70 MW Battery Energy Storage System (BESS) in Sweden

The system consists of 12 sets of 5.015 MWh prefabricated battery modules and 12 sets of 2.5 MW integrated inverter-booster modules. Featuring a standardized prefabricated design, it offers comprehensive capabilities including high-efficiency charging and discharging, stable grid connection, intelligent operation and maintenance, and safety protection. It is capable of performing core functions such as supporting grid connection for photovoltaic power plants, peak shaving and valley filling, frequency and voltage regulation, and power smoothing.

30 MW/60 MWh Lithium Iron Phosphate (LFP) Energy Storage Plant Project in Simo, Finland

II. Project Scope and System Configuration

2.1 Overall System Configuration

Topology Diagram of a 30 MW/60.18 MWh Energy Storage System

This project will install an energy storage system with a capacity of 300 MW (10% of the total PV installed capacity) and a 2-hour duration, consisting of one 30 MW/60.18 MWh energy storage system, which includes:

12 sets of 5.015 MWh battery prefabricated modules

12 sets of 2.5MW integrated inverter and step-up units

AC high-voltage side output: 35kV

35kV feeders: 2 circuits

2.2 Step-up Transformer Configuration

This project involves the construction of 12 × 2500 kVA step-up transformers with a voltage rating of 35/0.69 kV and a connection configuration of D, y11.

2.3 Energy Storage Power Converter (PCS) Configuration

This project involves the installation of 12 energy storage inverters, each with a rated capacity of 2,500 kW.

2.4 Energy Storage Battery Configuration

This project involves the construction of 12 containerized energy storage battery units, each with a capacity of 5.015 MWh.

III. Selection of Energy Storage Batteries

3.1 Comparison of Battery Performance

This project compares the technical performance of lithium-ion batteries, lead-carbon batteries, and vanadium redox flow batteries—all of which are commercially mature and widely used. The key characteristics of each battery type are as follows:

Table

Battery Type	Ternary lithium-ion battery	Lithium iron phosphate battery	Lead-carbon battery	All-vanadium flow battery
Rated voltage	3.7V	3.2V	2V	1.0~1.6V
Charging ambient temperature	0℃~50℃	0℃~50℃	-23℃~60℃	30~35℃
Discharge ambient temperature	-30℃~50℃	-20℃~50℃	-23℃~60℃	30~35℃
Energy density (Wh/kg)	150～200	90～150	35～50	25～40
Maximum charging current	2C	2C	0.2C	0.25C
Maximum discharge current	4C	3C	0.4C	0.5C
Depth of discharge	80%～90%	80%～90%	60%～80%	100%
DC conversion efficiency	90%～95%	90%～95%	70%～85%	70%～80%
Cycle life (1C, 80% EOL)	100% DOD > 3000 cycles; 90% DOD > 4500 cycles	100% DOD＞3000 cycles；80% DOD＞5000cycles；20% DOD＞30000 cycles	Up to 4,200 cycles at 70% DOD	100% DOD＞10000 cycles
Capacity Recovery Characteristics	Once degraded, it cannot be restored	Once degraded, it cannot be restored	Once degraded, it cannot be restored	Available for streaming
Battery cost (RMB/kWh)	1600～1800	800～1200	600～1200	3500～4500

3.2 Technical and Economic Comparison of Battery Types  

3.2.1 Determining the Battery Type

Lithium-ion Batteries

High energy and power densities; supports up to 4C discharge; wide charge/discharge rate range; excellent high-rate discharge performance; strong charge acceptance capability, facilitating system frequency and voltage regulation; cycle life of up to 5,000 cycles at 80% DOD or higher; charge/discharge efficiency of up to 95%.

Lead-carbon battery

Deep-cycle batteries have a longer service life than standard lead-acid batteries, but their charge and discharge rates are low (0.2C for charging and 0.4C for discharging), making them unsuitable for frequency and voltage regulation applications.

Vanadium flow battery

Although they have a long cycle life, their low charge/discharge rate, low energy density, large footprint, and high cost do not meet the requirements of this project.

Standard lead-acid battery

The charge/discharge rate is too low, failing to meet service life requirements.

After comparison, only lithium iron phosphate and ternary lithium batteries are suitable for this project.

