DC Coupling - Large Scale BESS

DC Coupling a Battery Energy Storage System (BESS) to Solar - Targeting the Capacity Market in Western Australia

Across our Australian power industry?we’re seeing a significant increase in large capacity BESS projects being connected to the grid, with a multitude of use cases.

AC coupling is by far the predominant means of connecting these systems to the grid for a number of?reasons, with flexibility of operation being front and center.

A high-level synopsis of what we’re looking at:

AC Coupled

Simply put, we use separate inverters and separate switchgear (and sometimes even separate power transformers) for the solar farm and BESS. The coupling is then done on the AC side. Each system can be controlled independently without curtailment, creating substantial flexibility in how the system is operated. For instance, the solar system at full generation has no impact on using the battery system for a number of?use cases (particularly for grid support functions such as peaking, voltage and frequency support).

DC Coupled

Here we’re coupling the solar and batteries on the DC side into a single inverter, with dedicated switchgear and transformer. Key to enabling this type of connection is the DC-DC convertor bringing the batteries into the inverter, and an inverter with the capability to accept this configuration.

?As we’re now effectively constrained in capacity behind the single inverter, we do lose the full flexibility of the BESS for grid support functions.

?However, there are a number of?key benefits, namely:

1. Renewable Energy Smoothing (RES):??Short, rapid changes and fluctuations in PV generation are balanced out by the battery?at the DC source.
2. Ramp Rate Control:?Much smoother / controlled ramp rate (for increasing and decreasing power to the grid).
3. Frequency:?Effectively adding independent frequency support to a PV system (via the stored active power component behind the inverter).
4. Equipment:?Less equipment (no separate inverters, switchgear and transformers needed for the BESS).
5. Losses:?Reduced losses as we’re only using one power circuit for the DC/AC conversion and the connection to the grid.
6. Clipping Capture:?As we’re typically oversizing PV onto the inverters (to maximise energy yield at the earlier and latter parts of the day), the inverter “clips” the excess power to the nameplate capacity of the inverter. This excess energy can now be stored in the BESS, whilst the inverter continues to operate at full capacity.

Clipping Capture is the key enabler for a Capacity Market Business Model at Frontier Energy's Waroona Solar Farm in WA which Incite Energy is assisting with development.

The BESS is able to?remain at capacity with energy from one of the 3 sources:

1. Curtailed PV to Grid,
2. Clipping Capture,
3. Off peak tariff charging from grid.

Economic algorithms are then applied to the Power Plant Controller (PPC) to optimise energy charging and discharging sequences.

Key Technical Challenges experienced:

Components:?Finding suitable system components was challenging as high-capacity DC Coupled configurations are still limited in the OEM market. It was critical that 6 core components had to be fully compatible across both primary (thermal) and secondary (control) systems. These were:

1. ?PV Collectors (Combiners)
2. Battery (incl. the Battery Management System).
3. DC-DC Converter
4. Inverters
5. Power Plant Controller
6. Communications?

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Layouts:?Whilst similar to?an AC coupled BESS, where we’d target minimising the DC cable lengths into the battery inverter, DC coupling requires fully distributing the batteries and DC-DC convertors throughout the cluster arrays of the solar farm itself. A key challenge here was the significant increase of high-capacity DC cabling into the inverter, from the PV Combiner boxes and now the Convertor inputs as well. As this is always a high-risk area due to the concentration of cabling at high operating temperatures, careful attention was required for cable layouts, depths?and routing.

?Control Componentry and Communications:?From the start, this was going to be one of the most critical aspects of the DC coupled system. It not only enables thermal control into the inverter (which is now effectively >2 oversized with DC input), but also forms the backbone of enabling the business case for a capacity market. Robust (and compatible) N-1 communications across all core components had to be allowed for.

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With DC coupling having a more targeted use case that is closely coupled to the energy market, we see a significant growth in this area of renewable energy penetration to the grid.

For standalone PV plants currently being designed, it can certainly pay dividends to future proof the layouts and system configurations by making allowance for DC coupling in the future. A minimum amount of spend can unlock a very cost-effective DC coupled BESS retrofit in future.

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