BATTERY ENERGY STORAGE FOR DISTRIBUTION APPLICATIONS

 Abstract—Energy storage systems are used in different ways to achieve energy management of electric power systems. Batteries are the most important devices to build energy storage systems as they are rechargeable. Battery Energy Storage Systems are therefore finding their way in Distribution Electric Power System applications. This paper   illustrates some of the ways in which Battery Energy Storage Systems are applied in Distribution Power System.

I.     INTRODUCTION

The Electric Power System (EPS) has evolved from being hierarchical with respect to power generation, transmission, and distribution to being integrated. The modern-day EPS incorporates bidirectional power flows, and electric power through Distributed Generation (DG), is also being generated at load centers within the distribution network. Battery Energy Storage Systems (BESS), both in stationary and mobile forms, are finding their application within this revolutionized EPS. Integration of BESS into the Distribution Electric Power System (DEPS) can be said to target two main goals, namely, Distribution Infrastructure Services (DIS) and Customer Energy Management Services (CEMS) applications.

 

Distribution Infrastructure means the physical equipment used to distribute electric power at voltages below 38,000 volts, including but not limited to poles, primary lines, secondary lines, service drops, transformers, and Meters [1]. Application of BESS to Distribution Infrastructure can take the form of Uninterruptible Power Supply for Control and Instrumentation, Distribution Upgrade Deferral, Voltage Support, and Outage Mitigation. DEPSs Control and Instrumentation applications require uninterruptible power supply to the Intelligent Electronic Devices (IEDs) in equipment like Remote Terminal Units (RTUs) and Programmable Logic Controllers (PLCs) to achieve operational continuity. Distribution Upgrade Deferral can involve installation BESS downstream from the nearly overloaded Distribution node to provide enough incremental capacity to defer the need for a large investment in distribution equipment reinforcement. BESS can also provide voltage support to DEPS by addressing issues relating to overvoltage and undervoltage conditions. BESS outage mitigation application can involve absorption of extra power during light load periods and supplying additional power during high load conditions.

 

BESS Customer Energy Management Services applications in DEPS on the other hand involve utilities meeting customer expectations of Power Quality, Power Reliability, Retail electric energy time-shift, and Time-of-Use (TOU) Cost Management and Demand Charge Management. Integration of renewable and distributed energy technologies onto the DEPS introduces challenges of maintaining power quality, as well as balancing supply and demand. At the same time, reducing and optimizing energy consumption is key to both keeping overall energy costs down and meeting sustainability goals.

 

II.     DISTRIBUTION INFRASTRUCTURE SERVICES APPLICATIONS OF BESS

A.     Uninterruptible Power Supply for Control and Instrumentation

Modern electrical power systems are highly automated and use microprocessors in IEDs imbedded in equipment like RTUs and PLCs. To deliver secure, continuous, reliable, and quality electric power supply to customers in the safest manner possible, the DEPS incorporates these electronic and logic devices within its architecture for Protection and Control applications. Uninterruptible power supply to these protection and control devises is of utmost importance. Battery storage therefore finds its way into these ancillary services for DEPS.

B.     Loss Minimization

Many DEPS have radial structure of their feeders. These radial feeders can have a large current to voltage ratio which results in high quantity of power losses in a distribution system. These power losses can be reduced by optimal allocation of Distributed Energy Resources (DER) complete with BESS. Power loss in each branch is the measure of squared value of current flowing into the branch, and energy storage shifts some of this current to a low demand period decreasing the resistive losses [2]. 

 

C.     Distribution Upgrade Deferral

DEPS expand at rates dictated by economic and demographic factors. Load forecasting and planning requirements for DEPS feeder upgrades or new installations become paramount to avoid exceeding the original thermal ampacity limitations of feeders’ line conductor or associated transformers. By utilizing the Battery Energy Storage for peak shaving, BESS on selected locations at the substation or along the distribution feeder would be used to relieve thermal stress to various pieces of equipment, such as substation transformers or distribution conductors [3]. Aging DEPS equipment can also have its lifespan extended by lowering the load it services using BESS. A key value proposition for this application is that a small amount of storage can allow the utility to delay the need for expensive, demand-growth-related DEPS equipment upgrades or reduce demand served by existing DEPS equipment such that the equipment’s life is extended.

 

The common methodology used for developing an effective Distribution Upgrade Deferral program involves development of engineering and financial model that provides guidelines for practical storage deployment and assesses business benefits of BESS as a potential solution for capacity and operational issues.  The methodology can rely on:

·         Detailed engineering analysis of the storage benefits on the utility’s distribution system.

·         Developed and validated repeatable and scalable models and control algorithms.

·         A specified timeframe of stochastic cost projections for selected battery storage technologies.

·         Detailed costs-benefit analysis for the battery type to be used.

 

It is crucial that the BESS must be located downstream from the affected equipment so that it would qualify as a DER. Usually upgrade deferral may only be applied for a very small portion of the year because peak demand may only be experienced during the most extreme peaks like the hottest days. When system upgrade becomes less expensive compared to BESS application if peak demand is growing quickly and requires large amount of storage needed to continue to defer the upgrade, the benefit would diminish rapidly after just a few years. Mobile BESS can be moved and used for deferral or life extension elsewhere. Stationary BESS may also be re-used for other benefits.

 

D.     Voltage Support

Variable energy resources like photovoltaic and wind can cause power and voltage fluctuations. If power generation from these resources significantly exceeds local load demand, it can lead to unacceptable voltage rise at a load bus. These voltage fluctuations can be problematic for DEPS Controllers to manage.

