Applications of Grid-connected Battery Energy Storage Systems

Battery energy storage systems (BESSes) act as reserve energy that can complement the existing grid to serve several different purposes. Potential grid applications are listed in Figure 1 and categorized as either power or energy-intensive, i.e., requiring a large energy reserve or high power capability. They can also be classified according to the deployment time scale, which ranges from milliseconds to hours. A general understanding of the services is helpful before analyzing how storage has been used for delivery.

 

Figure 1. An overview of the different ancillary services provided by BESS. Image used courtesy of IEEE Open Journal of the Industrial Electronics Society

 

Power Quality

Voltage waveform

Power quality indices are used to measure how much the voltage and current waveforms differ from a perfect sinusoidal waveform. Distortions can be temporary, like when loads or generators are turned on or off, or they can be constant at steady states, like when non-linear loads or power electronic interfaced generators are running. Energy storage has been looked into for this purpose and has been shown to be a good answer.

 

Power fluctuation

The rise of intermittent power sources has also brought up the problem of power fluctuations in the network. Solar irradiation and changes in wind speed can cause distributed generation (DG) plants to change power quickly and in large amounts, which can hurt the network. In this situation, energy storage can be added to the DG plant to help smooth out the short-term changes in power. When BESS is used this way, it adds an extra cost to the RES plant, lowering the system's income. In this case, one way to compensate for the lost money could be to give the plant owners financial incentives to reduce power fluctuations.

 

Battery energy storage system. Image used courtesy of Adobe Stock

 

Continuity of Service

In addition to measuring the voltage waveform and the changes in output power, the continuity of service is also kept an eye on. System average interruption frequency index (SAIFI) and system average interruption duration index (SAIDI) are used to figure out the Distribution System Operator’s (DSO) bonus pay. Also, national grid codes can require DSOs to pay fines or make payments to users if service is interrupted. To improve service reliability on distribution grids, energy storage systems can be put in place to make black start procedures easier and let the distribution feeder work on its own.

Both of these problems happen when one or more faults cause a part of a distribution network to stop working with the main transmission grid. In the case of blackouts, storage systems could be added to plans for fixing the grid, making the process of getting power back on faster. Also, a lot of DG and storage systems could make it possible to run safely even when the islanding isn't planned. In a hypothetical islanding procedure, BESSes will be needed to monitor and reduce the fault-caused transient and sudden load-generation imbalance so that the switch from being connected to the grid to being off the grid is smooth.

 

Voltage Control

Several devices, such as tap changers, capacitor banks, voltage regulators, and static VAR compensators, can change the voltage in distribution grids. BESSes can help shape the future of voltage management by adding flexibility to distribution grid management. The use of storage units in the voltage control scheme has been shown to work well from a technical point of view.

Figure 2 shows the voltage profiles of one of the two main feeders of the IEEE European test network. These profiles are evaluated and plotted in Figure 3(a), showing the voltage profiles before and after the addition of PV generators. This lets anyone see how the PV generators can cause overvoltages at the network's end. Figure 3(b) shows how BESS could help reduce overvoltages. The colored lines show the voltage profiles when the BESS system is turned on to reduce the overvoltage. The different colors show where the energy storage is located in the network. In each case, a star is placed on the node where the BESS is located.

 

Figure 2. IEEE European Test Feeder schematic—highlighted with a star the three nodes considered for locating the energy storage units in the analysis of Figure 3. Image used courtesy of IEEE Open Journal of the Industrial Electronics Society

 

Figure 3. Voltage Profiles along the network: (a) with and without PV generation and (b) with PV generation and with storage units used to reduce the overvoltage; the storage units are located in the node marked with a star—the nodes numbers are referred to the numeration of Figure 2. Image used courtesy of IEEE Open Journal of the Industrial Electronics Society

 

To limit how much DGs affect the grid voltage, national and international energy regulators have made it a requirement for DGs connected to distribution grids to follow Q(V) or cos(V) droop curves. In the most recent versions of the national technical standards, such as the Italian standards CEI 0-16 and CEI 0-21 and the German standards VDE-AR-N 4110 and VDE-AR-N 4105, these requirements have also been added for energy storage systems. This service must be done automatically and simultaneously as the main function. It helps to balance out the overvoltage in the distribution network feeders by taking in reactive power and balancing out the undervoltage by putting out reactive power.

