Choosing the right generator set: factors to consider

Load calculation

An accurate electrical dimensioning is necessary when purchasing a generator set. To find the right balance several factors need to be considered: current and future power needs, equipment elements, legal regulations or location of the set. They are crucial for a right choice.

Why correct generator selection is essential

Each energy-related project implies a different challenge and different compliance requirements. Electrical dimensioning is growing increasingly complex.

In the past, dimensioning calculations were based on the arithmetic sum of power ratings together with some corrections of the power factor depending on the number of engines included in the installation. The type of load was simpler, closer to just-resistive loads. These days such scenario is unthinkable. The electrical devices which connect to an installation have a very high load of electronic components; the energy generation becomes more complex with the appearance of harmonics, inductive and capacitive loads.

That’s why GENESAL ENERGY’s salesforce receives continuous technical training and is closely assisted by the engineering department in order to offer the customers an optimal solution.

Acquiring an undersized generator will leave the end user with insufficient power at given times. If a generator set is required to provide an amount of power for which it was not designed, it will suffer voltage and frequency drops causing overheating and excessive wearing of the engine parts also affecting the alternator coating.

Acquiring an oversized generator means the fuel consumption will increase. If the generator is operated at low loads, there is a drop in the working temperature, which leads to poorer combustion thereby affecting the induction as a well as the anti-emissions systems such as filters and valves. This fumes in the exhaust alongside with suspended and unburned particles can, in the long term, cause power loss and the build-up of soot in the intake & exhaust.

To prevent both scenarios, the engine should function at an average of 70% of its rated power avoiding working for long periods of time at less than 30% of its capacity.

How to calculate the power your genset requires

Measuring right

The first and foremost step is defining which equipment will be connected to the generator set and knowing the necessary power delivery for the project. In other words, knowing the total amount of power needed by the electrical devices connected to the genset, applying simultaneity rates when possible and always responding to the customer’s requirements and needs.

However, a simple mathematical operation will not give us enough information. There are other factors which may affect the result of this calculation. Namely technical aspects and usage or environmental conditions.

It is necessary to have a more thorough control over the greater loads in the installation like electric motors and discharge or LED lighting, all electrical loads which are difficult to feed because they involve electrical fast transients which can cause peak consumptions lasting for split-seconds, which badly designed generator sets will not be able to supply.

Sizing the generator set

In order to size the generator set and decide on its configuration, the team of engineers will take as a basis the electrical, mechanical or civil engineering project. It specifies:

The necessary power delivery and load types required in order to cover existing and future needs, diversity factors, step loads, etc.
The elements of the equipment (manual or automatic start, group and grid parallel operation, soundproofing, auxiliary fuel storage, heaters, etc.)
The statutory provisions on electricity, noise, gas and particle emissions, transportation and road safety (for mobile generator sets).
The location where the generator set will be installed (indoors, outdoors, height above sea level, dusty environment, etc.)

Once this information is gathered by GENESAL ENERGY’s engineers they start to forge their project complying with the customer’s needs, thus adapting not only to the technical requirements, but also to the actual use that the genset is intended for.

Type of application and frequency of use

When sizing a generator set, we need to consider the appliable type of load and frequency of use:

Continuous Power (COP) For operating 24hours/365 days according to ISO8528. These sets can operate an unlimited number of hours at a 100% of its rated power. This type of equipment is fit for permanent parallel operation with grid as in cogeneration (CHP) plants or for continuous resistance-heating requirements.
Long Term Power (LTP) Sets aimed to operate up to 500 hours per year at a 100% of its rated power. This type of sets is often used in parallel with grid for peak-shaving.
Prime Power (PRP) Sets capable of operating 24hours/365 days. These sets can operate an unlimited number of hours with the possibility of a 10% overload for 1 hour every 12 hours. The average load for a period of 24 hours cannot exceed the 70% of the PRP rated power of the set. This is the most common type of generator set for almost every industrial or commercial application given that most of the loads applied to the set are variable, this is, they connect and disconnect according to the needs of the installation.
Standby Power (STP) Sets suitable for operating variable loads up to 200 hours per year within an average of 70% of the STP rated power every 24 hours. This type of sets is often found in emergency industrial and commercial applications.
Data Centre Continuous (DCC) In data centres complying with Tier III and Tier IV regulations (acc. to Uptime Institute) only unlimited-operation generators can be installed. According to ISO8528 this is only achievable through COP sets, but for variable loads as those found in data centres it means oversizing of the genset. In order to avoid this, DCC rated sets can operate an unlimited number of hours with these variable loads.

