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This thesis has proposed a new approach for PV inverter ramp-rate control using an integrated energy storage device. Unlike the traditional moving average method, which is biased by the previous values and does not directly control the ramp-rate at a desired level, the proposed method can limit the ramp-rate within a desired level. An inverse characteristic of the desired ramp-rate with the PV panel output ramp-rate is proposed to improve the fluctuation mitigation performance during the ramping event. Once the

ramping event is over, the desired ramp-rate is switched to a droop characteristic with SoC so that the available capacity is taken into account while controlling the ramp-rate.

Simulation results show that the mitigation of the PV output fluctuation achieved by the proposed ramp-rate control strategy is comparable to those obtained using the moving average method, while not using the energy storage all the time. Also, during very high ramp-rate events, the proposed strategy can provide tighter control of the ramp-rate due to the inverse characteristic. The mitigation of voltage fluctuation in weak networks caused by PV output fluctuation using the proposed method is also demonstrated. The validity of the proposed strategy has been verified in a dynamic environment using a dynamic model of the PV-storage integrated system developed in this chapter. The results from a laboratory experiment using a PV inverter, a battery storage system, and an electronic load demonstrates that the proposed approach is able to control the PV inverter ramp-rate by providing appropriate support from the storage device.

A

PPENDIX

TABLE7-I

SIMULATION PARAMETERS OF THE PROPOSED CONTROL STRATEGY

Parameter Value

|RRlim|,  5 W/sec, 0.005

SoCref, SoCLB, SoCUB, DBSoC 70%, 2.5%, 5%, 2.5%

MARRmin, MARRmax, DBMARR MARR, 5 W/sec, (5-MARRmin)

SoCmax, DoDmax 100%, 40%

TABLE7-II

PARAMETERS USED FOR DYNAMIC SIMULATION

Parameter Value

Tinv,Tsto,Tmeas 20 mS, 5 mS, 3 mS

KP, KI 2.30, 470

 

R

EFERENCES

[1] W. Jewell and R. Ramakumar, "The Effects of Moving Clouds on Electric Utilities with Dispersed Photovoltaic Generation," IEEE Transactions on Energy Conversion, vol. EC-2, pp. 570-576, 1987.

[2] E. C. Kern, E. M. Gulachenski, and G. A. Kern, "Cloud effects on distributed photovoltaic generation: slow transients at the Gardner, Massachusetts photovoltaic experiment," IEEE Transactions on Energy Conversion, vol. 4, pp. 184-190, 1989.

[3] C. Trueblood, S. Coley, T. Key, L. Rogers, A. Ellis, C. Hansen, and E. Philpot, "PV Measures Up for Fleet Duty : Data from a Tennessee Plant Are Used to Illustrate Metrics That Characterize Plant Performance," IEEE Power and Energy Magazine, vol. 11, pp. 33-44, 2013.

[4] J. Johnson, B. Schenkman, A. Ellis, J. Quiroz, and C. Lenox.Sandia National Laboratories. Initial Operating Experience of the La Ola 1.2-MW Photovoltaic System [Online]. Available:

http://energy.sandia.gov/wp/wp-content/gallery/uploads/SAND2011-8848_Lanai_DOE_Report_FINAL.pdf

[5] NREL. 1-Second Global Horizontal Irradiance Oahu, Hawaii [Online]. Available:

http://www.nrel.gov/midc/oahu_archive/

[6] S. Abdollahy, A. Mammoli, F. Cheng, A. Ellis, and J. Johnson, "Distributed compensation of a large intermittent energy resource in a distribution feeder," in IEEE PES Innovative Smart Grid Technologies, Washington, DC, USA, 24-27 Feb. 2013, pp. 1-6.

[7] S. Sayeef, S. Heslop, D. Cornforth, T. Moore, S. Percy, J. K. Ward, A. Berry, and D.

Rowe.CSIRO. (June 2012). Solar intermittency: Australia's clean energy challenge.

