Guidelines for calculating the AC backup generator for DC systems

The AC backup generator, motogenerator, or power plant, is the main AC support for any installation. It consists of an engine, usually diesel, with a generator coupled to its shaft.

Obviously, we are referring to fixed generators, with capacities greater than 20 kVA.

To give you some context, we invite you to read our post Power equipment for telecommunications. Part 2 and we also recommend the article Guidelines for Calculating AC/DC Rectifiers for DC Systems.

This engine rotates at the appropriate speed to produce AC electricity with the required frequency and voltage in the country where it is located. Below, we present the calculation guidelines you should consider when sizing it. Let’s get started!

Parameters to consider for the installation

Before selecting and installing a generator set for AC backup in DC systems, you have to take the following into consideration:

• Nominal AC voltage of the building
• Altitude above sea level at the installation site
• Ambient temperature, as there is generally no air conditioning service in the spaces where it will be locate

As for the sizing of the power plant

At this point, you must be very careful and have a clear understanding of the following:

• Have an estimate of the increase in electrical load over a period of 5 to 10 years. If you don’t have this data, plan for a reserve of at least 25%
• Percentage of connected load. Major manufacturers insist that the connected load should be at least 50% and at most 80% of its nominal load
• Magnitude of the electrical load to be backed up by the generator, in kVA
• Type of electrical load to be served
• Operating regime. That is, whether it is continuous, operating more than 2 hours daily every day. Or standby, only operating during power outages or less than 2 hours daily for 20 days a month

Además de lo anterior, debes disponer de la siguiente información para el cálculo:

• Diagrama unifilar actualizado del sistema de potencia de la localidad
• Estudio de carga a conectar
• Medición de energía eléctrica en el tablero que será respaldado por el motogenerador. Bien sea el tablero principal o el de cargas esenciales
• Información estadística de la calidad del servicio eléctrico en la localidad. Con el fin de conocer la frecuencia y duración de fluctuaciones de voltaje, interrupciones de servicio, entre otros.
• Otros como localización geográfica, condiciones de acceso, contaminación del ambiente y más.
• También es importante estar enterado de todas las regulaciones gubernamentales que rigen en la localidad. Referidas a la instalación y operación de las plantas eléctricas.

In addition to the above, you must have the following information for the calculation:

• Updated single-line diagram of the local power system
• Load study to be connected
• Electrical energy measurement at the panel that will be backed up by the generator, either the main panel or the essential loads panel
• Statistical information on the quality of electrical service in the locality. To know the frequency and duration of voltage fluctuations, service interruptions, among others
• Other factors such as geographical location, access conditions, environmental pollution, and more
• It is also important to be aware of all government regulations governing the locality regarding the installation and operation of power plants.

How do I select the final capacity?

When calculating the capacity of the genset in different DC backup scenarios, it should be the highest among:

• The capacity in kVA to power the total load to be backed up without loading it more than 80%
• The capacity in kVA to withstand the starting of motors from different equipment, including hydropneumatic pumps, air conditioners, and others. This without exceeding the maximum voltage drop established
• If there are loads operating with thyristors, such as rectifiers, it should not exceed 50% of the nominal capacity of the genset.

In our next post, we will discuss the Criteria for projects involving equipment associated with gensets. It will be very interesting!

Closing…

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The project for the installation of a backup AC generator set for DC systems must follow very specific guidelines. Although, looking at it closely, all of them are necessary to achieve a good result. So our advice is to comply with everything!

On the other hand, we recommend our Course on sizing and designing DC power systems for telecommunications and critical systems. So that you can acquire the skills and knowledge necessary to project in this somewhat unknown factor. To find out about its content, click here.

By checking our website energydcac, you will find relevant and extensive content on the topic of DC/AC energy. Visit it and see!