Exploring the Differences Between On-Grid, Off-Grid, and Hybrid Battery Energy Storage Systems

June 8, 2024

In recent years, battery energy storage systems (BESS) have emerged as crucial components of modern power systems, offering a range of benefits from grid stabilization to energy cost optimization.  Among the various types of BESS configurations, three main types of BESS are outlined below.

On-grid, Off-grid, and Hybrid Battery Energy Storage Systems Functionality Breakdown

Each electrical/mechanical configuration has its own set of advantages and applications, making them suitable for different installation and service scenarios.  Besides the batteries themselves the other key components that will determine the functionally and use of the complete battery energy storage system are the PCS and STS.

  1. A Power Conversion System (PCS) for Battery Energy Storage Systems (BESS) is a critical component that manages the flow of electrical energy between the batteries and the grid. It consists of power electronics, control systems, and monitoring devices that enable efficient and safe operation of the BESS.
  2. A Source or Static Transfer Switch (STS) is a critical component in power systems that have multiple power sources, such as grid power, backup generators, and renewable energy sources.  The main function of an STS is to automatically transfer the load from one power source to another in the event of a power outage or when there is a need to switch between power sources.

On-Grid Battery Energy Storage Systems:

On-grid BESS are connected to the main power grid and primarily serve to enhance grid stability, support renewable energy integration, and provide peak shaving services.  These systems are designed to operate in parallel with the grid, either injecting or absorbing power as needed to balance grid demand and supply.

Power Conversion System (PCS):  The PCS in on-grid systems is typically grid-following, meaning it synchronizes its output with the grid’s voltage and frequency.  This allows the system to seamlessly inject or absorb power without causing disruptions to the grid.  Typical applications for this are as follows:

Typical On Grid Battery Energy Storage Applications:

    1. Voltage Synchronization:  Grid-following PCSs continuously monitor the grid’s voltage waveform. They adjust the output voltage of the BESS to match the grid’s voltage, ensuring that the energy injected into the grid is at the correct voltage level. This synchronization is crucial for maintaining grid stability and preventing voltage fluctuations
    2. Frequency Synchronization:  Similarly, the PCS monitors the grid’s frequency and adjusts the BESS output frequency to match. In most regions, the standard grid frequency is 50 Hz or 60 Hz, and the PCS ensures that the BESS output aligns with this frequency. This synchronization is essential for maintaining the grid’s overall frequency stability.
    3. Active and Reactive Power Control:  Grid-following PCSs also provide control over the active (real) and reactive power output of the BESS. This control allows the BESS to provide ancillary services to the grid, such as frequency regulation, voltage support, and reactive power compensation.

 

Single Line Diagram of On Grid BESS

 

Off-Grid Battery Energy Storage Systems:

Off-grid BESS operate independently of the main power grid and are commonly used in remote areas or as backup power systems.  These systems rely solely on the stored energy in their batteries and renewable energy sources (if available) to meet their energy needs.  By providing grid-forming capability and advanced control features, the PCS enables off-grid systems to operate autonomously and efficiently, making them ideal for remote or isolated locations.

PCS:  The PCS in off-grid systems is designed to be grid-forming, meaning it can independently establish a stable grid voltage and frequency.  This allows the system to operate in isolation from the main grid, ensuring a reliable power supply. An off-grid Power Conversion System (PCS) is a crucial component of off-grid battery energy storage systems (BESS) that operate independently of the main power grid.  Unlike on-grid systems, which synchronize their output with the grid’s voltage and frequency, off-grid PCSs must establish and maintain a stable grid voltage and frequency autonomously.

Here are some key aspects of an off-grid PCS:

    1. Grid-Forming Capability:  One of the most critical features of an off-grid PCS is its ability to act as a grid-forming device. This means that the PCS can independently establish and maintain a stable grid voltage and frequency without the presence of an external grid reference. This capability is essential for ensuring a reliable power supply in off-grid applications.
    2. Voltage and Frequency Regulation:  Off-grid PCSs regulate both the output voltage and frequency of the BESS to ensure compatibility with connected loads. The PCS continuously monitors the voltage and frequency and adjusts its output to maintain stable power conditions.
    3. Load Balancing and Energy Management:  Off-grid PCSs also manage the distribution of power between the battery bank, renewable energy sources (such as solar panels or wind turbines), and connected loads. The PCS optimizes the use of available energy sources to ensure a reliable and efficient power supply.
    4. Communication and Control:  Off-grid PCSs are equipped with communication interfaces and control systems that allow operators to monitor and control the system remotely. This capability is essential for managing energy storage and ensuring system performance.
    5. Protection and Safety Features:  Off-grid PCSs incorporate various protection mechanisms to ensure the safety of the system and connected equipment. These may include overvoltage protection, overcurrent protection, and short-circuit protection, among others.

 

Single Line Diagram of Off Grid BESS

 

Hybrid (Auto-Switching) Battery Energy Storage Systems:

Hybrid BESS combine the features of on-grid and off-grid systems, allowing them to operate both connected to the main grid and in islanded mode (disconnected from the grid). These systems automatically switch between grid-connected and off-grid modes based on predefined criteria or external signals.

