What is the Purpose of Energy Meters in an Electrical System?

Energy metering in electrical systems utilise devices that record the electrical energy load in units of Kilo-Watt hour (kWh). Power utilities use this information to bill electricity users.

[Smart power meter from Schneider Electric]

The principals behind Energy Meters

Energy meters generally measure load over a set period of time. An energy meter thus generally records the instantaneous power consumption of a load and then integrates the result over a predetermined time frame. The energy is then displayed in kWh units. In Mathematical format, the power of the load is depicted as follows:

For single phase systems:

For three phase systems:

Where θ is the phase angle of the load

The energy is then:

The instantaneous voltage and current measurements are used to determine the power usage of a load at any given time.

Types of Energy Meters

There are various different forms of energy meters, from Electro-mechanical Induction type meters to Electronic energy meters. The inner working of these meters is generally the same as most systems require some sort of power measurement, power recording and power integration over time.

The main difference between meter types is the method used to measure current. The current can either be measured directly (Direct Reading) or by the use of a current transformers (CT’s). In the direct measurement methodthe current supplied to a load is directly measured by either mechanical or electronic methods. In the case of a current transformer the current carrying conductor is placed in the center of a current transformer. A proportionally smaller current is then produced in the secondary winding of the transformer which is proportionally smaller to the primary current. The size of the secondary current is dependent of the turns-ratio of the transformer. The secondary current is then recorded. This allows for the measurement of very large currents while not subjecting the meter to them.

[A schematic representation of a current transformer. Image created by Author.]

Why use a Current Transformer?

The current transformer might seem unnecessary at first, due to the transformer adding complexity to the system. The CT is however vital in some energy metering systems.  High currents could damage direct measurement instruments of a similar size and therefore CT’s are  used in energy meters rated at 150A and higher.

The difference between Building Management Systems (BMS) and Energy Management Systems (EMS) in energy metering.

Let’s first define these two concepts. A BMS is usually a hierarchic control scheme where a central server or computer gathers data about various processes within a building. This ranges from energy management to HVAC processes and uses proprietary software to do so. An EMS will usually  monitor the energy usage of a building or set of buildings using energy metering or sub-metering. The EMS is also controlled by a server or computer.

There is usually a misconception that a BMS will provide the same capabilities as an EMS when managing the energy consumption of a building. This is however not true as the two systems serve different goals and should be used to complement each other. The BMS is normally used as a real time data collection service which helps to manage the instantaneous requirements of a building. The BMS is also usually used as a means to balance the real time state of a system.

The EMS however, helps one to identify the long term energy requirements of a building as well as predict future load profiles. The EMS is thus a better solution when one wants to optimise the energy use and cost of a building over a long period of time. The EMS thus gives a better overview of the energy requirements of a system.

The emergence of smart energy meters which save data either to non-volatile memory or cloud based computer servers have greatly enhanced the use of modern day energy and building management systems. These energy monitors can be readily installed in most buildings, allowing the relatively fast setup of an EMS or BMS.

For energy meter price please visit our meter section here.

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

jannes9000@gmail.com

What is the Difference between a Pure-Sine wave and a Modified-Sine wave with respect to Inverters?  

In this regard, a sine wave or sinusoidal wave is a voltage supply in the form of AC (Alternating Current). In South Africa, a household plug socket supplies a voltage of ±230 Volts AC. Where ‘230’ is the amplitude of the wave.

Figure 1 below represents what a sine wave looks like measured over a period of time.Figure 1: 230V Sine wave.

As depicted in figure 1, the amplitude alternates between +230V and -230V. The frequency of alternation of the supply in South Africa is 50 Hertz, meaning 50 times per second.

How is the voltage from the battery converted to AC?

The voltage supply from a battery connected to a general household inverter will be in the range of 12-24 Volts. However, this voltage is DC (Direct Current) and must be converted to AC in order to run household appliances, hence and Inverter is required.

Figure 2 (a) depicts a general DC voltage of 24 Volts. (b) Follows a square wave created from the DC voltage that switches from positive to negative at specific time intervals.  (c) The square wave is then transformed into a modified sine wave.

