLTAGE REGULATION METHOD
V = V0 + ( JVP + JVQ tan(cos −1 pf ))ΔP
A. Determination of Voltage Sensitivity The relationship between voltage and DG power generation can be expressed with the help of Jacobian matrix which can be obtained through power flow results. It describes the linear relation between real and reactive power to voltage. The linearized power flow equation is given as [6]
⎡ ΔP ⎤ ⎡ J Pθ ⎢ ΔQ ⎥ = ⎢ J ⎣ ⎦ ⎣ Qθ
J PV ⎤ ⎡ Δθ ⎤ J QV ⎥⎦ ⎢⎣ ΔV ⎥⎦
B. Proposed Voltage Control Algorithm Normally voltage deviation of more than ±5% from normal voltage is considered as voltage instability problem. Due to large variation of loads in distribution systems, the threshold limits of voltage are usually violated. Re-dispatch of the real power output of DGs can be calculated based on the voltage sensitivities using a NLP optimization technique [13-14].
(1)
where ∆ denotes small variation in variables. The Jacobian matrix gives the linear relation between small changes in voltage angle ∆θ and ∆V voltage magnitude with the small changes in real and reactive power ∆P and ∆Q. Equation (1) can be re-written as
⎡ Δθ ⎤ ⎡ Jθ P ⎢ ΔV ⎥ = ⎢ J ⎣ ⎦ ⎣ VP
Jθ Q ⎤ ⎡ ΔP ⎤ JVQ ⎥⎦ ⎢⎣ ΔQ ⎥⎦
When all the DGs operate in UPC mode, then only active power output of all the incorporated DGs can be increased or decreased. For this mode, the objective function can be formulated as
(2)
(3)
(9)
⎧⎪Vl ≤ V0 + JVP ⋅ ΔP ≤ Vu ⎫⎪ ⎨ ⎬ ⎪⎩ ΔP ≤ Psurplus ⎭⎪
(10)
where ΔPiUPC is the change in active power of ith DG and
Equation (3) denotes the impact of active power output of multi-DGs on the system voltage, ΔVPDG is the voltage
ΔP is the vector of ΔPiUPC . Vl is the lower bound voltage and Vu is the upper bound voltage. Psurplus is the surplus capacity of the DG and i=1 to n, where n is the total number of DGs connected to the system.
variations with change in active power output of DGs. If ∆P=0 then equation (2) is given as
ΔVQDG = JVQ ΔQ
Max : Min{ΔPiUPC } Subject to
If ∆Q=0 then equation (2) is given as
ΔVPDG = JVP ΔP
(8)
(4)
For controlling the voltage for positive deviation from the normal voltage ΔPi is negative value i.e. the active power output of DG is to be reduced by ΔPi amount. For controlling the voltage for negative deviation from normal voltage, ΔPi is positive value i.e. ΔPi amount of real power is to be increased.
Equation (4) denotes the impact of reactive power output of multi-DGs on the system voltages, ΔVQDG is the voltage variations with change in reactive power output of DGs. The reactive power impact can be either positive or negative depending on the generator power factor. The capacitive power factor leads to voltage rise and inductive power factor to a voltage drop. The amount of reactive power output to be regulated can be determined as
When all the DGs are operated in PFC mode, the active power and reactive power of all the DGs has to be increased or decreased simultaneously. For this mode, the objective function can be formulated as
ΔQ = Q0 − Qpre
Max : Min{ΔPi PFC }
(5)
Subject to
2
(11)
⎧Vl ≤ V0 + ( JVP + JVQ tan(cos −1 pf )) ⋅ ΔP ≤ Vu ⎫ ⎪ ⎪ ⎨ ⎬ ⎪⎩ΔP ≤ Psurplus ⎪⎭
Load is randomly varied during simulation at time t=1s and the re-dispatch of DGs real and reactive power output at time t=2 s is obtained using the proposed control algorithm for the all the cases of voltage limit violations. The voltage control algorithm determines the amount of active power re-dispatch for each DG using single NLP technique. The voltage profile of all the buses is observed and the voltage violations are checked after application of the proposed control algorithm.
(12)
Both these objective functions (9) and (11) are solved using NLP technique. Fig. 1 shows the flowchart for adjustment of DG power for optimal regulation of voltage.
