Understanding the Roles of PCS in BESS

Battery energy storage system (BESS) plays an important role in the power generation process. The overall performance of BESS is decided by a combination of several key components: Batteries, Battery Management System (BMS), Power Conversion System (PCS), Energy Management System (EMS) and Fire Suppression System. PCS as a core device connects batteries and the power grid or the load, and controls battery charging and discharging. So, it’s essential to understand the specific roles of PCS before pairing it with the BESS.

PCS is a four-quadrant operating converter with controllable AC and DC sides, enabling bidirectional AC/DC conversion of electrical energy. It bridges the smooth bidirectional energy flow between the batteries and the power grid. The primary role of PCS is to?convert the DC power generated or stored in the batteries into AC power that can be fed into the grid when discharging; while during charging, it converts AC power into DC for storage in the batteries.

In principle, PCS enables the charge of batteries when there is surplus generation in low load hours by converting the AC power to DC, as well as the discharge of stored energy to the grid or load if needed by converting the DC power to AC. In this approach, it aims to achieve efficient power conversion and smooth bidirectional energy flow in various on-grid, off-grid as well as hybrid applications.

PCS can operate in three working modes, namely off-grid mode, on-grid mode and hybrid mode. PCS in on-grid BESS is mostly grid-following,?allowing the system to seamlessly “inject” or “absorb” power without causing disruptions to the grid. Typical on-grid battery energy storage applications include voltage synchronization, frequency synchronization, and active and reactive power control. On-grid BESS is designed to operate in parallel with the grid, enhancing grid stability plus supporting renewable energy integration and peak shaving. However, off-grid BESS operates independently from the main power grid and is commonly deployed in remote or isolated areas or as the backup power. The PCS in off-grid BESS is designed to be capable of grid forming, voltage and frequency regulation, load balancing and energy management, remote communication and control, as well as necessary protection and safety precautions. PCS in hybrid systems is a combination of on-grid and off-grid modes, and capable of both grid following and grid forming. It requires advanced control algorithms to ensure seamless transition between the two and needs to interact with the STS (Static Transfer Switch), ensuring power supply continuity during transitions.

What are the prior factors to evaluate a PCS? First of all, power conversion efficiency is a crucial factor to decide the operating cost of the BESS. Higher energy efficiency definitely contributes to higher economic efficiency in BESS operation. Secondly, precise power control allows the BESS to quickly respond to the grid demand, significantly improving grid stability and reliability. Thirdly, intelligent energy control helps optimize energy consumption and energy dispatch based on real-time grid and battery status. As importantly, modular PCS design facilitates flexible system configuration and easy scalability, individually tailored for different applications and practical needs.

PCS accounts for 5%-20% of the BESS cost. There’re two common types of PCS: centralized PCS and string PCS by the system topology. The latter is smaller in size, with lower conversion rate but higher power density. Centralized PCS has comparably higher conversion rate and larger size. Both of them can be used in residential (2-15kW PCS), commercial & industrial (30-100kW PCS) and utility energy storage applications (>186kW PCS). However, centralized PCS can reach 1250kW-2000kW in power and it’s more intended for utility scale energy storage systems. By cooling design, it can be broadly divided into two categories: traditional fan cooled PCS and the next-gen liquid cooled PCS.

Grid forming, large capacity, and liquid cooling are currently three major technical trends in PCS development. As BESS energy density continues to rise from kWh to MWh and even GWh, PCS power capacity is also growing to simplify system integration and saving footprint.?Liquid cooled PCS features 0 noise, high reliability and efficiency, and therefore often pairs with liquid cooled batteries to achieve superior reliability and durability when being deployed in various challenging outdoor environments with high altitude, high temperature and high levels of humidity, salt fog or dust. In addition, the grid-forming ESS technology addresses the safety and stability issues. It enables rapid frequency and voltage regulation, increase inertia and short-circuit capacity, and suppresses wide-frequency oscillations, which plays a pivotal role in grid stability and reliability. By harnessing the stability and flexibility of BESS, the grid-forming solution offers a pathway to a more sustainable and reliable energy future.

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