Figure 2: a) 24V DC, b) 24V square wave, c) 24V- 3 step modified sine wave.

 

The inverter then amplifies the modified sine wave to 230V allowing compatible home appliances to be powered directly from the inverter.

What is the difference between a Modified Sine Wave and a Pure Sine Wave?

As shown in figure 1, a sine wave is a smooth alternating voltage. Figure 1 in essence is a pure sine wave. But when dealing with inverters the sine waves are not generated by a motor/generator/turbine but rather a DC voltage is modified electronically to produce a sine wave as in figure 2. Therefore manufacturers will refer to the inverters output voltage as a “Modified Sine Wave”. This is more of an estimation of what a sine wave should be.

However, some manufacturers distinguish their inverters that produce much less distortion than the wave shown in figure 2 (c) by calling it a ‘Pure sine wave inverter’. I.e. more steps are used between the positive and negative alternation. The more steps used, the smoother the signal and the more ‘pure’ the wave appears to be. However the more steps involved, the more complex the inverter will be and hence more expensive.

Figure 3 shows a comparison of a sine wave , a simple modified sine wave and a pure sine wave that some complex inverters are capable of.

Figure 3: Overlay of a Sine wave, Modified Sine wave and Pure Sine wave.

For general household appliances such as a TV, PC, lights, etc… a modified sine wave is perfectly suitable.

What devices will not operate on a Modified Sine Wave?

Any appliances resistive in nature (e.g light bulb) or use a switch mode power supply (e.g motor) will run on a modified sine wave.

Devices with a built in clock that derive their time from the incoming frequencies may not function correctly. The function that provides the switching can be triggered at the steps instead of at the zero crossover point. This applies to biomedical devices that are used to monitor heart rate and oxygen cylinders.

Some cheaply made power supplies may also not function correctly. This will present itself in the charger heating up beyond its normal operating temperature.

A device called a line conditioner can be used between the output of the inverter and the non-operational device if needed. A line conditioner or power conditioner provides a “clean” AC power to sensitive electrical equipment [2]. This is an additional device that is separate from the inverter. Currently Switchman Products does not supply line conditioners.

For more information on Inverters and change over panels, please contact joshb@switchman.com

What is the Purpose of the Home/Office Inverter System? 720W / 1440W

INVERTER SUMMARY

  • Long run battery backup to run computers, TV, DSTV, lights, security and garage doors for 3 to 8 hours.
  • Suitable for home and office.
  • Plugs directly in to a wall plug – just plug your appliances in to the Inverter for uninterrupted power.
  • Automatic recharging when Eskom power is restored.
  • Add an additional battery to double the battery time.
  • Different power ratings available depending on your requirements. From 1 KVA to 2.4 KVA
  • Standard plug and play Inverters come complete with batteries.

1000­­ VA / 600 Watt Inverter with 1 battery will power a TV, DSTV, Computer a few lamps and cell phone charger for up to 4 hours.

2400 VA / 1400 Watt with 2 batteries will power 3 or 4 TV’s or 3 or 4 computers, DSTV a few lamps, cell phone chargers and an Internet router for up to 4 hours.

These units can also be used to power intercoms, gate motors and garage doors. The system can also provide a backup supply during extended outages for vital equipment such as alarm systems and security cameras. Figure 1 depicts some of the appliances that are/aren’t compatible with an Inverter.

Figure 1: Reference for some of the appliances that can/cannot be used with an Inverter. 

HOW DO THESE INVERTERS WORK?

The unit is permanently connected to a wall plug so that while mains power is present the built-in battery charger recharges the batteries and keeps them fully charged until a power failure occurs. The equipment requiring backup is also permanently connected to the Inverter via the connector provided. In case of a power failure the backup system automatically switches over to the Inverter, which will continue to provide power to the equipment. This is extremely fast and standard equipment like TVs, DSTV decoders, fans, routers etc. are unaffected. The systems can also backup computers but it is possible that a very small percentage of computers could reset during the switchover time.

When mains power returns, the whole procedure is reversed and the unit will switch back to mains power and will automatically start re-charging the batteries. Your equipment remains connected to the system even when power is restored. The whole process is fully automated.