A. Results of voltage control algorithm for DG in the UPC mode For UPC mode, three DGs (Type I) are considered to be connected in the system. The penetration level of DGs has been set to 23% for this mode. The voltages of all the buses are within the normal operating limits for base case load at this penetration. The simulation results obtained at various loading scenarios is shown in Table I. Table shows the application of the proposed voltage control algorithm with active power redispatch applied for different loading scenarios having buses with voltages below the threshold limits. TABLE I. RESULTS OF PROPOSED VOLTAGE CONTROL ALGORITHM (FOR UPC MODE) Re-dispatch of active power output Loading No. of buses (+∆Pi) of DG (in MW) Scenario below the (in % of DG at bus threshold DG at bus DG at base case) limit 18 32 bus 33 15 0.2126 0.0606 0.0606 100 8 0.0585 0.0170 0.0169 Fig.1.
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Flowchart for adjustment of DG power for optimal regulation of voltage
III. SIMULATION RESULTS
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The proposed voltage control algorithm is tested on IEEE 33bus radial distribution systems having 33 nodes. A single line diagram of the test system is shown in Fig. 2. The total load is 3.715 MW and 2.3 MVAr [15]. The number of DGs connected to the system is three. Two cases have been studied: (a) with three DGs (type I) connected to the system and supplying active power only, (b) with three DGs (type III) are connected to the system and supplying both active and reactive power. The location of DGs in the system is assumed to be known and in this paper it is considered to be located at bus 18, 32 and 33 [16]. Each DG is assumed to have the information about the sensitivity matrix, acceptable voltage limits, their sizing and siting and their surplus power capacity. For generating different loading scenarios, the real and reactive power demand at load buses is varied randomly from base case to 160% of the base case in steps of 10%. A large number of dynamic simulations are performed for all these loading scenarios using PSAT [17].
130 140
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Voltage in p.u.
0.98 0.97 0.96 0.95 0.94 0.93 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Bus Number
Fig. 3. Voltage profile of all the buses before and after control at base case with DGs in the UPC mode
Fig. 3, shows the voltage profile before and after application of proposed method at the 100% of base case corresponding to the case having 15 buses below the threshold value. As shown in Fig. 3, the proposed voltage control algorithm is able to regulate the voltage of all these buses
Fig. 2. Single line diagram of 33-bus radial distribution system.
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within the threshold limits. Fig. 4, shows the active power redispatch of all the three DG obtained for this mode at base case loading. For the same case, voltage profile of one of the violated bus (bus 18) out of the 15 buses is shown in Fig. 5. At time t > 1s when load is varied the voltage at the bus suddenly reduces to a value lower than 0.95 p.u making the system unstable. At t=2s when the DGs output power is re-dispatched using the proposed algorithm, the voltage profile settles to the new value within stable operating limits as shown in Fig. 5.
active power output of DG in MW
1
active power before control
0.7
active power after control
0.8 0.7 0.6 0.5 0.4 0.3 0.2
Bus number
0.5
As shown from Table I, it can be observed that there is a voltage limit violation for the 21 buses for this loading scenario. Also the voltage control algorithm fails to regulate the voltage of all these buses. This is due to the fact that the active power output of DG cannot be increased beyond the surplus active power of DG. Thus for loading level greater than the capacity of DG other control actions such as load shedding may be implemented. However it is observed that the proposed alogorithm is able to regulate the voltages of the majority of the buses for this case also.
0.4 0.3 0.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Bus Number
Fig. 4. Active power output re-dispatch of DGs at base case in UPC mode 1 VBus18
0.99
B. Results of voltage control algorithm for DG in the PFC mode For DG in PFC mode, type III DG has been considered to be connected in the test system. The penetration level of DGs has been set to 17% for this mode so that voltages of all the buses are within the normal operating limits for base case load. Random load variation has been performed for each loading conditions using dynamic simulations.
0.98 0.97
Voltage in p.u.
0.9
Fig. 7. Active power output re-dispatch of DGs for 140% of base case load for DG in UPC mode.
0.1
0.96 0.95 0.94 0.93 0.92
TABLE II. RESULTS OF PROPOSED VOLTAGE CONTROL ALGORITHM (PFC MODE) Re-dispatch of active power output Loading No. of buses (+∆Pi) of DG (in MW) Scenario below the (in % of threshold DG at DG at DG at base case) limit bus 18 bus 32 bus 33 9 0.1670 0.0481 0.0480 100 11 0.2351 0.0681 0.0680
0.91 1
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5 time (s)
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Fig. 5. Voltage profile of bus 18 at base case in UPC mode. 1 voltage before control
voltage after control
0.99 0.98
Voltage in p.u.
active power before control
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
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active power after control
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Active po w er o utpu t o f DG in M W
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0.93 0.92
140
0.91 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Bus Number
Fig. 6. Voltage profile of all the buses before and after control at 140% of base case load for the DG in UPC mode.
