Design Engineering For Battery Energy Storage Systems: Sizing, Selection and Operation

BESS Design & Operation

In this technical article we take a deeper dive into the engineering of battery energy storage systems, selection of options and capabilities of BESS drive units, battery sizing considerations, and other battery safety issues. We will also take a close look at operational considerations of BESS in electrical installations.

This article is the second in a two-part series on BESS – Battery energy Storage Systems. Part 1 dealt with the historical origins of battery energy storage in industry use, the technology and system principles behind modern BESS, the applications and use cases for such systems in industry, and presented some important factors to consider at the FEED stage of considering BESS in a project.

Feel free to read the first part of BESS series here:
BESS (Battery Energy Storage Systems) in LV and MV Power Networks: Practical Guide

Let’s get into the details of design engineering for Battery Energy Storage Systems (BESS)!

Table of contents:

1. Glossary of Terms
2. BESS Design and Engineering:
   1. Grid Connection
   2. Dimensioning of Batteries
   3. Division of Batteries
   4. Battery Energy Management: State of Charge and Health
   5. Blackstart Considerations
   6. Battery Safety Design
   7. Battery Room
   8. Direct Connected DC link:
      1. Direct DC Link Advantages
      2. Direct DC Link Disadvantages
   9. Indirectly Connected DC Link:
      1. Indirect DC Link Advantages
      2. Indirect DC Link Disadvantages
   10. BESS Operational Considerations:
       1. Pre-Conditioning for Operations Mode
       2. Thermal Events and Damage Inspection
       3. Emergency Shutdown – ESD
       4. Evacuation Planning
       5. Changing Control Parameters With Age
       6. Shutdown of Batteries
3. BONUS (PDF) 🔗 Download Handbook for Power System Operation and Control

1. Glossary of Terms

This article will be very heavy in the use of acronyms. Here is a quick guide:

Table 1 – Glossary of terms

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2. BESS Design and Engineering

These are the FEED and detailed design considerations that must be made when deciding on how best to integrate BESS into a design.

2.1 Grid Connection

The grid connection point should be decided early in the design phase. It may be decided to split the BESS into two or more distinct units for connection at multiple points in the network. This can be done to allow multiple sections to function independently with BESS support, as well as provide redundancy in system design.

The type of connection should be decided early. If the BESS shall connect to a LV or MV connection point. Most battery systems will not exceed 1500 V DC, as this would bring them into the HV classification range and entail increased equipment and operational demands.

Additionally, it may be difficult to find DC switchgear rated to such high voltages and current.

When connecting to an LV network, the BESS can be treated similar to a generator incomer, though energy flow will be bi-directional. Depending on the AC drive configuration, it may be possible to connect the BESS directly to the network before the output is modulating, and have the drive perform a ‘flying synchronisation’. Otherwise, conventional synchronising will be needed.

If connecting to a MV network, it will be necessary to install a transformer to interface the LV drive to the MV network. It may also be desirable to have an isolation transformer for direct LV connection also.

Connecting via a transformer is a more complicated undertaking. Several things must be considered:

#1 Will the BESS energise the transformer prior to connection to the MV network?

1. If yes, this will then require conventional synchronisation techniques for live/live connection.
2. If no, it may not be possible to ‘black start’ the facility from the BESS.

#2 How will sycnhronisation be controlled?

1. Options to use conventional pulse based synchro relays for frequency and voltage matching.
2. Option to use VT direct measured voltage feedback from transformer primary and MV bus to allow the BESS controller to perform its own synchronising.

#3 Decision on transformer winding ratios so that the BESS can stay connected at all expected MV bus voltage operating points:

1. Consider the voltage regulation of the transformer during full load charging and discharging of the BESS.
2. Option to select a more optimised voltage ratio between grid and BESS AC output. This may allow for lower DC link operating voltages than a direct connection.

Figure 1 – Single-line diagram of a BESS comprised of two phase shifted AC drives, connected to an AC 11 kV substation via a transformer

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2.2 Dimensioning of Batteries

One of the most impactful design elements of BESS is the dimensioning of the battery component. What is important to consider is the required power draw or charging current, and the energy requirements. While these two factors are highly correlated, there is the ability to tune for one or another.

When designing and selecting a BESS the project engineer will deal with a battery specialist who will try to select the correct battery package for the application. This will involve creating a usage profile for the system, with an assumed program of charge and discharge cycles.

This is just an illustrative example. But based on these requirements, a battery size and configuration will be specified that can sustain the necessary C rating (power flow), DOD, and still fulfill these requirements at its end of life.

Figure 2 depicts an example of a battery usage profile. This may be a typical process profile, which can be combined with other use profiles, along with information about the daily frequency, DOD, and yearly occurrences, to arrive at a lifetime usage profile of the battery.

Figure 2 – Example of a battery usage profile

It is important for owners and operators to understand that battery life is based on several assumptions. The load profile at design stage, consistent temperature of the battery room, and a lack of battery abuse, to name but a few.

Battery abuse may include storage at very hot or cold temperatures, excessive over or under charging, or subjecting the battery to voltage spikes.

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2.3 Division of Batteries

Different suppliers will have differing terminology. However, the “battery” is often used as a general term to refer to a common collection of battery arrays. The lithium-ion interface unit will make a cell. The electrochemical reaction inside this cell generates a voltage that is typically in the range of 2.2 – 4.4 volts at the extremes of SOC.

Though the exact values should be provided by the supplier for the actual cell chemistry that is being purchased.

These cells are then assembled in series into a battery module to generate a more useful voltage. This will differ between systems, but a typical battery module voltage is around 50 volts. These modules are then arranged again in series to create a string.