HOW LONG WILL THE INVERTER SYSTEM RUN WHEN THERE IS A POWER FAILURE?

The amount of backup time is determined by the size of the connected battery bank and the load on the system. By removing half of the load, the backup time can be extended to double. For example, if the system is setup to power two computers for 4 hours, by removing one computer the backup time can be extended up to 8 hours. For added functionality, the system includes an LCD display that provides visual feedback of battery charge and load allowing the user to monitor usage without concern.

HOW LONG DOES IT TAKE TO CHARGE THE BATTERIES WHEN MAINS POWER RETURNS?

The recharge time of the batteries depends on how much power was drawn from the batteries during a power failure. The recharge times can take up to 8 hours from a fully discharged battery.

HOW TO USE THE INVERTER WISELY

In order to get the best value out of the inverter all non-essential appliances should be switched off. This will reduce the load on the battery and extend its running time. Always keep the inverter plugged into the wall socket to allow for automatic recharge when the mains power returns.

For more information and pricing on Inverters please contact joshb@switchman.com or visit www.switchmanproducts.co.za

What Exactly is a Current Transformer?

The basic principals and uses of Current Transformers

A current transformer (CT) is a device which falls under the broad category of instrumental transformers.  These transformers are ordinarily used to measure the current flowing through a load supplying conductor.

The working of Current Transformers

Current transformers, just like normal transformers, have a primary and a secondary winding. The primary winding, supplying a load, is normally a current carrying conductor consisting of one or two turns either wound around or passing through a hollow core. The secondary winding of the transformer consists of many turns supplying a small load or meter. The current produced in the secondary is thus directly proportional to the current flowing through the primary winding. This makes the device ideally suited to measure currents within a conductor while not subjecting the device to the load current.

[A schematic illustration of a Current Transformer. Image create by Author]

The current flowing in the secondary winding of a current transformer is usually specified and thus has a rated value. The magnitude of the current is normally 1 A or 5 A. The current flowing in the primary winding of the transformer is also normally rated at a set value. The current rating of the current transformer is given as: Ip/Is. Thus for a current transformer which carries a current of 100 A in the primary winding and a current of 5 A in the secondary winding will have this current rating (or CT Ratio): 100/5.

The current transformer thus has a set primary and secondary current rating and thus should have a set turns-ratio (N). The turns-ratio of an ideal transformer is defined by the following definition:

From the given definition it is clear that if the primary winding only consists of one turn that the secondary current will be inversely proportional to the number of secondary winding.

Example:

If we have a 100/5 current transformer we have:

Thus if NP is one turn, then the secondary winding needs 20 turns when the we solve for NS .

The direction and orientation of the secondary winding with regard to the primary will affect the direction of flow of the secondary current. It is thus of great importance that the CT be installed in the correct orientation as to prevent unintended damage to the CT or metering device.

The transformer secondary winding should never be open-circuited as a very large voltage will be developed across the secondary terminals which could electrocute personnel or damage equipment.

 

The Burden of a Current Transformer

The current transformer normally has a rating called its burden. The burden of a CT is the maximum load the CT can operate under without being damaged or losing accuracy. The burden is normally given as a value in ohm or VA.

Accuracy of a Current Transformer

The real current transformer does contain non-idealities just like its normal counterpart. These non-idealities thus affect the accuracy of the CT. The accuracy of the CT is normally given as a percentage and CT’s are divided into accuracy classes. Both the Accuracy and class of the CT should be displayed on its nameplate.

The Types of Current Transformers

Current transformers can in all shapes in sizes. The three main types of current transformers and their uses are listed below.

  • Bar-type Current Transformer: The bar-type CT is a type of current transformer where the primary winding is a bus bar which is directly connected in series with the current conducting element which is supplying a load. The bar-type CT thus only has one turn on the primary side as there is only one bus bar positioned inside the core of the CT. These transformers have high levels of insulation as they are directly bolted onto the current carrying element. Bar-type CT’s thus have fairly high current ratings.
  • Window-type Current Transformer: The window-type CT mainly consists of the core and a secondary winding without a primary conductor. The primary conductor supplying a load thus needs to be disconnected when installing the window-type CT and placed through the CT. This CT also has a lower voltage rating as compared to the bar-type CT. The primary winding thus needs to be highly insulated to avoid damaging the device. The construction of the window-type CT thus also allows the addition of additional turns to the primary winding if needed.
  • Wound-type Current Transformers: The wound-type CT’s have a primary and secondary winding wound a toroidal structure, similar to the construction of a traditional transformer. These CT’s are really used as they operate at much lower currents and ratings when compared to normal CT’s.