3.2.2 Determining the Type of Lithium Battery

Table

Project	lithium iron phosphate	lithium-ion	Conclusion
Safety	Decomposes at around 800°C, produces no flammable gases, and is relatively safe	It decomposes at around 200°C and releases oxygen, which can easily lead to thermal runaway, making it relatively unsafe.	Lithium iron phosphate offers superior safety
Temperature tolerance	Discharge performance is poor at temperatures below -20°C	Discharge performance is poor at temperatures above 50°C	There isn't much difference in this project environment
Charge/discharge rate	Charge at 2C; discharge at 3C	Charging: 2C; Discharging: 4C	The project has a current density of approximately 0.5 C, and both conditions are met.
Cost-effectiveness	Reasonably priced	About 1.3 times that of lithium iron phosphate	Lithium iron phosphate offers better cost-effectiveness

Based on a comprehensive evaluation, lithium iron phosphate batteries have been selected as the energy storage solution for this project.

5.015 MWh lithium iron phosphate battery prefabricated module

3.3 Liquid Cooling System Design Proposal

Currently, the two mainstream thermal management methods for lithium-ion batteries are air cooling and liquid cooling; hybrid systems using phase-change materials are not yet mature. This project has selected a liquid cooling system, and its key advantages and design requirements are as follows:

Advantages of Liquid Cooling Systems

High cooling efficiency, saving over 30% in floor space, and suitable for large-scale, long-duration energy storage

Even cell temperature distribution reduces operational energy consumption by 30%–50% and extends battery life

Easy to transport and install, suitable for large-scale energy storage applications

Design of a Liquid Cooling System

The battery container is equipped with an integrated liquid-cooling unit, which is located at one end of the container

Cooling pipes cover each battery pack, ensuring high cooling efficiency

Dual-compressor redundant design to improve system availability

Features both heating and cooling functions, with constant-temperature control of the coolant

Temperature control range: 18°C to 30°C, temperature control accuracy: ±2°C

Protection rating: IP54; noise level ≤70 dB(A) at a distance of 1.0 m (in accordance with GB/T 21361-2008)

Components of the Liquid Cooling System

It comprises a fluid supply unit, a cooling and heat dissipation unit, a control and protection unit, and a piping system; the fluid supply, cooling and heat dissipation, and control and protection units are all housed within the liquid cooling cabinet. The system employs a closed-loop circulation design and is equipped with a high-level water tank for pressure stabilization, a PTC heater, and an automatic fluid replenishment function.

IV. Main Electrical Schematic

4.1 35 kV Section

This project connects to the 35 kV busbar of the newly constructed 220 kV step-up substation via two 35 kV lines.

4.2 Energy Storage Battery Section

Twelve new energy storage subsystems will be constructed, each serving as a 2.5 MW/5.015 MWh energy storage node; the AC sides of the 12 subsystems will be connected in parallel and fed into the 35 kV busbar of the 220 kV step-up substation via two 35 kV cables.

5.015 MWh containerized energy storage battery

Configuration for each subsystem:

1 unit of a 2500 kVA three-phase dry-type, self-cooled, double-winding transformer

1 unit of a 2500 kW energy storage power converter (PCS)

1 set of 5.015 MWh energy storage batteries

4.3 Access Cable Lines

High-voltage compartment of the energy storage station: ZR-YJV22-26/35-3×120mm² in series

Total connections: 2 circuits ZR-YJV22-26/35-3×150mm²

Installation method: Cable trench + Direct burial

4.4 Station Power System

Each energy storage unit is equipped with one 100 kVA, 0.69/0.4 kV auxiliary transformer, with power supplied from the low-voltage side of the on-site step-up transformer.

Install a UPS (uninterruptible power supply) on-site to ensure power supply to critical loads

4.5 Neutral Point Grounding Methods

The grounding configuration of the 35 kV system for this project is consistent with that of the 220 kV step-up substation.

V. Short-Circuit Current Calculation and Selection of Major Equipment

5.1 Calculation of Short-Circuit Current

35 kV electrical equipment: Short-circuit current withstand level of 31.5 kA

690V Electrical Equipment: Short-circuit current withstand level 50 kA

5.2 Selection of Major Equipment and Conductors

5.2.1 Integrated Inverter and Booster Module

There are a total of 12 units, each housing equipment such as step-up transformers, PCS units, and high-voltage feeder cabinets.

