In a Battery Energy Storage System (BESS), thermal management within the Power Conversion System (PCS) is far more than an auxiliary subsystem designed simply to dissipate heat. As power density, switching frequency and the dynamic demands of bidirectional power conversion increase, temperature control becomes a key design consideration, closely linked to electrical architecture, modularity and operational continuity.

This is the context in which the design of the SIEL Lumina PCS 2000 should be understood. The platform features 200 kVA power conversion modules, with up to ten modules per unit, delivering a total rated AC power of 2,000 kVA. The conversion stage uses SiC MOSFETs and diodes in a three-level NPC-1 topology, operating at a switching frequency of 25 kHz. In an architecture of this kind, managing module temperatures is an integral part of power management itself.

From Cooling to Thermal Management
The Lumina PCS features a forced-circulation, closed-loop liquid cooling system using a water-glycol mixture, integrated into the PCS modules. The circuit includes a pumping system and a dry cooler, while temperature and operating conditions are continuously monitored by the control system. The converter also incorporates electronic protection against both overheating and hydraulic circuit faults.

In Lumina, liquid cooling is not merely a design choice aimed at achieving high volumetric power density—approximately 2.8 MW/m³. It also enables the converter to operate without derating at ambient temperatures of up to 50°C, in outdoor installations with an IP54 protection rating. For a BESS, this approach has implications that extend well beyond the PCS itself. The converter must follow variable charge and discharge profiles and respond to grid requirements, while operating conditions can change throughout the day and according to the services being delivered. Thermal management therefore becomes an integral part of the system’ s ability to handle different operating points in a controlled and predictable manner.

Power Management Becomes Thermal Energy Management
The relationship between thermal management and modular architecture is particularly significant. The ten 200 kVA modules do not necessarily need to operate simultaneously. The Master-Slave control logic dynamically determines how many modules are required to meet the requested active and reactive power, keeping the remaining modules available in a standby-ready state.

Electrical power management and thermal management are therefore coordinated within the same system-level control strategy. The converter activates the number of modules required by the current operating point, while ensuring that active modules remain within their designed operating conditions. Module activation and deactivation are managed through hysteresis and power ramps, avoiding abrupt transitions. Periodic master reassignment also helps distribute operating hours across the different modules. For plant operators, this architecture offers a way to meet power demands without treating the PCS as a single, indivisible block. Modularity allows power conversion capacity to adapt dynamically to the BESS operating conditions, while thermal management supports the operation of each individual module. The result is an architecture designed to adapt to the plant’;s actual operating profile, rather than being optimised around a single operating point.

Thermal Management: Performance Optimisation and Easier Maintenance
The converter’s thermal management strategy is designed to optimise conversion efficiency by keeping module operating temperatures as stable as possible. This helps limit thermal stress associated with large temperature gradients, which can occur in converters relying on forced-air cooling. The hydraulic circuit control system dynamically adjusts coolant flow to keep power semiconductors within an optimal operating temperature range for conversion efficiency.

One of the most distinctive features of Lumina is the integration of liquid cooling into a high-power modular converter while retaining the ability to remove individual modules during operation. This is made possible by sophisticated hydraulic components, including no-spill quick-disconnect couplings. These enable maintenance operations, such as module replacement, to be carried out without shutting down the entire machine and without the risk of coolant leakage during disconnection.

An Architectural Function, Not Just a Cooling System
In Lumina, thermal management should therefore be viewed as a fundamental element of the overall PCS design. Liquid cooling, combined with SiC power semiconductors, a three-level conversion topology and 200 kVA modularity, provides an integrated approach to the challenges of power density, control, efficiency and operational continuity.

This is the key point: in a PCS designed for BESS applications, managing heat is not simply about preventing overheating. It is about engineering the system's operating conditions so that power conversion can sustain the performance levels envisaged by the design over time. For plant operators, the value of this approach lies in having a PCS that integrates power conversion, dynamic power management, thermal control and modular maintenance within a single architecture. Thermal management thus becomes more than a means of dissipating heat: it is a design enabler that contributes to system availability, operational flexibility and the effective management of the BESS as a whole.

For more information, please visit: www.sielups.com