Switchman-Products offers a full range of Current Transformers, visit the website here for more details.

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

jannes9000@gmail.com

 

Why are Surge Arrestors so important?

The use of Surge Arrestors in electrical power systems

Surge Arrestors fulfill important roles in industry and even residential areas. These devices protect voltage sensitive equipment connected to power lines from voltage surges induced by lightning strikes or switching within a power line.

How a Surge Arrestor works

The surge arrestor basically acts as a voltage controlled resistor. This means it changes its resistance according to the voltage which is developed across it. The surge arrestor contains a Metal Oxide Varistor (MOV) disk. This disk is a semi-conductor.  The MOV allows the arrestor to have an extremely high resistance at low voltages. When the voltage across the device rises to dangerous levels, the resistance of the MOV disk rapidly drops. The arrestor thus acts as a short circuit when high voltages are developed across it. This allows the arrestor to divert high voltage surges to ground protecting other devices connected to the power line.

Due to the working of the device, it needs to be connected in parallel to all the equipment which needs to be protected. The device also has low power requirements when operating under normal conditions due to the high resistance of the device under these conditions. The surge arrestor will thus ensure that the voltage of equipment is kept at a set value and does not rise above this specified value. This is known as voltage clamping.

[The internals of a Surge Arrestor with the MOV disks visible]

I have a Lightning Rod, why do I need a Surge Arrestor?

A common misconception is that lightning rods can fully protect all devices connected to a power line. This is however not true. Lightning rods merely divert lightning away from power lines. If the lightning strikes the power line directly, the lightning rod is powerless to stop the developed voltage surge from damaging the line and equipment. Also a lightning rod can’t protect power lines from voltages surges produced during switching within a line.

This is where a surge arrestor is necessary. The surge arrestor will nullify any voltage surges within the power line. It is thus recommended that a surge arrestor and lighting rods are installed to ensure full protection of electrical equipment and power lines.

Surge Arrestor Selection and Installation

When selecting a surge arrestor, it is very important that one keeps in mind the rated voltage of the system which needs to be protected. This is important as different surge arrestors have different rated voltages at which they operate. This is due to the fact that surge arrestors are rated to clamp circuits at a certain voltage and can only protect circuits up until a maximum allowable voltage. Surge arrestors are also rated to divert a set amount of energy. If arrestors divert more energy than this maximum energy, they will be damaged. Before installation every surge arrestor needs to be carefully chosen to ensure complete over-voltage protection.

When you are ready to install your chosen arrestor, it is also important to keep all the connection lines to the arrestor as short as possible. This is due to the fact that during an over-voltage situation long lines leading to the device can actually amplify the voltages developed across the equipment and this lessens the efficiency of the surge arrestor.

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

jannes9000@gmail.com

Switchboard Manufacturers Test Assembly to SANS 61439-1&2

Due to increased inquiries and requests from customers in the electrical industry, it has become advantageous to transition from the current SANS 60439-1 to the new SANS 61439-1&2.

Switchboard Manufacturers KZN have tested their upgraded assembly system at the South African Bureau Of Standards (SABS) NETFA Laboratory in Bronkhorstspruit. 

Shane O’Reilly and Andrew MCarthy lead the team that designed and developed the assembly that passed the tests with ease.
The tests were completed under the auspices of the Short-Circuit Laboratory Manager, Seth Mnisi.
The following tests were completed:
1) Strength of Materials-10.2
  • Resistance to corrosion.
  • Thermal stability and resistance to abnormal heat and fire of insulting materials.
  • resistance to ultra-violet (UV) radiation.
  • resistance to mechanical impact.
  • durability of marking.
  • lifting and transport.

