Electricity + Control August 2016

ENERGY + ENVIROFICIENCY

I n C o n v e r s a t i o n W i t h

The third important imperative driving the implementation of mi- crogrids, according to Duarte, “is to reduce the carbon footprint of electricity generation as a whole”. “The management software allows us to make decisions on a millisecond basis as to how to generate the electricity needed in the cleanest way possible,” he says. Talking about advancing renewable penetration, he says that in spite of the rise in installed renewable capac- ity in South Africa as a result of the REIPPPP, the penetration of renewables in terms of supporting load demand remains low. “Current generation capacity is at around 43 GW and we now have some 3 000 MW of installed renewables. This translates to an installed penetration of around 6.0%,” he says, comparing this to Germany, where up to 78%of daily electricity demand could come from renewables. “But high renewable penetration introduces power supply volatility, which creates difficulties for systemoperators, who need to balance the grid via deflections and stabilisation strategies. “While all renewables are associated with volatility, the battery Components of ABB’s Longmeadowmicrogrid system Four diesel generators of 700 kVA each allow for a maxi- mum generation from fossil fuel of 2,8 MW. Each of these has its own controller allowing them to be brought online individually. “We only ever use two because demand at our Longmeadow facility seldom exceeds 1,0 MW,” says Duarte. “On the PV side, we have a 750 kWp system based on mono-crystalline panels. We use a 630 kW transformation centre to convert the PV power, which matches the maxi- mum capacity we can get out of the cells at any time, due to differently inclined morning/afternoon panels, etc.” The dc power generated from the panels is passed through a single PVS 800 630 kW ABB inverter to generate the ac supply. The battery bank and PowerStore controller is a 1,0 MW system with 380 kWh of energy storage. It uses Samsung Li-ion batteries, which can be discharged down to 20%. “When we model the microgrid, we look at the loading and the biggest impact at any one time and we base the storage needs on minimising this impact. For financial reasons, it is impractical to simply size the storage system to deliver, say, 1,0 MW of power for four hours to cover load shedding. Instead, we use the batteries to give us enough time to allow the diesels to come in and take over generation, approxi- mately 15 to 20 minutes on full load. Ideally, we prefer the batteries to be cycled between 40% and 88% of capacity and we only use excess PV to recharge them,” Duarte reveals.

storage and flywheels embedded in microgrids are an ideal way of managing this. Distributed microgrids, at suburb level for example, can significantly increase overall renewable penetration, while mak- ing the whole system more stable and reliable. Even if hundreds of microgrids are interconnected, each one balances itself, so the grid itself is not destabilised in any way by the variations in renew- able energy generation,” he assures. Modelling local load profiles Another distinguishing feature of the microgrid is that emphasis is placed on modelling the genera- tion needs based on the load profile of the facility or area to be supplied. “There is a concerted and upfront effort to balance the generation/supply equation. It is not just a matter of putting up a PV sys- tem, connecting it to the distribution boards and hoping it will generate as much power as possible,” Duarte argues. ABB also offers power consulting, which results in upfront investment cost savings by ensuring a reliable consumer-oriented systemand power quality. Operational cost savings are also achieved: by optimising network configurations and the intelligent use of mod- ern automation equipment; andmaintenance cost reductions through the implementation of reliability centred maintenance. At the starting point of this offering is a grid study to determine the prevailing load and connecting standards. “If the load turns out to be lower than the generation capacity of the chosen solution, then the initial CAPEX investment will never be used to its potential. Conversely, if the renewable component of a chosen system is too small, then the likely return on investment will also be low, as will the emission reductions. “As part of our grid study, we also determine how to comply with local regulations. Whether in rural Africa or here in Longmeadow, systems must all comply with power quality requirements and safety regulations,” he adds. Adding to this offering is the a steady state analysis – how much power is needed under normal operating condition, which governs the overall capacity (kVA) of the microgrid – and a dynamic analysis model is also needed: “The effect of step loads being introduced, the need for critical loads to retain their supply and the impact of partial supply outages all need to be taken into account,” Duarte explains. “This helps to size the battery store or flywheel capacities, for example. It also helps to identify ways of expanding the system, when the need arises.” Depending on the size of the system and the variety and number of generation sources, the complexity of microgrid increases. To cater for this, visualisation and automatic control functionality has to be introduced – “and this is where ABB really excels,” believes Duarte. From the analyses performed during the consulting phase of a project, ABB is able to make specific recommendations about the

Electricity+Control August ‘16

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