The Tesla Microgrid solution to power Jouberton Community Health Centre

Jobourton’s Community Health Care Centre’s low voltage distribution boards are currently being built by Switchboard Manufacturers JHB. The facility will use a Tesla Microgrid to supply the facility with a reliable source of power if the grid supply were to fail.

The Tesla Microgrid turnkey solution has been used in many different environments around the world to supply reliable off-grid power to a vast array of different facilities.

[A typical PV array.]

What is the Tesla Microgrid?

The Tesla Microgrid is a turnkey solution provided by Tesla to enable a facility the capability of creating its own independent power grid, called a microgrid. The microgrid provides power with the use of a diesel generator, PV arrays and Tesla’s own Powerpacks which operate as battery supplies. These systems provide reliable power solutions to facilities which either have limited or no access to grid based power supplies.

The Tesla microgrid achieves this with the use of a set of controllers which monitor the power quality of the grid supply, generator, PV array and Powerpack. When one or more of these supplies aren’t outputing the desired power, the other supplies take over to support the facility. These controllers thus ensure that there is a continuous supply of power even when grid based power fails.

[Block diagram representing the interactions between the Microgrid controllers and the Distribution board. Image created by Author]

The Proposed System

The proposed system to be built can be seen in the above image. A 200kVA diesel generator, a PV array of 28 panels and a Tesla PowerPack system will make up the supplies to the microgrid. The control system consists of the Islanding Controller, Microgrid Controller and the Site Master Controller. These controllers ensure normal operation of the microgrid.

The flow of control in the Microgrid

As can be seen in the block diagram above, the microgrid control is a fairly complex system. The islanding controller is responsible for cutting the grid supply when it detects that the grid supply is under-performing. The Islanding Controller establishes the microgrid by opening the Islanding MCCB. By opening this MCCB the facility is completely cut off from the grid supply. To re-establish the grid supply the Islanding Controller needs to first ensure that the microgrid voltage is synchronised with the grid voltage before engaging the Islanding MCCB. Equipment damage could incur if the voltages are not synchronised.

The Microgrid controller controls the generator and PV inverters. Thus, as the name suggests, the Microgrid Controller ensures that the microgrid is operating optimally. The Site Master Controller controls the entire power system and processes input from the Microgrid and Islanding controllers. The Site Master Controller also controls the Powerpack inverters and thus engages the battery supply when need be. The Site Master Controller also thus ensures that all the inverter outputs are synchronised with each other and the grid supply and that the power produced is of optimal quality.

The Distribution Board

The main distribution board shown in the diagrams above is similar in working to a solar distribution board, as the flow of power is in reverse to a normal distribution board. This is an important factor to note as certain circuit breakers and current transformers (CT’s) are direction sensitive. The distribution board takes the various power sources feeding into it and then feeds them all onto a single bus-bar. The bus-bar then feeds into eighteen smaller distribution boards which serve the various buildings of the Health Centre. The Main distribution board and the smaller distribution boards will all be made by Switchboard Manufacturers. The other systems in place are either provided by Tesla or by the supplier of the PV contractor.

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.

jannes9000@gmail.com

How do Solar Distribution Boards Differ from Regular DB’s?

With the steady decline in solar panel prices it has become an attractive option for business owners to install solar panel power systems to ensure a stable power supply. Solar distribution boards, sometimes called combiner boards, form an integral part of this solar supply system. The working of Solar distribution boards and the differences between regular distribution boards are discussed further below.

The basic construction of a distribution board

The basic distribution board (DB) consists of a steel enclosure which contains circuit breakers connected to DIN rails or bus-bars. This allows for the distribution of power within an electrical power system. In a normal distribution board the system is fed from a main circuit breaker (CB) which in turn feeds other smaller circuit breakers which control their respective load circuits. In the below image the main circuit breaker feeds the smaller line circuit breakers in a hierarchical fashion, this is a good example of a traditional distribution board.

[Diagram of a typical distribution board. Image created by Author.]

The basic working of Photovoltaic panels

Photovoltaic (PV) panels or solar panels, as they are commonly known, generate electrical power when exposed to sunlight. A group of solar panels are usually used to produce power and are arranged in a PV array. The PV array outputs a DC voltage and current corresponding to the amount of solar radiation which reaches the PV panels. This voltage depends on the arrangement of the panels, but it is usually 12V. This DC power is then converted to AC power with the use of a DC to AC inverter to produce 220VAC. The inverter then feeds this power to the solar distribution board. It is sometimes necessary to implement more than one PV array. Fortunately the solar DB can accept more than one inverter output.

As one would expect the power produced by the PV array is highly sensitive to changing weather conditions. This is why electric power systems utilizing PV power usually rely on grid based supplies as well. The power system thus relies on a dual power supply. These two power supplies need to be synchronised in order to protect the electric circuitry from damage due to mismatched voltages. This is done by synchronising the inverter output voltage to the grid voltage. The PV array can thus not completely supply an entire business without the support of grid based power, but it greatly reduces the amount of grid based power the business uses. Your kWh meter will thus have a much lower reading at the end of the month.

For an entirely off-grid solution it is wise to invest in generators and backup battery supplies to ensure that a continuous and steady supply of power is available.

Why is a Solar Distribution board upside-down?

In a solar distribution board the flow of power is sometimes referred to being ‘upside-down’. This is due to the fact that the feed of power is in the reverse direction compared to regular distribution boards. Multiple smaller lines of power feed into a bigger line of power. The smaller lines come from the PV array inverters. Thus in the solar distribution boards a collection of smaller circuit breakers feed into a large main circuit breaker, which feeds a load.

The direction of power flow is important as this determines the direction in which measurement devices such as current transformers (CT) and energy meters should be placed. In the solar distribution board the direction should thus be reversed. Certain circuit breakers are also direction specific and should thus also be installed in the correct direction of current flow.

The below image displays the flow from the small CB’s into the main CB. The solar distribution board can then feed into another DB which has feeds from a grid supply or other sources of power.

 

[A schematic representation of a Solar Distribution Board working in conjunction with utility power. Image created by Author.]

The solar distribution board is thus used to distribute power from separate PV arrays into a single main line which supplies a load.

For more information on Distribution Boards, visit the Switchman Website or email joshb@switchman.com .

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.

jannes9000@gmail.com