2) Degree of Protection of Enclosures- 10.3 

Validate protection against direct contact with live parts, as well as protection against ingress of solid foreign objects and liquids, in accordance with IEC 60529.

3) Clearances and Creepage Distances – 10.4 

Verify that the clearance and creepage distance enable the assembly to withstand the following:

  • Exceptional, transient overvoltage (lighting, HV operations),
  • Operating voltage and temporary overvoltage.

4) Protection against electrical shock and integrity of protective circuits- 10.5

Verify that:

  • The effective continuity between the exposed conductive parts of the assembly and the protective circuit.
  • The short-circuit withstand strength of protective circuit.

5) Incorporation of switching devices and components-10.6-

Ensure the compliance of equipment implementation in accordance with the rules of manufacture and EMC regulations, if applicable.

6) Internal electrical circuits and connections- 10.7

Verify the conformity of implementation and dimensioning of internal circuits and connections. The following should be carefully checked:

  • Short-circuit withstand strength.
  • Temperature-rise withstand.
  • The section of the neutral conductor.
  • Identification of conductors.

7) Terminals for external conductors-10.8

Verify the compliance of implementation and dimensioning of terminals for external conductors.

8) Dielectric properties-10.9

Test each type of circuit in the assembly to ensure:

  • Power-frequency withstand voltage.
  • Impulse withstand voltage.

9) Verification of temperature rise -10.10

Ensure:

  • Thermal stability of the loaded assembly,
  • That the temperatures are controlled on accessible parts, connections and equipment devices.

10) Short -circuit withstand strength-10.11

In comparison to a tested reference design or by testing, verify the level of withstand assigned to the reported short circuit current (unless excluded)

11) Electromagnetic compatibility -10.12

Verify EMC requirements via tests, Except if:

  • The incorporated devices and components comply with ECM requirements for the environment that has been specified;
  • Their installation and cabling comply with the specifications of the manufacturers.

12) Mechanical operation – 10.13

Verify via tests the mechanical operation of removable parts (including any insertion locking). Enclosures, partitions and fastenings should be able to withstand the wear-and-tear of normal use under short circuit condition.

All of the above test were completed and comply with SANS/IEC 61439-2012/2011 Edition 2- part 1 & 2 

50kA Busbar Support Test at The Apollo

SWITCHBOARD MANUFACTURERS KZN TESTS 50KA BUSBAR SUPPORTS

Switchboard Manufacturers KZN put their upgraded 50kA Busbar Support System to the test at the South African Bureau Of Standards (SABS) NETFA Laboratory in Bronkhorstspruit. 

[The testing staff at NETFA and Switchboard Group. Centred Seth Mnisi, right- Shane O’Reilly, Left- Josh Berman]

Shane O’Reilly and Andrew MCarthy lead the team that designed and developed the composite supports that were able to pass the tests with ease.
[Busbar System connected up to the 127MVA transformer. Output potential: 100kA]
The tests were completed under the auspices of the Short-Circuit Laboratory Manager, Seth Mnisi, who treated the Switchboard team to a tour of the facility.
The Busbar Support System was able to withstand a short-circuit test to SANS / IEC 61439-2:2012/2011 Ed.2 up to 65kA rms per shot.
The incredible machinery coupled with the friendly and highly qualified staff keep this facility at the top of its game.

The Basics of Contactors

The Workings and Uses of Contactors

A contactor is a type of relay which is used to conduct larger currents. The contactor is used to open and close the connection to devices which require regular switching such as lighting, motors and heating. The contactor can be normally open, but closes when activated or vice versa. Contactors can be either used in AC or DC applications, but AC is more commonly used.

[A Schneider Electric four pole contactor.]

The basic design of a contactor

The contactor consists of three basic elements:

  • The spring which allows it to return to a set position.
  • The contacts which make or break the circuit.
  • The coil when energised,  is used to move the contacts into the open/closed (O/C) position.

The contactor usually has a three or four pole switch in three phase circuits or a double pole switch in single phase circuits, which in its natural position is open. The switch consists of stationary and moveable contacts usually coated with a silver alloy. The two contacts are held apart with the use of the spring. The moveable contacts are connected to an armature. A separated circuit which operates at a much lower voltage (usually 220Vac/380Vac/440Vac or 12Vdc/24Vdc) is used to power a coil wound around a magnetic core forming an electromagnet. When the electromagnet is energized it attracts the armature which then brings the two contacts together closing the circuit. The switch can either be held closed by keeping the electromagnet energized or by mechanical methods.