Step-up Transformer Specifications

Type: Three-phase indoor dry-type double-winding transformer

Capacity: 2500 kVA

Rated voltage: 37±2×2.5%/0.69 kV

Wiring group: D, y11

Impedance voltage: Uk = 6.0%

Cooling method: AN (AF)

Energy Storage Power Converter (PCS) Specifications

Rated capacity: 2,500 kW, maximum 2,750 kVA

Maximum DC voltage: 1500 V; Full-load voltage: 1000 V to 1500 V

Maximum DC current: 1375 A × 2

Maximum system efficiency: 99%

AC current harmonics: THDi < 3%

Power factor: Adjustable from -1 to +1

Protection rating: IP54

Operating temperature: -30°C to +50°C

Cooling method: Intelligent air cooling

Communication: Modbus-TCP/IP, Ethernet

Dimensions: 6000 × 2900 × 3000 mm, Weight: <20 tons

Complies with standards: GB/T 36276, GB/T 34131

High-voltage feeder panel

35 kV indoor metal-clad fixed switchgear, rated voltage 40.5 kV, rated current 1250 A, rated breaking current 31.5 kA.

5.2.2 Energy Storage Battery Compartment

The system utilizes lithium iron phosphate batteries in an outdoor prefabricated container configuration. Each battery module has a capacity of 5.015 MWh; with a total of 12 modules, the system has a total capacity of 60.18 MWh and a total power output of 30 MW. Key parameters of a single battery module:

Rated charging and discharging power: ≥5.015 MW

Available capacity: ≥5.015 MWh

Battery energy conversion efficiency: ≥90%

Battery compartment operating temperature: -30°C to 50°C

Single cell: 3.2 V/314 Ah, operating voltage 2.5–3.65 V, cycle life ≥ 10,000 cycles (25°C, 70% SOH), standard discharge rate 0.5C

Battery system efficiency: ≥92%, calendar life: ≥10 years

Battery rack: Corrosion resistance compliant with ISO 9223-1992 C3; seismic resistance: Grade 7

Cycle life: 7,000 cycles at 70% EOL (95% DOD)

VI. General Electrical Layout

The energy storage system has a total capacity of 60.18 MWh and is located on vacant land north of the 220 kV step-up substation.

Total number of cabins: 24

12 battery prefabricated modules: Dimensions 6250 × 2896 × 2438 mm; battery modules arranged back-to-back

12 integrated inverter-booster enclosures: Dimensions 6000 × 2900 × 3000 mm; each contains a high-voltage compartment, a low-voltage compartment, a transformer, and a PCS

The 35 kV feeder switchgear is located in the 35 kV switchgear room, and the secondary equipment is housed in a container in the central control room.

The energy storage area is enclosed by a PVC fence

Cables connecting compartments are laid in cable trenches

VII. Overvoltage Protection and Grounding

7.1 Overvoltage Protection

Surge arresters are installed on the 35 kV outgoing circuits of the step-up bay to protect against lightning surge waves.

7.2 Protection Against Direct Lightning Strikes

The metal shell of an outdoor energy storage container forms an equipotential surface (Faraday cage); reliable grounding is sufficient, and no additional direct lightning protection is required. If the container is made of non-metallic material, a lightning protection strip must be installed on the roof.

7.3 Grounding

The outdoor main grounding grid of the energy storage station is designed in conjunction with the grounding system of the 220 kV step-up substation and connected via equipotential bonding.

VIII. Fire Alarm System

Install one automatic fire alarm system throughout the facility, integrated into the intelligent monitoring and auxiliary control system.

 

Detection Configuration: Install heat and smoke detectors in the high-voltage compartment, step-up compartment, and battery compartment; use explosion-proof models in the battery compartment.

 

Fire Protection Power Supply: Dual-circuit 220V AC power supply; the fire alarm control panel is equipped with a 12V DC battery, providing a backup runtime of ≥2 hours in the event of a power outage.

 

Interlock Control: In the event of a fire, non-fire-related power sources (such as lighting, air conditioning, and fans) are shut off, and the system interfaces with the video surveillance system.

