AC Coupling in Utility-Interactive and Stand-Alone Applications: Page 11 of 16

Multicluster Box. For large ac-coupled stand-alone systems, SMA manufactures the SMA Multicluster Box (MCB), an offgrid ac-distribution hub that combines and manages a variety of ac-generation sources in large-scale Sunny Island multicluster systems. While a Sunny Island inverter cluster can be used in grid-tied battery-backup applications, the MCB was developed for standalone 3-phase 208 Vac systems only. A preconfigured ac-distribution board in the MCB allows you to easily connect all the ac components in the stand-alone grid, including batteries, ac generator and renewable energy sources, loads and Sunny Island inverters.

Smartformer. SMA will soon be releasing the recently ULlisted Smartformer, a 120/240 Vac autoformer designed for systems that utilize a single Sunny Island inverter in conjunction with a single Sunny Boy inverter. The autoformer provides stepup and step-down options to supply loads with 120 Vac and 240 Vac and allows coupling of a Sunny Boy inverter with a single Sunny Island inverter.

Developed for off-grid village electrification projects, SMA’s multicluster system platform (see Figure 5, below) integrates Sunny Island inverters in groups of three, referred to as clusters. The inverters in each cluster share a dedicated battery bank and are configured for 3-phase ac output. Two, three or four 3-phase clusters, each consisting of three Sunny Island inverter/ chargers, can be connected in parallel via the MCB for system inverter capacities of up to 110 kWac. Additional clusters can be connected to the MCB’s distribution board at any time, enabling expansion of renewable energy generation capacity. Limiting factors of the multicluster system are the number of inverter connections possible on each MCB ac-distribution board and the ampacity of their switching devices. With the exception of the master cluster, maintenance or replacement of individual inverters can take place during system operation—the only operational difference during system maintenance is that the total output of the system is correspondingly lower.

In ac-coupled multicluster systems, SMA Sunny Boy inverters communicate with Sunny Island battery-based inverters via a shielded RS485 communication cable. When the batteries are completely charged and the ac-load demand is low, the Sunny Island uses frequency-shift power control to gently throttle down the power output of the grid-direct SMA inverters. These frequency shifts prevent battery overcharging and are independent of any other communication functions between the inverters. Unlike the operation of ac-coupled solutions from other manufacturers that use frequency shift to completely drop ac-coupled grid-direct inverters off-line, SMA’s fully integrated approach allows the ac-coupled string inverters to remain online during frequency shift while operating at lower power levels (see Figure 6).

Most ac-coupled systems that utilize SMA’s battery-based and grid-direct inverter products do not require any additional components such as relays and remote disconnects to manage power production when ac production exceeds the on-site load. In addition, if the battery reaches a preset low state-of-charge threshold, a load-shedding contactor can be programmed to open to prevent an over-discharge of the battery, which could cause the individual cluster to disconnect from the system. With a portion of the loads off-line, the ac-coupled charging sources continue to charge the battery bank. When a sufficient battery state of charge is achieved, all loads are automatically reconnected to the ac-distribution system.

The MCB includes several heavy-duty contactors. Each generator contactor connects grid-forming generators, such as those used in high-power diesel generating plants. When the generator voltage and frequency parameters are within limits, the Sunny Island synchronizes with the generator voltage and frequency reference and uses both generator power and ac renewable energy sources to either supplement loads or charge the battery banks. If an inverter cluster fails or is switched off, the generator contact automatically closes and the generator is directly connected to the loads. If the generator fails, the system quickly disconnects it and maintains power to the loads via the batteries and the available renewable energy sources. This system redundancy ensures that power is still available for the ac loads, even when one component fails.

An example of an off-grid, ac-coupled multicluster power supply system is on Scotland’s Isle of Eigg. The island is part of the Scottish Inner Hebrides chain and is approximately 12 square miles with a population of 90. Due to high costs, the island has not yet been connected to the mainland’s powerdistribution grid, which is about 10 miles away. Until 2008, diesel generators supplied the island with electricity and the entire ac-distribution network had to be taken off-line when a generator required maintenance.

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