[A schematic representation of a simple contactor. Image created by Author.]

The contactor contacts are closed as fast as possible to avoid arcing between the two closing contacts. The arc between the two contacts needs to be extinguished as fast as possible as this could damage the contacts in the long term. The contacts are coated in silver as this prolongs the use of a contact before failure. In today’s contactors the contacts are brought together at such a speed that the contacts bounce off each other. This bouncing can cause secondary arcs which can also cause damage to the contacts. It is thus desirable for contactors to have as little bounce as possible, as contactor bouncing decreases the life expectancy of the contactor.

It is for this reason that contactors should only be purchased from a reputable authorised supplier to ensure that a premium quality product is used for your installation.

Contactor Ratings

The IEC rates contactors based on the design philosophy followed to determine the life-expectancy of the contactor. The following categories are used by the IEC to rate contactors.

It is important to follow the guidelines set out by the manufacturer when installing a contactor. Most contactors will fall into one of the listed ratings.

Contactor Applications

As can be seen in the table contactors are mostly used to operate motors. These contactors are generally of the three or four pole type due to most motors utilise three phase power. The two pole contactors are mostly used to switch lighting and heating in a building due to these systems utilizing one phase power.

For more details on contactors and their installation please contact joshb@switchman.com ,  Alternatively visit our website www.switchmanproducts.co.za

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand. jannes9000@gmail.com

Wire Gauge Selection – The Do’s and Don’ts

How to correctly size wires and why it is important

The cross-sectional size of a wire is a very important factor which needs to be considered when designing electrical circuits, especially when said wire needs to conduct large quantities of current. This is due to the fact that any physical wire will have some measure of resistance and will thus dissipate energy in the form of heat. It is thus the job of any circuit designer that this dissipated energy is within safe levels and that the load being supplied will operate efficiently.

 

Selecting an Appropriate Gauge for your Current Rating

As stated before the cross sectional area of your chosen wire will determine how much current a cable can safely handle. This is due to the following relationship between resistance (R), length of cable (l), resistivity (ρ) and cross-sectional area (A).

From the equation it is easy to determine that at larger cross-sectional areas, a cable will have a smaller resistance and thus dissipate less energy.  It is thus a generally accepted consensus that larger diameter wires can conduct larger values of current. This brings us to wire gauge tables. These tables are used to determine the size of a cable needed for a certain application.

Referring to the below table we can see that if we have a current rating of 27A or smaller we can use the 2.5mm2 wire gauge.

[Data obtained from South Ocean’s Bare Copper Earth Wire]

Sizing the cable for Voltage Drop

Just as in any resistor, the wire will also have a voltage drop developed across it. This voltage drop can thus hinder certain loads if it is too large. The general rule of thumb is to size a wire such that it will have a voltage drop of less than 2.5% of the source voltage. The size of the voltage drop depends on the length of cable as well as the magnitude of the current carried by the cable. To calculate the voltage drop developed, we simply take the voltage drop constant given by the table, multiply it by the current and length of the cable.

For example if we have a cable which is 10m long and conducts 20A and has a cross-section of 2.5mm2. We have a voltage drop of 3.6V.

(Voltage Drop = 20A x 10m x 0.018V

Working Example:

We want our cabling system to supply a load of 10kW at a voltage of 230V and a cable length of 10m. This gives us a current of 43.5A when we divide the power by the voltage.

We should however design the cable to carry an extra 20% of current in the case of an emergency. We thus design the cable to carry a current of 52.5A.

The choice of a 10mm2 wire should be sufficient for our needs.

We now need to see if the voltage drop is within sufficient limits.

Let’s calculate the voltage drop across the wire:

The voltage developed across the cable is thus within acceptable limits as it is less than 2.5% of the source voltage.

For further information contact Joshb@switchman.com

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

Jannes is completing a 6 week learnership at Switchboard Group.

jannes9000@gmail.com

 

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