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WHAT IS A MICRO GRID?

A micro grid is a local energy grid with generating, control, storage and load management capability, which means it can connect and disconnect from the utility grid and operate continuously and autonomously. There are other types of non utility class micro grids as well, we will ignore those for now.

 

HOW DOES A MICRO GRID WORK?

To understand how a micro grid works, we first have to understand how the grid works and what are the issues with it.

The grid connects energy consumers to central power sources, which allow us to use appliances, heating/cooling systems, machines and motors and other electronics. But this interconnectedness means that when part of the grid needs to be repaired, or are damaged, everyone is affected. In the local case, Eskom, because of reasons, are unable to provide a stable grid or supply enough energy when demanded.

This is where a micro grid comes into its own. A micro grid generally operates while connected to the utility grid, but importantly, it can be isolated (islanding) from the utility grid and operate on its own using local energy generation and storage, for reasons.

A micro grid can be powered by distributed generators, batteries, and/or renewable resources like solar or wind or hydro. Depending on how it’s generated and how its requirements are managed, a micro grid might run indefinitely.

 

LEGAL FRAMEWORK

In South Africa, there are almost no micro grids. This is because of the ideology and moronic outlook of the policy makers. As we all know, we have some serious issues with the national generation and distribution of energy in SA, let’s leave it at that.

 

Because the Utility and the policy makers cannot see the forest for the trees, the deck gets stacked against private generation and distribution. No competition allowed, no matter the cost to society, the environment or you. The collectivist centralist ideology and outlook of the state, as well as cronyism and graft is a toxic mix that produce the inevitable failures as we witnessed to date.  

 

More to the point, micro grids in South Africa are severely restricted. You are only allowed to consume energy on the same property where it was generated. You are not allowed wheeling energy at all. Grid feed in may be legally and technically possible, but it is severely dis-incentivised. Firstly by the low tariffs paid (theft if you asked me) and the secondly by some of the ridiculous arguments made as to why it will damage the grid infrastructure. Also tax disincentives are used. Think the proposed tax on SSEGs…. In countless other countries, grid feed in works and is not a problem. The UK is a good example, it may have its own issues yes, but you are incentivised to feed into the grid.

 

This incentive can happen by tax rebates, tariff incentives or by other means. It actually assist in stabilising the grid. Alas here we are.

 

POWER SOURCES

There are a few potential sources available for use to generate the power. Solar PV, wind and hydro being of the more popular type. Most of these sources are intermittent. This poses some challenges.

 

STORAGE

There are two economically viable storage systems, batteries and hot water. There are others. Batteries are the most sensible for the bulk of storage as it can supply loads at any time. Hot water storage makes sense as it is easy to store lots of energy for later use as hot water.

 

DISTRIBUTION

Buildings are already wired and loads can accept electrical power from the grid. Micro grids can plug into the utility and/or building so can support the same loads. We are really concerned with consuming the energy generated locally so we disregard the grid for now.

 

EVENTUALLY, MICRO GRIDS

A multitude of Micro grids can strengthen utility class grid resilience and help mitigate grid disturbances as well as function as a grid resource for faster system response and recovery. Some micro grids ma do the opposite. Unlike grid tied systems with no storage, when micro grids are allow to feed in to the utility grid, they can support utility grids… take that….

 

Micro grids are an excellent way to ensure resiliency and energy independence for their operators.  Micro grids are also ideally suited to critical infrastructures. Generator sets, gas or petroleum fuelled, can support micro grids and further bolster resiliency. Black start capability is vital for micro grids. This ensures that depleted storage systems will not result in a deadlock preventing startup of the system. A micro grid consists of a battery backed inverter charger, similar to an off grid system or hybrid system. To this we add a String inverter, same as for a grid tie system. 

 

So what you ask is deadlock then? Glad to explain. Picture this:

image.thumb.png.e1e7b33c80fb81389c022b930f92e587.png

The inverter charger above in the middle, consume energy from the battery to generate a grid locally at 50Hz. If it is tied to the utility grid, it will synchronise with the utility grid and switch to the utility to support the loads. When the utility fails, it can normally function as a UPS and switch over to its own local grid and continue to function as normal. This transition may be seamless or not, it depends on inverter architecture and configuration.

 

To the left of that are the Solar panels with the String inverter. Take note that this is not fed by the utility grid directly, but via the inverter charger. When the utility grid fails, the string inverter will continue to work as its supplied by the inverter charger’s grid. This is really where the magic happens.

 

Now under utility grid failure and continued load support from batteries, we deplete the storage system. If we rely solely on the utility grid and solar to keep the batteries charged, we may encounter issues. When the batteries eventually runs out of energy due to no charging, the inverter charger will shut down. This in turn will trigger the string inverter to also shut down as its grid dissapeared. Since the string inverter depends on a grid to function, it will cause a deadlock when the Solar PV system cannot start because the batteries are drained and the inverter charger cannot be powered on, so no grid and no charging of batteries. In this scenario, we have to wait for the utility to return. Our energy independence is non existent. 

 

Solving the deadlock problem can be easily done. Use a generator to supply the grid to the inverter charger and the system will start, called black start capability. Eskom suffers the same issues and there are only a few plants that have this black start capability. We do not know if the procedures are in place, hence the dread inside the utility to loose the grid, and the rolling blackouts.

 

The generator solution is however not ideal. It is mostly manual, even if you have an automatic start generator, and most small systems are manual start anyway. A better solution is to provide automatic black start capabilities by adding a Solar PV charge controller and a few panels to directly charge the batteries. In the above picture, it’s the Solar PV system on the right with the charge controller. This system will start charging as soon as we have Solar available.

 

A combination of Solar charge controller and generator is the most robust solution as even during prolonged dark days because of the weather, we can keep the batteries charged, irrespective of utility grid. Energy independence gained, complete.

 

Micro grids can be very efficient as most energy from the string inverter are directly converted and used by the loads. Batteries are always charged by the charge controller and the energy is produces as a DC voltage/current and used to charge the DC battery. The excess string inverter energy can also be used, with less efficiency,  to charge the batteries as well. These are called AC and DC coupling. Either may be used independently, or in combination.

 

SUMMARY

This is a serious simplification of a micro grid. The micro grids we are discussing here have a single source and are owned by a single person or business. There are numerous items we have not looked at. Most of the control and monitoring of micro grids for our application are build into the equipment. They take care of Voltage issues like droop due to load, frequency drift are compensated for and can be manipulated to control other equipment. These micro grids are distinct from other types such as distributed micro grids where there are more than one source and many different consumers.

 

Micro grids are always a custom configuration and or design for each user or community

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