GENESAL ENERGY generator sets always display PRP/STP rated power in their technical specifications sheets as they are the most common type of application.

Environmental and usage factors in generator sizing

The location where the genset is installed is a decisive factor. GENESAL ENERGY provides solutions for:

Saline environments.
Marine generators.
Mines and dusty environments (IP 45).
Nuclear zones.
Earthquake-proof electrical panels.
Extreme temperatures (-32°c to 50°c).
Mobile generators reinforced for military use.

Elevation of the installation

This factor may sound secondary but if it is not considered, it may shatter the engineering work entirely. Engines need oxygen in order to function and the oxygen concentration decreases as the altitude of the genset’s location increases.

This cause the fuel-oxygen mix inside the combustion chamber to be poorer in oxygen, decreasing the engine’s power output beyond 1000 metres of altitude by 10% and more. This applies only to atmospheric aspiration engines as turbocharged engines are not heavily affected by height.

Type of load connected

Another key step in sizing a generator set is knowing the type of loads it will be powering. Some devices as pumps or engines using convertors are subject to transients during start-up.

Special attention must be paid to the following types of loads:

Uninterruptable Power Supplies (UPS). The rated power of an alternator should be oversized depending on the installation.
Large engines loads. They can increase the starting power needs by up to six times, depending on the type of start-up.
Lighting loads. Specially lighting with sodium vapour and metal halides and, more recently, LED lighting, due to the amount of transients generated by the circuitry that composes them.
Variable frequency Devices (VFD). They can lead to an oversizing of 40 to 100%.
Loads particularly sensitive to transients. Including power electronics or devices – like computers – that have many electronic components.
Loads which require voltage and frequency variations for their operation.

Start-up current needed to counter a power outage

The initial loads simultaneously connected to a generator directly affect its sizing. Especially dynamic loads such as direct starting motors or soft starter motors. Generally:

Electric motors with star-delta start: the ‘start-up current’ equals 3 times the rated or operating current.
Direct starting motors: draw 6 times the nominal current.
Starter motors with variable frequency devices: pay special attention to its data sheet, because the ramp and speed increase is normally configurable.

Generators should be sized bearing in mind that single or simultaneous start-up of the sets cannot cause any voltage or frequency drip preventing the motor’s start.

Distribution of single-phase loads in a three-phase system

There could be single-phase loads connected between a phase and neutral within a three-phase system. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded with single phase loads. Failure to balance loads can cause Eddy currents, overheating and the eventual destruction of the alternator. To avoid this outcome, generator sets are equipped with load imbalance’s protection.

There are several solutions to correct excessive load imbalance:

Oversize the set to endure higher imbalances.
Use a single-phase generator set, if possible.
Try to even loads among phases.

Depending on the above, the three-phase generator set should be sized to bear the maximum load per phase.

As we have seen, sizing a genset is an intricate process.

It is much better to leave it in capable hands. Genesal Energy has more than 20 years of experience in the energy distribution sector. And we continue to innovate in order to provide our customers with personalized solutions of utmost efficiency.

Example: How to rate the power of a generator set?

We will end up with an example on how to assess the power of a generator for a 12-storey apartment building, which needs to support the following equipment:

1 15 kW Lift: start with variable frequency drive (VFD), 380 V.
2 Water pumps: 10 HP each, star-delta start (S-D), 380 V.
1 Heating pump: 5 HP, direct start (DOL), 380 V.
1 Pressurized staircase for emergency, 15 HP, direct start (S-D), 380 V.
Common aisles lighting: 5.5 kW (55 light points of 100 W each), 220 V.
Entrance hall lighting: 1.5 kW (15 light points of 100 W each), 220 V.
Vehicle entrance gate: 0.5 kW, direct start (DOL), 220 V.

We will now proceed to devise the load table and determine the starting power for each device. In this case, we will consider the scenario including the highest possible demand for calculating the genset: one with an elevator and another with a pressurized staircase, given that both cannot function simultaneously.

REQUEST THE LOAD TABLE (spanish)

Now we have given you this example, but each project is unique. Request our load chart in the form below if you want more information about choosing the right power for a generator set.