 

Peak Shaving and Load Smoothing

Peak shaving and load smoothing involve making the generation and load profiles flatter so that the grid sees less power at its highest point. In real-time, this scheme can help solve network congestion by preventing the conductors from being overloaded by the peak power of both the generator and the load. Also, in a planning horizon, network improvements like rewiring a feeder or replacing a transformer could be avoided or put off by installing energy storage systems.

In this case, energy storage could be a good idea because DSOs are required to ensure that the network infrastructure is good enough to handle both the load's nominal power and the connected generators' nominal power. Along with putting off the upgrade, peak shaving and load smoothing may also help reduce network losses. In this way, BESS operation can further reduce system losses by making the load and local generation more similar in shape.

 

Frequency Control

In the ancillary service market, generators connected to the transmission networks offer frequency control as a paid service. In recent years, this service has also been offered by generators and energy storage systems connected to the distribution network. Generators and BESS use a droop control that watches for frequency imbalances and responds to them by changing the power output. Table 1 shows the main parameters for some European countries' primary frequency control logic.

Figure 4 demonstrates how the droop control logic works. Frequency control is a valuable feature of energy storage systems. Energy storage systems might be limited by their maximum and minimum state of charge (SoC). Several ways to control the SoC have been suggested to solve this problem. Depending on the country, the droop logic is set up with different parameters that define or don't define the deadband and change the amount of droop. The way people get paid is often through tenders, where they bid on regulating power and the price that is needed.

Table 1. Primary Control Parameters in Some European Countries

Parameter	DE	FR	ES	NL	BE	GB
Full Availability	30 s	30 s	30 s	30 s	30 s	30 s
Droop	-	3-6%	7.5%	10%	-	3-5%
Dead Band	10 mHz	10 mHz	-	-	10 mHz	15 mHz
Full Deployment	200 mHz	200 mHz	200 mHz	200 mHz	200 mHz	200 mHz
Service Procurement	Tenders	Bilateral contracts	Not renumerated	Tenders	Tenders	Tenders

 

Figure 4. Example of P-f curves for primary frequency control—the curves are made according to the data in Table 1. Image used courtesy of IEEE Open Journal of the Industrial Electronics Society

 

Energy Arbitrage

Energy arbitrage is buying and selling energy on the spot energy market. Since the electricity sector is separated in most countries, energy arbitrage can only be done by a business user. This can be done with a BESS+DG or BESS+load system, where the storage unit moves the energy production or generation to make the most of price changes in the energy market. Energy arbitrage could be used to create a business case, but the prices on the central European spot market may not be high enough if that is the only source of income.

Considering the day-ahead-market price data for Italy and the UK in 2018, the lowest and highest daily prices are found. These are shown in Figure 5, along with the biggest daily price difference and the average difference over the year. The average daily price difference can be less than 50 €/MWh. 

 

Figure 5. Analysis of day-ahead market prices of the year 2018 for Italy (a) and the UK (b). Image used courtesy of IEEE Open Journal of the Industrial Electronics Society

 

Key Takeaways of Grid-connected BESS

This article has discussed the various applications of grid-connected battery energy storage systems. Some of the takeaways follow.

• Grid applications of BESS can be categorized by energy use and implementation speed. Energy storage in the DG plant can also reduce power fluctuations.
• Energy storage systems can simplify black start procedures and let the distribution feeder function independently, improving distribution grid reliability.
• BESSes can shape voltage management by adding flexibility to distribution grid management, which has been shown to work technically.
• Technical, economic, and regulatory research may examine how to combine multiple services effectively. Research should focus on optimizing battery features and providing complementary services.
• Flattening generation and load profiles reduces network congestion. Energy storage systems avoid feeder rewiring and transformer replacement.
• Generators and energy storage systems connected to the distribution network can ignore paid frequency control.
• Energy arbitrage—buying and selling energy on the spot energy market and moving energy production or generation to take advantage of price fluctuations—can be done with a BESS+DG or BESS+load system.
• Research should demonstrate how to best use the battery's characteristics by creating a comprehensive service delivery plan. 
• Combining multiple services may be studied in the future. Multiple stakeholders also improve business case success.

This post is based on an IEEE Open Journal of the Industrial Electronics Society research article.

 

Featured image used courtesy of Adobe Stock