Characterising the effect of high penetration solar intermittency on Australian electricity

networks. [Online]. Available:

https://publications.csiro.au/rpr/download?pid=csiro:EP121914&dsid=DS1

[8] H. Sugihara, K. Yokoyama, O. Saeki, K. Tsuji, and T. Funaki, "Economic and Efficient Voltage Management Using Customer-Owned Energy Storage Systems in a Distribution Network With High Penetration of Photovoltaic Systems," IEEE Transactions on Power Systems, vol. 28, pp.

102-111, 2013.

[9] N. Kakimoto, H. Satoh, S. Takayama, and K. Nakamura, "Ramp-Rate Control of Photovoltaic Generator With Electric Double-Layer Capacitor," IEEE Transactions on Energy Conversion vol.

24, pp. 465-473, 2009.

[10] T. Kinjo, T. Senjyu, N. Urasaki, and H. Fujita, "Output levelling of renewable energy by electric double-layer capacitor applied for energy storage system," IEEE Transactions on Energy Conversion, vol. 21, pp. 221-227, 2006.

[11] T. Monai, I. Takano, H. Nishikawa, and Y. Sawada, "A collaborative operation method between new energy-type dispersed power supply and EDLC," IEEE Transactions on Energy Conversion, vol. 19, pp. 590-598, 2004.

[12] K. S. Tam, P. Kumar, and M. Foreman, "Enhancing the utilization of photovoltaic power generation by superconductive magnetic energy storage," IEEE Transactions on Energy Conversion, vol. 4, pp. 314-321, 1989.

[13] S. Rahman and K. S. Tam, "A feasibility study of photovoltaic-fuel cell hybrid energy system,"

IEEE Transactions on Energy Conversion, vol. 3, pp. 50-55, 1988.

[14] T. D. Hund, S. Gonzalez, and K. Barrett, "Grid-Tied PV system energy smoothing," in Photovoltaic Specialists Conference (PVSC), Honolulu, HI, USA, 20-25 Jun., 2010, pp. 2762-2766.

[15] J. Traube, L. Fenglong, D. Maksimovic, J. Mossoba, M. Kromer, P. Faill, S. Katz, B. Borowy, S.

Nichols, and L. Casey, "Mitigation of Solar Irradiance Intermittency in Photovoltaic Power Systems With Integrated Electric-Vehicle Charging Functionality," IEEE Transactions on Power Electronics, vol. 28, pp. 3058-3067, 2013.

[16] S. G. Tesfahunegn, Ø. Ulleberg, P. J. Vie, and T. M. Undeland, "PV Fluctuation Balancing Using Hydrogen Storage–a Smoothing Method for Integration of PV Generation into the Utility Grid,"

Energy Procedia, vol. 12, pp. 1015-1022, 2011.

[17] L. Xiangjun, H. Dong, and L. Xiaokang, "Battery Energy Storage Station (BESS)-Based Smoothing Control of Photovoltaic (PV) and Wind Power Generation Fluctuations," IEEE Transactions on Sustainable Energy, vol. 4, pp. 464-473, 2013.

[18] C. J. Coe, A. N. Hurst, M. I. Hardin, M. C. Such, and R. T. Jennings.(Nov. 2011, US Patent 20110273129). Managing Renewable Power Generation [Online]. Available:

https://docs.google.com/viewer?url=www.google.com/patents/US20110273129.pdf

[19] C. A. Hill, M. C. Such, C. Dongmei, J. Gonzalez, and W. M. Grady, "Battery Energy Storage for Enabling Integration of Distributed Solar Power Generation," IEEE Transactions on Smart Grid, vol. 3, pp. 850-857, 2012.

[20] A. Ellis, D. Schoenwald, J. Hawkins, S. Willard, and B. Arellano, "PV output smoothing with energy storage," in Photovoltaic Specialists Conference (PVSC), Austin, TX, USA., 3-8 Jun., 2012, pp. 1523-1528.