PCS:  The PCS in hybrid systems must be capable of both grid-following and grid-forming operation, depending on the system’s mode. This requires advanced control algorithms and power electronics to ensure seamless transitions between modes.

STS:  Hybrid systems require sophisticated switchgear, known as Source or Static Transfer Switches (STS), to enable automatic switching between grid-connected and off-grid modes.  These switches must be fast and reliable to maintain power supply continuity during transitions (around 0.4 milliseconds)

Here’s how a hybrid PCS typically works and how it needs to interact with a Source Transfer Switch (STS):

    1. Grid-Connected Mode:  In grid-connected mode, the hybrid PCS operates like a traditional on-grid PCS, synchronizing its operation with the grid’s voltage and frequency.  It can provide grid support functions such as frequency regulation and voltage support, as well as charge and discharge the battery as needed to optimize energy usage.
    2. Off-Grid Mode:  In off-grid mode, the hybrid PCS operates autonomously, establishing and maintaining a stable grid voltage and frequency independent of the main grid.  It relies on the energy stored in the battery and any available renewable energy sources to power connected loads.
    3. Switching Between Modes:  The hybrid PCS needs to work in conjunction with a Source/Static Transfer Switch (STS) to switch between grid-connected and off-grid modes.  The STS is a key component that allows the system to seamlessly transition between these modes without interrupting power supply to connected loads.
    4. STS Functionality:  The STS is responsible for disconnecting the BESS from the main grid and connecting it to the off-grid system when switching to off-grid mode.  It must be fast and reliable to ensure that the transition is smooth and does not disrupt power supply.
    5. Control and Coordination:  The hybrid PCS and STS need to be carefully coordinated to ensure that the switch between modes is triggered at the right time and under the right conditions. This requires advanced control algorithms and communication between the PCS and STS.
    6. Safety and Protection:  The hybrid PCS and STS must incorporate various safety and protection features to ensure the safety of the system and connected equipment during mode transitions. This may include overvoltage protection, overcurrent protection, and short-circuit protection.

 

Single Line Diagram of Hybrid BESS

 

Here are some main uses for a hybrid or off-grid BESS and PCS:

    1. Remote Area Electrification:  Hybrid or off-grid BESS and PCS are used to provide electricity in remote areas where extending the main power grid is expensive or impractical. This includes powering remote communities, research stations, and off-grid industrial sites.
    2. Telecommunications Towers:  BESS and PCS are used to provide backup power for telecommunications towers located in remote areas. They ensure uninterrupted communication services, even during grid outages.
    3. Islanded Microgrids:  In islanded microgrids, BESS and PCS work together to provide a reliable and stable power supply to a localized area. They integrate renewable energy sources such as solar panels and wind turbines, reducing reliance on fossil fuels.
    4. Off-Grid Homes and Cabins:  BESS and PCS are used to power off-grid homes and cabins, providing a reliable and sustainable energy source for lighting, appliances, and other electrical devices.
    5. Mining and Remote Industrial Sites:  BESS and PCS are used in mining and remote industrial sites to provide a reliable power supply for operations. They help reduce reliance on diesel generators, lowering operating costs and environmental impact.
    6. Emergency and Disaster Response:  BESS and PCS are used in emergency and disaster response scenarios to provide backup power for critical infrastructure such as hospitals, shelters, and communication centers.
    7. Energy Management and Peak Shaving:  BESS and PCS are used for energy management and peak shaving in hybrid systems that combine multiple energy sources. They store excess energy during off-peak hours for use during peak demand periods, reducing energy costs.
    8. Grid Resilience and Stability:  BESS and PCS can improve grid resilience and stability by providing ancillary services such as frequency regulation, voltage support, and reactive power compensation. They help maintain grid stability during fluctuations in supply and demand.

In conclusion, each type of battery energy storage system (BESS) and its associated Power Conversion System (PCS) offers distinct advantages and applications:

  1. On-Grid BESS and PCS:  These systems are vital for enhancing grid stability, supporting renewable energy integration, and providing peak shaving services. The grid-following PCS ensures seamless integration with the grid, enabling the BESS to inject or absorb power as needed.
  2. Off-Grid BESS and PCS:  These systems are ideal for remote areas or as backup power systems. The grid-forming PCS allows the BESS to operate independently of the main grid, providing a reliable power supply without interruption.
  3. Hybrid BESS and PCS:  These systems combine the features of on-grid and off-grid systems, providing flexibility and resilience. The PCS in a hybrid system must be capable of both grid-following and grid-forming operation, working in tandem with a Source Transfer Switch (STS) to enable automatic switching between grid-connected and off-grid modes.

Each type of BESS and PCS serves a unique purpose, ranging from grid stabilization to off-grid power supply.  Understanding the differences between these systems is crucial for selecting the right solution for specific applications, ensuring reliable and efficient operation in diverse energy scenarios.