Loss and load reduction by coordinated control of community energy storage and OLTC

Transcription

1 International Journal of Smart Grid and Clean Energy Loss and load reduction by coordinated control of community energy storage and OLTC Jae Won Cang, Ki Hoon Kang, Gyu Sub Lee, Jin O Lee, Seung-Il Moon a * Department of Electrical and Computer Engineering, Seoul National Univ. #13, 1 Gwanangno, Gwana-gu, Seoul, 8826, Sout Korea Abstract enewable energy sources are usually integrated wit a distribution system troug a small-size-generator wit low voltage level. As a consequence, distributed energy storage systems ave been connected to a distribution system to elp wit renewable energy resources integration. Community Energy Storage () is a small distributed energy storage connected to te secondary part of a transformer and is an advanced energy storage system close to te customer. provides many benefits suc as pea saving, load sifting and voltage control. Meanwile, distribution system operators conduct Conservation Voltage eduction (CV) to reduce power consumption in te system. To implement CV on te system, flat voltage profile is required. is distributed in te distribution system and is capable of controlling system voltage. In tis paper, a control metod of is proposed in order to implement CV. Based on day-aead load forecasting, power consumption including load demand and active power loss is reduced by te optimal scedule of output power and On-Load Tap-Canger (OLTC) operation. Wit a case study using Matpower, tis paper validates te effect of te proposed metod. Keywords: Community energy storage, conservation voltage reduction, loss reduction, load reduction, OLTC 1. Introduction Nomenclature E : Primary bus voltage of transformer grid E : Sending part voltage of distribution system S E : eceiving part voltage of distribution system E : Bus voltage connected wit C P, Q : Active/reactive power from transmission system to distribution system grid grid P, Q : Load demanding active/reactive power load load P, Q : Active/reactive power from sending end to receiving end P, Q : Excanged active/reactive power wit, X : Equivalent resistor/reactance of distribution line L L X : Equivalent impedance of transformer Tr : Turn ratio of transformer P : Power loss of system at time Loss P, i : Grid angle : Active power of i at time Vmax, V : Maximum and imum voltage of system allowance V : Voltage of bus at time tap, tap : Maximum and imum of tap position tap : Tap position of OLTC at time max * Manuscript received December 1, 215; revised April 2, 216. Corresponding autor. Tel.: ; address: jwcang91@gmail.com doi: /sgce

2 Jae Won Cang et al.: Loss and load reduction by coordinated control of community energy storage and OLTC 87 Q, i,max : eactive power of i at time P, P : Maximum and imum active power of max, SOC, SOC : Maximum and imum of SOC SOC : State of carge of i at time i Z p, I p, P : Active power of load ZIP coefficient p Zq, Iq, P : eactive power of load ZIP coefficient q P : Injected active power to bus at time Q : Injected reactive power to bus at time P : Generated active power of bus at time Q : Generated reactive power of bus at time GK, GK, P : Consumed active power of bus at time LK, Q LK, P L, K : Noal consumed active power of bus at time : Consumed reactive power of bus at time Q : Noal consumed reactive power of bus at time L, K N : Number of time intervals B : Number of buses consisting of a distribution system N C : Number of connected to a distribution system N enewable Energy Sources (ES) are directly integrated into a distribution system because tey are small and te output voltage is low [1]. An Energy Storage System (ESS) is integrated wit a distribution system to mitigate te output of ES. A distributed ESS is also used to control grid voltage and supply te reserve for frequency regulation [2]-[3]. ecently, ESS is installed similar to Community Energy Storage () due to te small scale of ES, wic is close to te consumer suc as a domestic Potovoltaic () panel [4]. is a type of ESS, wic as small capacity and is connected to te secondary part of a distribution transformer. Because is located near te customer, reliability of is more credible and efficient tan te reliability of ESS. is tan ESS. In addition, is widely spread and as easy to control voltage wit small capacity. Tus, is ceap to maintain and as a or influence on te grid [5]-[7]. Te operation of for reducing pea load in a distribution system integrated wit generation and optimization of output for load leveling is presented in [8], [9]. In [1], using predicted load and price information, a metod for maximizing operational advantage of is used to acieve an economic profit. Also, imizing te fluctuation of system voltage due to intermittent generation wit is proposed [11]. Tis paper proposes could be used for CV. CV is a metod tat reduces te wole active power of a load maintaining a low voltage level wit a range of allowance. Te earlier versions of CV use a Step Voltage egulator (SV) for te feeder, sunt capacitor (SC) and On-Load Tap-Canger (OLTC) of te transformer to flatten te voltage of te system. is distributed to a system and can control voltage by using output controlling. Tus, flattening voltage using could be used for CV. In tis paper, we consider integrated wit a distribution system of wic voltage is controlled by te OLTC and wic reduces a voltage of a connected point of te system troug active/reactive power. Terefore, we propose coordinated control for reducing te loss and load power wit CV. Te voltage for eac bus, and all sceduling intervals for te State of Carge (SOC) and output must be ept witin a range of constraints. Te interior point solver of MATPOWE is used for te simulation results. Tis paper is organized as follows. Section 2 provides teoretical analysis of output control. Section 3 proposes output sceduling. Section 4 provides simulation and experimental results. Section 5 is te conclusions. 2. Teoretical Analysis of Output Control In tis section, a system voltage equation is derived wen is connected wit a distribution system.