150 160
Fig. 6 and Fig. 7 show the voltage profile and active power redispatch of all the three DGs for sample loading at 140% of base case.
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The sample results for each operating scenarios is shown in Table II. It shows the application of the proposed voltage
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control algorithm with active power re-dispatch applied for all the loading scenarios having bus voltages outside the normal operating limits. When all DGs are in PFC mode, re-dispatch of both active and reactive power of DG has to be done simultaneously without changing the power factor of the involved DG.
For the same case, voltage profile of one of the violated bus (bus 17) out of the 17 buses is shown in Fig.11. At time t=1s when load is varied the voltage at the bus suddenly reduces to a value lower than 0.95 p.u. making the system insecure. At t=2s when the DGs output power is re-dispatched using the proposed algorithm, the voltage profile settles to the new value within secure operating limits.
1
Voltage in p.u.
voltage before control
voltage after control
0.99
1
0.98
0.99
0.97
0.98
VBus18
0.97
Voltage in p.u.
0.96 0.95 0.94 0.93
0.96 0.95 0.94 0.93
0.92 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0.92
Bus Number
0.91
Fig. 8. Voltage profile of all the buses before and after control at 120% of base case load for the DG in PFC mode
0.9 0
active power after control
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The Fig. 12 shows the voltage profile before and after application of proposed method at the 160% of the base case corresponding to the case having 21 buses voltages below the operating value. As shown in Fig, the proposed voltage control algorithm is able to regulate the voltage of all these buses within the operating limits.
active pow er before control
1 0.99
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voltage before control
voltage after control
0.98 0.5
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Vo ltag e in p .u .
Active power of DG in MW
2
Fig. 11. Voltage profile of bus 18 for 120% of base case for DGs in PFC mode
0.8 0.7
1
time (s)
Fig.8 shows the voltage profile before and after application of proposed method at 120% of base case corresponding to the case having 17 buses below the operating value. As shown in Fig, the proposed voltage control algorithm is able to regulate the voltage of all these buses within the operating limits. The Fig. 9 and Fig. 10 shows the active and reactive power redispatch of all the three DGs obtained for this operating condition at the 120% of base case loading.
0.4 0.3 0.2
0.96 0.95 0.94 0.93 0.92
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0.91 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
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Bus Number
0.89 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Fig. 9. Active output re-dispatch of DGs for 120% of base case load for DGs in PFC mode
Bus Number
Fig. 12. Voltage profile of all the buses before and after control at 160% of base case load for the DGs in PFC mode
0.35 reactive power after control
reactive power before control
1 0.9
active power after control
active power before control
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Active power of DG in MW
Reactive power of DG in MVAr
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Bus Number
Bus Number
Fig. 10. Reactive output re-dispatch of DGs for 120% of base case load for DGs in PFC mode
Fig. 13. Active output re-dispatch of DGs for 160% of base case load for DGs in PFC mode
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Reactive Power of DG in MVAr
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reactive power after control
reactive pow er before control
[2]
0.6 0.5
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Bus Number
[5]
Fig. 14. Reactive output re-dispatch of DGs for 160% of base case load for DGs in PFC mode [6]
The Fig. 13 and fig. 14 shows the active and reactive power re-dispatch of all the three DG obtained for this operating condition at the 160% of base case loading. It can be observed that in the PFC mode the proposed method is able to control the voltage at the loading beyond the DGs surplus capacity. This is due to the reason that in this mode the real and reactive power output of DGs are regulated simulataneosly. Thus proposed control method is able to control volatge satisfactory at 160% of the base case also.
[7]
[8]
[9]
IV. CONCLUSIONS The integration of Distributed Generation in a distribution system poses various challenges to the system operator during operation such as stability problems and voltage regulation issues. This paper proposes an algorithm for optimal control of voltage by regulating the real and reactive output of the DGs. The algorithm is able to calculate the amount of re-dispatch of DGs for regulating the voltage with only single NLP technique. The proposed approach works efficiently for single DG as well multi-DG connected systems to regulate the voltage. For UPC mode, it is observed that the proposed approach is not able to control the voltage when the loading level increases beyond the surplus power of DGs. This is due to the fact that the active power output of DG cannot be increased beyond the surplus active power of DG. For such operating conditions other control methods such as load shedding may be an alternative approach to control the voltage. However, in PFC mode the proposed approach is able to control voltage for worst loading cases also as real and reactive power output of DGs are regulated simultaneously. The results show that proposed voltage control algorithm for the system connected with DGs in UPC mode and in PFC mode is successful in regulating the voltage up to its maximum power output loading.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
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