[21] M. J. E. Alam, K. M. Muttaqi, and D. Sutanto, "Mitigation of Rooftop Solar PV Impacts and Evening Peak Support by Managing Available Capacity of Distributed Energy Storage Systems,"

Accepted for a future issue of IEEE Transactions on Power Systems.

[22] H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li, and Y. Ding, "Progress in electrical energy storage system: A critical review," Progress in Natural Science, vol. 19, pp. 291-312, 2009.

[23] B. Espinar and D. Mayer.International Energy Agency. July 2011. The role of energy storage for mini-grid stabilization [Online]. Available: http://www.iea-pvps-task11.org/HTMLobj-187/Act_24_Final.pdf

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Chapter 8

 

M ITIGATION OF N EUTRAL C URRENT AND

N EUTRAL P OTENTIAL R ISE IN 4- WIRE

M ULTIGROUNDED LV N ETWORKS UNDER

U NBALANCED A LLOCATION OF R OOFTOP

S OLAR PV U SING E NERGY S TORAGE

 

A

BSTRACT

An unbalanced allocation of 1-phase rooftop photovoltaic (PV) units have the potential to exacerbate the existing neutral current and neutral potential in low voltage (LV) 4-wire multigrounded distribution networks. In spite of balanced loads, a high PV unbalance can create considerable neutral current and neutral potential rise. Traditional mitigation strategies have certain limitations to mitigate the combined effect of load and PV unbalance. To address this unbalance effect, energy storage is applied as a potential strategy against the neutral current and neutral potential under a high penetration of unbalanced rooftop PV allocation. Two novel mitigation approaches are proposed: one is using distributed storage devices connected to each PV unit, suitable for use in an existing network, and the other is a Community Energy Storage (CES) system connected to all the PV systems installed in an LV feeder, suitable for new networks or future grids. A power balancing algorithm is developed to perform the balancing operation while minimizing power drawn from the energy storage. A charge/discharge control strategy is developed that will continuously balance and dynamically adjust the power exchange with the grid in a real time and mitigate the neutral current and neutral potential rise. An Australian LV distribution system is used to verify the proposed approach and results are presented.

8.1 I

NTRODUCTION

Neutral current flow in low voltage (LV) 4-wire multigrounded distribution networks is a known problem among distribution system research community and protection engineers. The fundamental frequency component of the neutral current is mainly produced by load unbalance [1] and unbalanced impedances of the network. The unbalanced phase currents do not sum up to zero, and the resultant neutral current flows through the neutral conductor and neutral grounding resistances causing health and safety concerns. Besides the fundamental component, harmonic components produced by the magnetization of distribution transformers and non-linear loads can increase the neutral current to be even higher than the phase current [1]. In a healthy distribution system, the neutral grounding resistances would be small and therefore, a reasonable amount of neutral current produced by inherent unbalances of a distribution feeder would not increase the neutral potential significantly. However, if the neutral grounding resistances elevate, for example due to bonding with a non-metallic water piping system [2], the neutral potential or Neutral-to-Ground Voltage (NGV) can become significant.

A high NGV value can damage sensitive electronic and computing equipment due to the common-mode noise effect [3]. Manufacturers of computing equipment have specified 0.5 V [3] as the limit of common-mode noise; a more stringent limit of 0.1 V is reported in [4]. NGV and other forms of stray voltages are also detrimental for human beings [5]

and farm animals [6]. To ensure a safe and reliable operation of distribution networks, neutral current and NGV rise have to be treated with due importance.