3 88 International Journal of Smart Grid and Clean Energy, vol. 5, no. 2, April 216 Also, we verified tat te control of active/reactive power of can control system voltage and loss. Fig. 1. connected to radial distribution system System voltage control and loss using output Fig. 1 sows a simplified distribution system for teoretical analysis. OLTC and can be used to control system voltage. also controls system voltage for controlling active/reactive power. We assume tat system voltage is near rated voltage and tat can supply/absorb active/reactive power as operator instructions. Also, we assume tat te loads consist of a constant power model and te cooper loss of te transformer can be ignored. Te receiving part, sending part voltage and power loss were represented by te following equations: E 4 2A 2 B B AC (1) A B E P X Q C X P Q , S 2( L L ), ( L L )( ) E 2E X X E ( ) ( Q cos P sin ) ( ) ( P Q ) (2) 2 grid 2 grid Tr Tr S r r E E P jq P I, P jq ( P P ) j( Q Q ) (3) 2 2 loss L L E P and Q were detered by power of te sending part and receiving part were decided by OLTC in te following equations: 1 2 load load P and Q, respectively. Consequently, voltage and active P, Q and te tap position of te E f ( P, Q ) f ( P, Q, tap) (4) E g ( P, Q ) g ( P, Q, tap) (5) S 1 2 P ( P, Q ) ( P, Q, tap) (6) loss Output Sceduling Sceduling wit OLTC and is performed using Matpower [12]. For optimal sceduling, te objective function and constraints ave to be detered.

4 3.1. Objective function Te object of tis paper is tat wen a load is canged wit time in a distribution system connected wit, power loss and load power are reduced to maintain te system voltage level wit te proper range. Unlie an earlier CV operation using OLTC, SV and SC, te output power control metod using wit OLTC is proposed. Te sum of load power and loss is te objective function. N B N L, Loss (7) 1 1 J Min( P P ) 3.2. Constraints Tere are several constraints for finding te optimal solution to an objective function. First, system voltage and OLTC ave constraints based on te operation of te distribution system. V V V for 1,2,, N and 1,2,, BN (8) Jae Won Cang et al.: Loss and load reduction by coordinated control of community energy storage and OLTC 89 max for 1,2,, N (9) tap tap tapmax Te output and SOC of te are given by te manufacturer. P, P, i P,max for 1,2,, N and i 1,2,, CN (1) SOC SOC SOC for 1,2,, N and i 1,2,, CN (11) i max Te maximum and imum reactive power of is detered by S P Q S P ,max, i, i,max, i S,max and P : for 1,2,, N and i 1,2,, CN (12) Te active power of and te SOC of are expressed as: SOC SOC P c for 1,2,, N and i 1,2,, CN (13) 1 i i, i For te next period of sceduling, te end and start values of te SOC are te same. SOC SOC for i 1,2,, CN (14) 1 N 1 i i Te buses are not connected wit te and ave constraints. P P P for 1,2,, N and G, L, Q Q Q for 1,2,, N and G, L, Te buses are connected wit te and also ave constraints. (15) (16)

