Grid interactive data centres – an emerging flexibility option

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Data centres with grid interactive uninterruptable power supply (UPS) offer the potential to support system stability and sector decarbonisation.

Data centres, and for that matter Bitcoin mining operations, are major growing loads on electricity grids. With their use of energy storage as backup in UPS systems, they also are a source of major storage growth opening the way to support the grid and grow the integration of renewables while providing an ROI for the owner.

A new white paper from Eaton and Microsoft points to the potential of modern static UPSs with bidirectional converters and built-in control features in providing the speed of response to tackle the fast frequency variations in the grid – unlike the standards UPSs that are a feature with backup generators in typical data centres.

With correct control algorithms and the capabilities of modern UPS technology, the use of energy from batteries can be seamlessly controlled while remaining connected to the power grid, the white paper states.

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There’s no need to disconnect the grid from a UPS to provide a demand response; this can be done by controlling battery discharge (or charging) in parallel with the grid.

With a bidirectional rectifier, power can be fed to the grid by operating in regenerative mode. The response given can be controlled and is independent of the UPS load level. The energy management of the UPS and battery can respond to the active power setpoint, calculated either by a secondary controller or built-in controller in the UPS, to respond to the frequency deviation.

The white paper cites two case study examples. One is an Eaton UPS in Dublin, Ireland, which is used to provide fast frequency response to the transmission system operator Eirgrid as part of aggregator Enel X’s virtual power plant.

The second is a UPS with a grid-interactive function for dynamic frequency regulation, which has been field-tested for one of Microsoft’s eastern US data centres in collaboration with the regional transmission organization PJM.

UPS reliability

The white paper notes that using a UPS for a secondary application such as frequency regulation or demand response raises reliability concerns. However, these can be mitigated by the UPS technology with the correct control methods and control algorithms.

For example, when using their batteries in parallel with the power grid, UPSs can revert full power back to the rectifier, or batteries, should the other circuit fail. Equally, a UPS can estimate the remaining energy in batteries and allocate enough capacity for the critical load, preventing the batteries from being discharged beyond defined limits.

The other key issue is battery degradation. In some markets, such as in Ireland and the Nordics, grid interactive operation can have a minimal impact on battery degradation due to the operation plan and payment method, while in electricity markets such as PJM in the US or in continental Europe, the battery degeneration has a significant impact on the total cost of ownership. Battery degradation modelling is challenging and requires detailed development and testing.

The battery technology is another issue and Li-ion is established to advance UPS technology, but there are many more nascent chemistries under evaluation.

A key element for the battery is to understand the market requirements, such as activation frequency, the local frequency quality indicating how often the battery is used and what the depth of discharge is, the white paper states.