 

Voltage regulation involves controlling voltage magnitudes at all buses of the distribution network to be within the permissible limits. In particular, quick and accurate voltage control becomes primarily important in networks with high PV generation because transient cloud conditions challenge traditional voltage control schemes and can cause frequent voltage and power fluctuations [4]. BESS can help in DEPS voltage regulation processes to solve the voltage deviation problems in LV distribution networks with high penetration of variable resources. For instance, a distributed control based on a consensus algorithm for buses voltage regulation can be applied, where a local control scheme is employed to regulate the state of charge (SoC) of each BESS within desired SoC range. A similar approach but using coordinated control strategy combining a local droop-based control method and a distributed control scheme based on consensus algorithms, with each of which having specific objectives in regulating the charge/discharge of BESS, can help ensuring that bus voltages remain within specified limits.

E.     Outage Mitigation

Natural disasters, which have been aggravated by climate change, can lead to large-scale power outages, and affect critical infrastructure in the process, causing social and economic damages. Improving power grid resilience can help mitigate the damages caused by these events [5]. Resilience has been defined as the ability to reduce the magnitude and/or duration of disruptive events, which includes the capability to anticipate, absorb, adapt to, and/or rapidly recover from a potentially disruptive event.

 

BESS can enhance DEPS resilience by providing localized support to critical loads during an outage. Mobile BESS can further provide operational flexibility to support geographically dispersed loads across an outage area.

           

III.     CUSTOMER ENERGY MANAGEMENT SERVICES APPLICATION OF BESS

A.     Power Quality

Power Quality (PQ) constitutes Voltage Quality, Current Quality, the Quality of Power Supply, and the Quality of Power Consumption. Electricity customers expect to be supplied with power of good quality in accordance with standards and guidelines for their specific region e.g., ANSI C84.1. Poor power quality is attributed due to the various disturbances like voltage sag, swell, impulsive, and oscillatory transients, multiple notches, momentary interruption, harmonics, and voltage flickers [6]. Very large and fluctuating loads on distribution feeders can cause voltage sags and variations which can also affect other customers on the same feeder. BESS can output active and reactive power at the same time and have the four-quadrant operation ability thus can play an important role in the power quality management of distribution network [7].

 

Most industrial automation devices are very sensitive to voltage variations and system harmonics. Customers’ behind-the-meter BESSs can be used to mitigate power quality issues for local loads, over and above Demand Charge Management applications.

 

B.     Power Reliability

Power reliability can be defined as the degree to which the performance of the elements in a bulk system result in electricity being delivered to customers within accepted standards and in the amount desired [8]. The degree of reliability may be measured by the frequency, duration, and magnitude of adverse effects on the electric supply [9]. Reliability quantifies the likelihood of a system to function as specified, under given conditions, over a given duration. BESS localized in load centers can be used to achieve power reliability for customers as an alternative source of supply to the main grid.  

C.     Retail electric energy time-shift

Distribution System Operators (DSOs) can optimize the use of DG and enable customers to participate in various Demand Side Management (DSM) programs like Demand Response (DR). DR is a wholesale market program that energy customers can participate in to earn money for reducing electricity use during peak times. In general, DR includes all planned electricity consumption pattern modifications by end-use customers that are intended to modify the timing and/or the level of their electricity consumption in response to incentive payments or to changes in the price signal over time [10]. DR is a critical measure that a utility can apply to maintain grid reliability and reduce peak electricity prices. However, customers, especially commercial and industrial, may not have the flexibility to adjust load if they are to participate in DR exercise the traditional way. BESS storage gives customers the flexibility to participate in DR programs without affecting their operations. BESS can function automatically by processing DR notifications and take over customer’s local load during utility’s peak hours. This would ensure that there are no disruptions to domestic routines, business and/or production processes of a customer.

D.     Time-of-Use (TOU) Cost Management and Demand Charge Management (DCM)

A customer can be charged for both electricity energy consumption with respect to time in kilowatt-hours (kWh) as well as peak power demanded in kilowatt (kW). The charge on peak power demanded is called Demand Charge (DC). On the other hand, utilities use a Time-of-Use (TOU) pricing structure by allocating higher electricity prices with periods of higher demand. Using storage devices, TOU management can reduce energy charges via energy time-shifting and price arbitrage, while DCM can reduce demand charges via peak load shifting [11].

 

A customer can use BESS for TOU management to reduce energy charges through energy time-shifting and price arbitrage, as well as DCM to reduce demand charges by peak load shifting. BESS can be charged during low electricity prices and discharged to offset energy use when prices are high. This would result in reduced net energy charges. Similarly, in DCM applications, storage devices are charged when demand is low – ideally when energy prices are also low – and discharged to mitigate the peak load when demand is high [11]. DCM with BESS is ideal for periodic peaking loads as they accumulate high demand charges.

 

IV.     CONCLUSION

Modern life and associated lifestyles require continuous, reliable, and secure power supply. The need for non-disruption to essential and critical services like healthcare, financial systems, telecommunication, emergency response, navigation, transportation etcetera exert the need for energy storage systems that ensure continuity of power supply. Battery storage technologies have become important in modern day electric power systems due to the need for replacement of fossil fuels with renewable energy. It is therefore critical that Battery Energy Storage Systems are applied to Distribution Electric Power Systems to support Distribution Infrastructure Services as well as Customer Energy Management Services.

V.     References

 

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