5 9 International Journal of Smart Grid and Clean Energy, vol. 5, no. 2, April 216 P P P P for 1,2,, N and G, L,, i Q Q Q Q for 1,2,, N and G, L,, i For applying CV, tere are constraints for te system load model. 2 L, L, ( p( ) p p) V V (17) (18) V V P P Z I P for 1,2,, N and 1,2,, BN (19) V V Q Q Z I P for 1,2,, N and 1,2,, BN (2) 2 L, L, ( q( ) q q) V V 3.3. Matpower for output sceduling Te interior point solver from Matpower is used to draw optimal coordinated sceduling, wic satisfy te above objective function wit constraints. Te variables used for te interior point solver are x [ P, Q, SOC, tap, V,, P, Q, P, Q ] (21) T, i, i i G, G, L, L, 4. Case Study For verifying a reduction of load power and power loss, a distribution system is formed based on te IEEE 13 test bus system. Te simulation system is composed of 13 buses and 32 tap positions of te OLTC. Te OLTC controls te voltage of Bus 2 in order to regulate te voltage of te wole distribution system. On Buses 5, 12 and 13, a generator and are connected. Four of te are connected wit te secondary part of a distribution transformer. Te capacity and maximum output are based on [13]. Te initial SOC state is.6 and te SOC is maintained from.3 to.9 for te life cycle. Te maximum output of te generator is.1mw. Simulation system is sown in Fig. 3. Fig. 2. Active power of total loads, load on Buses 5, 12 and 13, and generator. Table 1. Parameters of te simulation system Distribution lines Transformer OLTC Loads s Modified IEEE 13-bus test system 115/4.16V, 5MVA, x=8%, r=1% At HV side, -1% to 1% regulation wit 32 steps ZIP_residential : Zp = 1.5, Ip = -2.31, Pp=1.81 Zq = 7.41, Iq = , Pq= 5.55 ZIP_industrial : Zp = 1.21, Ip = -1.61, Pp=1.41 Zq = 4.35, Iq = -7.8, Pq= 3.72 ated power : 25W Capacity : 5W Efficiency : 1% Maximum SOC :.9 Minimum SOC :.3 Initial SOC :.6

6 Jae Won Cang et al.: Loss and load reduction by coordinated control of community energy storage and OLTC 91 We assume tat load prediction is performed every our and te load is comprised of constant power, constant current, and constant impedance load. ZIP coefficient for Industrial load and residential load is based on [14]. Te parameters are sown in Table 1. Also, load and information is sown in Fig. 2. Te load consists of an industrial load and a residential load. Bus 5 is connected wit te as a residential bus. Buses 12 and 13 are connected wit te as an industrial load. Two cases are compared for verification of te study. Case 1 is te multi s connected system. Case 2 is te control for te system voltage using te coordinated control of OLTC and a single ESS connected to a distribution system. Capacity and maximum output of a single ESS is te same as te capacity and maximum output of a wole s. Location of ESS is decided as Bus 13, wic can mostly decrease te consumed power and loss branc. Case 1 and case 2 of te simulation systems are sown in Fig ESS Fig. 3. Modified IEEE 13 test bus (multi connected system and single ESS connected system). Table 2. Sceduling results for Case 1 and Case 2 Case Case 1 Case 2 Case1 Case2 (Case1-Case2)/Case1 [%] Total load[mw] Loss[MW] Total load+loss[mw] Table 2 sows te optimal sceduling results for Case 1 and Case 2. In te situation were multi s are connected to a distribution system, te consumed power is lower tan te consumed power of a single ESS connected case. Te total load, loss, and sum of total load and loss fell by.114%, 4.56%, and.199%, respectively. Fig. 4. Voltage of eac bus at te eaviest load time and te tap position of te OLTC. Fig. 4 sows tat te system voltage of te OLTC and multi connected s is lower tan te voltage of single connected ESS at te eaviest loading time. It is impossible to cange to a lower tap position because te voltage of Bus 5 is at a imum tolerance range. As ESS is connected to bus 13, it is