In addition to the legacy of load and network unbalance, a high penetration of single phase rooftop solar PV units can worsen the classical neutral current and neutral (or earth) potential. Residential rooftop PV system size is primarily proposed by the customer during the application process. Due to a rapid growth of the number of PV connection requests, distribution utilities in Australia are not always in a position to perform detailed analysis of the impact of each PV connection, especially if the PV system is small (less than 5 kW) [7]. The main attention is given on the transformer size so that the proposed connection does not cause exceeding the limit. Therefore, a direct control to accomplish a balanced allocation of PV systems to different phases of a feeder may not always be practiced. Hence, a highly unbalanced allocation of PV units may appear which can create a large neutral current and hence, the NGV can rise to

exceed the acceptable limit.

Several methods for the mitigation of neutral current and neutral potential rise have been reported in the literature [6, 8-11]. The balancing of loads [8-9, 11] is a traditional mitigation method used for the reduction of neutral current produced by the load unbalance, although in practice it is very difficult to have 100% load balance [11]. Other forms of traditional mitigation techniques, such as re-sizing of neutral conductor [8, 11], improving grounding system [8, 11] or installation of equipotential planes [6, 8, 10], may need significant additional material and can increase installation costs. Further, the variability of PV generation would make it difficult to balance the combined effect of load and PV unbalance using the traditional mitigation techniques that are mainly of

“static” type, and does not change with the variation of PV output. Therefore, new mitigation strategies need to be developed to balance out the combined unbalance produced by rooftop solar PV units and load demands.

Energy storage has been used in [12] to mitigate voltage unbalance in LV networks.

In a previous work [13], the authors have explored the application of distributed energy storage devices integrated with rooftop PV systems for the mitigation of neutral current and NGV problems. However, the detailed method for charging/discharging control was not shown in [12]. The concept of Community Energy Storage (CES) system [14] is becoming popular among the utilities [15]. It entails utility deployment of modular energy storage systems where one CES system is connected with multiple customers in a feeder to serve as a back-up power source, and also for different ancillary support including the buffering of customers’ PV/wind energy and time shifting of energy for PEV charging [15]. With an appropriate control strategy, the CES system can also be used for mitigating neutral current and neutral potential rise problems.

This thesis proposes the application of energy storage devices for mitigation of neutral current and neutral potential rise in LV distribution networks under an unbalanced allocation of 1-phase PV resources. At first, the shortcomings of the traditional mitigation strategies in providing effective solutions for neutral current and neutral potential rise caused by PV units will be discussed. Considering the limitations of the traditional strategies, two new solution approaches are developed in this thesis:

one is the distributed energy storage based mitigation strategy for existing distribution networks and the other is the CES based solution approach suitable for new networks or

future grids. The main contributions of this thesis are to develop an algorithm to minimise the power consumption/delivery by the energy storage while mitigating the neutral current and NGV rise problems, and to develop a charging/discharging strategy using the proposed power balancing algorithm to provide a dynamic mitigation effect under variable load and PV unbalance. The proposed approach will be analysed for their effectiveness in mitigating the neutral current and neutral potential problems in the context of Australian LV distribution systems.

8.2 N

EUTRAL

C

URRENT AND NEUTRAL

P

OTENTIAL

C

REATED BY

U

NBALANCED

A

LLOCATION OF

R

OOFTOP

S

OLAR

PV

UNITS

A three-phase 4-wire multigrounded LV feeder containing PV integrated households is considered in Fig. 8.1. The neutral current produced at any given bus can be obtained from the expression below.

 

   

re re

 

im im

, , ,

PV L PV net L

net net net N

c b a V p

V j V V

Q Q j P I P

I I I I

n p n p

p p p

p p

c b a n

(8.1)

where, INnis the neutral current at the given bus; Ineta ,Inetb ,andInetc are the net currents in phase a, b and c; PL, PPV are the active load and active power of the PV inverter at the respective phases; QL, QPV are the reactive power of the load, and reactive power of the PV inverter at the respective phases; Vre and Vim are the real and imaginary components of the respective phase and neutral voltages.

INn

Ineta Inetb Inetc

 

Fig. 8.1. Neutral current in a 4-wire multigrounded LV feeder produced by unbalanced load and PV allocation.