ATS panels in sites with unstable electricity grids

The envisaged surges and instability in the main grid has to be taken into account in order to prevent future incidents, switching failures and subsequent outages in the supply and overrun costs in the installation.

Antonio Moreno, HIMOINSA Sales Engineer

The characteristics of the site in which an emergency generator set is to be installed as an alternative source of energy in the event of a main grid supply failure, determines to a great extent the choice of the switching system. Above and beyond the actual power that is supplied and the voltage and frequency of the grid, the location and the layout of the installation are factors that must be properly calibrated before reaching a decision. A bad choice may lead to malfunctions, power outages and unexpected cost overruns in the installation. All the technologies that currently exist in the market are perfectly capable of performing a failover satisfactorily, but are they all appropriate for locations where the electric grid is unstable? What parameters should be taken into account before opting for one type of switching or another?

Types of switching and standard solutions depending on the amperage

An ATS panel is a device that comprises a power output and two energy inputs; one of these corresponds to the main energy source (usually the electric grid) and the other to an emergency source of energy (usually a generator set). While the grid continues to work normally, that is the input that is activated on the ATS panel. When the grid fails or the parameters are outside the established range, the system will disengage

this input from the grid and it will activate the input of power from the generator set so that the supply can continue until the network stabilises and the process can be reversed.

Nowadays, several technologies are available for general failover operations and, more specifically, for switching from a grid to a generator set:

A PAIR OF CONTACTORS

Comprising two electromechanical switches that establish or interrupt the flow of current as soon as voltage is sent to the coil; this is the simplest switching system on the market.

A PAIR OF ISOLATORS OR MOTORISED BREAKERS

This makes it possible to manage the two energy supplies in order to run an alternative or simultaneous supply. This feature makes it a type of switching that is very useful in projects where the emergency source can be synchronised with the main source. Motorised breakers allow power to be restored from the main grid without a zero-crossing, thereby avoiding any outage while switching from one source to another.

MOTORISED SWITCH

Unlike the previous two, this is a single device that activates one or other power source through an internal engine control unit. In addition to its being easy to assemble, its main advantages reside in its high durability and the large number of manoeuvres that it allows.

The selection of the most suitable type of failover usually depends on the size of the installation and the budget. Thus, depending on the amperage, the following standard solutions normally apply:

Problems that arise in ATS panels in sites with unstable electricity grids

The transfer of the power supply between the grid and the generator set may be altered when the installation is located in a site with an unstable electricity grid. HIMOINSA’s Engineering Team has detected that frequent power outages and in particular over-voltages cause damage and provoke failures in the operation of the ATS panels that employ contactors with VAC coils and motorised switches with VAC engine control units.

As far as using contactors is concerned, we have seen a significant increase in incidents in installations. The constant surges in the grid end up damaging the contactor coils and stop them from operating, thereby preventing the failover when there is a grid supply failure.

A large number of incidents have also been observed in the case of isolator pairs or motorised breakers with VAC engine control units. In this case, the damage caused to the engine control unit or the opening coils by overvoltages stops the switch from changing position even though it receives the order to do so, which consequently leads to a supply failure. The problem is compounded when the voltage that is used approaches the voltage limits of the engine control unit’s tolerance range established by the manufacturer.

The most appropriate switching solutions when the primary energy supply is unstable

Switching failures lead to outages in the end energy supply, with the consequent economic losses that this may cause to the consumer, not to mention the extra costs involved in repairs which increase the cost of the initial investment.

The most common solution in these cases is to include voltage surge arresters in the design of the installation. However, although this may resolve the problem initially, in the medium term these devices also deteriorate as a result of surges in the grid.

Therefore, to reduce such incidents and to ensure the quality and profitability of the installation, HIMOINSA recommends certain specific ATS panels when there is evidence that they are going to be located in a region with frequent electricity supply problems.

1. For an amperage of up to 3200A, we recommend using motorised switches with VDC engine control units rather than VAC units, in order to isolate them from voltage fluctuations. In addition, the engine control unit must be 12 VDC or 24 VDC depending on the auxiliary voltage of the generator set.

2. For switches above 3200A, we recommend the use of a pair of motorised magneto thermal breakers or motorised isolator switches with 24 VDC coils and engine control units.

In this way, using a VDC power supply, the elements of switches that are more sensitive to fluctuations in the electricity grid, such as coils and engine control units, are isolated from such fluctuations, thereby providing the installation with greater durability.

The next question is: what source must the VDC supply come from, for these more sensitive elements? One key factor shall exclusively determine the most appropriate answer for each project: the layout of the installation and the distance between the ATS and the generator set. To determine whether the distance is excessive or not, we will take as a parameter the section of cable needed for the expected consumption. We will also take into account a drop in voltage within the most restrictive limit recommended for the element of the installation in question.

When the distance between the ATS panel and the generator set is such that the section of cable required is less than 6-10 mm², we recommend that the power supply should come directly from the auxiliary electric system of the generator set, i.e. from the batteries of the generator set.
When the above option is not feasible and the generator set is too far away from the ATS panel, we recommended using a proprietary VDC supply system in the ATS panel, with a battery charger and batteries.

Conclusions

The actual power that is supplied, the voltage, the frequency of the grid and the budget are not the only circumstances that influence the emplacement of one or another type of switching. The envisaged surges and instability in the main grid has to be taken into account in order to prevent future incidents, switching failures and subsequent outages in the supply and overrun costs in the installation. By isolating the elements that are most sensitive to variations in voltage, such as coils and engine

control switching systems, we can prevent these failures and ensure that ATS panels in locations with unstable grids work properly.

HIMOINSA manufactures ATS panels that carry out the transfer of the power supply between the grid and the generator set; we can provide you with the advice you need as to the installation that best suits the needs of your project.

Antonio Moreno Carrasco. An Industrial Engineer specialised in electrical installations. He has been working in the Commercial Engineering Department of HIMOINSA since 2013, carrying out technical and financial feasibility studies for special projects and advising on the quality and costs of engineering solutions in order to adapt the product to the needs of each project.

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