7 92 International Journal of Smart Grid and Clean Energy, vol. 5, no. 2, April 216 difficult to compensate for te voltage of Bus 5. However, as in te multi s connected cases, connected wit Bus 5 can compensate for te voltage of Bus 5. Te tap position of te OLTC can also be low and te system voltage can be maintained low. Fig. 4 also sows te tap position of te OLTC for te two cases. Te tap position for multi s connected cases is lower tan te tap position of a single ESS connected case for most of te time. Because te distribution of in te system compensates eac voltage of te bus effectively, system voltage can be low and te tap position of te OLTC can be low. 5. Conclusions ESS is distributed in a distribution system in order to directly connect te ES wit a distribution system for mitigating te output of ES. Distributed ESS not only mitigates te output of ES, but also affects te control of system voltage, frequency regulation and demand response. ecently, te ES is closed to customers. ESS is also installed as a near te customer. taes a small load and is installed on a secondary part of a distribution transformer. is available to reduce te pea load, system operation cost and control system voltage. Earlier versions of CV control system voltage wit OLTC, SV, and SC. Because is distributed in a system and can control system voltage using output control, we notice tat could also be used for CV. Tis paper proposed a coordinated control metod of OLTC and output for CV, troug ability of reducing te voltage of an integrated branc in order to decrease consumed power. In oter words, based on te predicted load, scedule of active/reactive power of output and te tap position of OLTC, load power and loss could be reduced. For sceduling derivation, tis paper used te interior point solver provided by Matpower. Also, te proposed control metod was verified by teoretical analysis and a case study was performed. Te case study sowed tat te proposed control metod improved te reduction of load power and loss power in comparison to a previous control metod. Consequently, te proposed control metod sowed tat could be used for renewable energy resources integration and could improve system operation efficiency troug CV. Acnowledgements Tis wor was supported by te Global Excellent Tecnology Innovation ( ) of te Korea Institute of Energy Tecnology Evaluation and Planning(KETEP), granted financial resource from te Ministry of Trade, Industry & Energy, epublic of Korea. eferences [1] Freris L, Infield D. enewable Energy in Power System, Jon Wiley & Sons; 28. [2] Stadler M, Kloess M, Groissboc M, Cardoso G, Sarma, Bozcalui MC, et al., Electric storage in California s commercial buildings. Applied Energy, 213; 14: [3] Scroeder A. Modeling storage and demand management in power distribution grids. Applied Energy, 211; 88: [4] Jung J, Co Y, Ceng D, Onen A, Argande, Dile M, et al., Monte Carlo analysis of plug-in ybrid veicles and distributed energy resource growt wit residential energy storage in Micigan. Applied Energy, 213; 18: [5] Nourai A, Sastry, Waler T. A vision & strategy for deployment of energy storage in electric utilities. Power and Energy Society General Meeting, 21:1-4. [6] Parra D, Gillott M, Norman SA, Waler GS. Optimum community energy storage system for energy time-sift. Applied Energy, 215; 137: [7] Zu W, Garrett D, Butowsi J, Wang Y. Overview of distributed energy storage systems for residential communities. Energytec, 212:1-6. [8] Huq KMM, Baran ME, Luic S, Nare OE. An energy management system for a community energy storage system. Energy Conversion Congress and Exposition, 212: [9] Meng F, Haugton D, Cowdury B, Crow ML, Heydt GT. Distributed generation and storage optimal control wit state estimation. IEEE Transaction on Smart Grid, 213; 4(4):

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