 

Alarm Facilities: Manual call points and dedicated fire telephones are installed in each fire compartment.

Signal Transmission: Fire alarm signals are transmitted to the main control building.

 

Containerized Energy Storage System Cooling System

IX. Power and Lighting Systems

Outdoor areas are equipped with floodlights and maintenance power distribution boxes.

 

LED fluorescent lights are used inside the cabin, and explosion-proof LED energy-saving lights are used in the battery compartment.

 

Emergency lighting: Includes standby lighting and evacuation lighting; the illuminance of evacuation lighting in main corridors is ≥1 lx.

X. Cable Installation

Cable fire protection complies with the requirements of the “Code for Design of Power Engineering Cables” (GB-50217-2018):

Power cables are laid in separate layers and separated by fire-resistant partitions.

Fire-resistant partitions are installed for critical control cable circuits.

Communication optical cables are laid in fire-resistant cable trays. 

Power, control, and communication optical cables are separated into distinct layers within the same cable trench.

XI. Relay Protection for Energy Storage Systems

Prefabricated Substation Protection
The high-voltage side is equipped with a load switch and a current-limiting fuse; the transformer is equipped with high-temperature alarms and over-temperature trip protection; the low-voltage side PCS circuit breaker is equipped with long-time, short-time, and instantaneous overcurrent protection

 

PCS Protection
Phase-to-phase short-circuit instantaneous protection, single-phase ground fault, overload, undervoltage, anti-islanding, temperature, overvoltage, and reverse DC connection protection

 

Monitoring and Interlocking
Cameras in the battery compartment and electrical compartment are integrated into the booster station’s video surveillance system; the fire suppression system is integrated into the station-wide fire alarm system

 

XII. Energy Storage Monitoring System (EMS)

  
12.1 EMS Energy Management System
A complete EMS system is configured, integrated with the substation’s computer monitoring system, and communicates with the grid dispatch center to achieve comprehensive monitoring, control, dispatch, and O&M management of the entire system. Core functions:

Human-Machine Interaction: Display of equipment status and power consumption information; parameter settings

Data Acquisition: Real-time collection of all data from BMS, PCS, and smart auxiliary equipment

Historical Storage: Storage of key data, status, faults, and operation logs

System Communication: Supports CAN, RS485, and LAN ports; protocols include CAN and Modbus RTU/TCP

Control Strategies: Executes charging and discharging based on PV output, peak/off-peak periods, and dispatch instructions

Access Control: Tiered operational permissions; operation logs are traceable

 

12.2 Data Acquisition and Monitoring
Designed for “minimal manning,” the system performs data acquisition, analysis, regulation, safety monitoring, incident guidance, and energy optimization management.

 

12.3 Data Acquisition and Processing
Acquires analog signals (current, voltage, power, frequency, etc.), digital signals (protection, alarms, status), and environmental parameters to implement out-of-limit alarms, SOE logging, and data preprocessing.

 

12.4 Monitoring and Alarms
Monitoring: Main electrical schematics, PCS/BMS status, curves, reports, and alarm information

Alarms: Categorized into incident alarms and warning alarms, distinguished by color; critical signals trigger automatic pop-up alerts

 

12.5 Control and Operation
Supports remote operation of circuit breakers, setpoints, and switches, featuring operation supervision, permission control, operation logging, and error notification functions.

XIII. List of Materials for Energy Storage Systems

List of Equipment and Materials for Energy Storage Projects

Table

No.	Equipment / Material Name	Specifications and Models	unit	Quantity
1	2.5 MW PCS Inverter and Booster Integrated Enclosure	Includes a 2500 kW PCS and a 2500 kVA transformer; dimensions: 6000 × 2900 × 3000 mm	set	12
2	5.015 MWh battery prefabricated module	Lithium iron phosphate, 0.5C, dimensions: 6250 × 2896 × 2438 mm	set	12
3	EMS Battery Management System	Complete set	set	1
4	35 kV power cable	ZRC-YJV22-26/35kV-3×120mm²	m	502
5	35 kV power cable	ZRC-YJV22-26/35kV-3×150mm²	m	276
6	Cable Termination	Compatible with 3×120mm²	set	20
7	Cable Termination	Compatible with 3×150mm²	set	4
8	Secondary cables and accessories	Complete set	set	1