THREE-PHASE GENERATORS FOR SINGLE-PHASE LOADS: DISTRIBUTE A LOAD.

Orefice Generators - Copyright 2017

When dimensioning the power of a generator, it is necessary to carefully evaluate the type of power to be supplied.

It's very comon that one has  a three-phase generator set and has to supply a single-phase load. How to proceed?

You must have heard that a three-phase generator of Power P = 3 kW can drive a single-phase power load of 1 kW because Pm = P / 3.

This condition is valid for all alternators with Star  windings connection and a voltage of 400 V, where to obtain the single-phase voltage of 230 V, we just power the load between a phase and the Star center.

“Single-phase loads powered by a three-phase generating set must be distributed in such a way that, at each phase, no more than the rated current of the generator is running.

Notice how we talk about power but also about current, because it is important to note that some loads at the time of insertion have a higher absorption, or higher currents. It goes without saying that an experienced technician can rely first on an experimental calculation, but then he must  measure the electrical loads distributed over the phases with the aid of a measuring instrument.

If you do not have a calibration tool, but you have a Generator branded Orefice with VEOTM control panel , you can easily monitor the absorption directly from the Display.

If you do not have a measuring instrument neither a generator  Orefice you will not be able to ascertain the correct distribution of loads between phases.

What if the loads are not properly distributed?

It may happen that the generator will go into distress and that the windings inside the alternator may overheat until they "burn" and interrupt.

To make a comparison, we imagine that electric load is a heavy shopping bag that we carry for hundreds and hundreds of meters with the same arm. We will reach our destination with a tired arm that is more subject to an injury. Dividing the load into two envelopes would not change the work to be done, and perhaps even fatigue, however it would be easier to carry the load to the target avoiding pain and risk.

Unbalanced loads are one of the major breakdown factors in an alternator and consequently in a generator set.

That's not all

An unbalanced load can damage the user connected to it, especially when it is a variable load. An electronic equipment user may not work properly.

That happens especially to Audio Services that hire or purchase a generator to which they then connect their panel without taking care to check how much current they are withdrawing from each phase.

Let's take an example to better explain what can happen:

· Generator with electric power Continuous power of 16 kW; Power factor 0,8; Rated voltage three-phase 400 V.

· Single-phase electrical load to be connected to a total of 8 kW divided as follows:

O Halogen lamps: 5 kW

O Other users with a transformer: 3 kW

Instinctlively the less experienced technician would feel free to connect all the halogen lamps under one stage and the remaining 3 kW under another phase. With this subdivision, considering a current of the halogen lamps that is at least 10 times the nominal current for an average time of 0,12 s, it will not only have problems with the lamps being switched on but will overload the generator for at least 0,12 s will not guarantee voltage and frequency values uitable for feeding the remaining 3 kW with the risk of causing damage to voltage values utside the limits.

The difference between a Generator with engine 3000rpm and 1500rpm

What's the difference between a generator set 3000 rpm and 1500 rpm?

A generating set per definition is a combination of an internal combustion engine and electric generator.

The most common engines are those Diesel and Petrol engines with 1500 rpm or 3000 rpm, means revolutions per minute. (The engine speed can also be lower than 1500).

So what is the difference between a  generator 3000 rpm and 1500 rpm one?

Technically we have already answered: one engine in one minute executes 3000 rotations, while the other in the same minute runs 1500, or half. It means, in other words, that if a speedometer measure the number of turns to the shaft of one and the other, we will get either 2 revolutions and 3 revs respectively.

This difference leads to obvious Consequences that should be known when buying and while using a generator:

Life Expectancy

An engine with 3000 rpm has a lower wait than the engine 1500 rpm. This is due to the strain difference to which it is subjected. Think of a car traveling at 80 km / h in the third gear and a car traveling at 80 km / h in fifth gear, both reaching the same speed but with a different mechanical stress.

If we want to give numbers, we can say that a generator set with diesel engine 3000 rpm reached 2500 hours of operation may need a partial or total review, while for a diesel engine 1500 rpm this may be necessary after The 10.000 hours of operation. (Indicative values).

Operating limits

Some say 3 hours, more 4 hours, or 6 hours of continuous operation.

A 3000 rev / min engine has a limit on running time, usually after a few hours of operation it would turn off to allow it to cool down and check the levels. This does not mean that it is forbidden to use it h24, but that continuous use is not appropriate. A high number of laps, for a prolonged time, is not ideal for a diesel engine.

Weight and Dimensions

The engine at 3000 rpm with equal power has smaller dimensions and weight than the 1500 rpm since it has different technical characteristics to reach the rated power. Usually these are air-cooled mono and two-cylinder engines.

....

Running Costs

The cost of the 3000rpm engine is lower and, consequently the generator’s cost too, and even the running cost is different: usually an engine working under stress tends to accumulate over time in number of failures and maintenance higher than the average.

The noise

The noise of a motor generator at 3000 rpm is usually higher, and even when it has an acoustic pressure similar to that of its half brother with engine 1500 rpm, the sound frequency is more annoying in the case of the motor 3000 rpm.

Taking these aspect into consideration you will have a better idea to select and use Your generator.

“Let’s cogenerate”! What is a "CHP", Cogeneration plant and how it works?

The world's first power plant (built in New York by Thomas Edison in 1882) was essentially a cogeneration plant because it provided electrical heat and power to Manhattan's buildings.

From a performance point of view, the generator is a machine that "sucks".

Indeed, all the generators have a poor performance because, to produce electricity, they waste enormous amounts of heat energy dispersed through the exhaust gas, the radiator and the natural irradiation of the hot parts of the engine.

The cogeneration plant, also called CHP, allows the simultaneous production of electrical energy and thermal energy in the form of hot water or steam.

For years, cogeneration has been an exclusive technology of companies and large plants that consume huge amounts of heat, but today it’s a technology increasingly closer to small and medium-sized companies and even the private sector, due to the evolution of energy costs which has made cogeneration systems more competitive than ever before, and a technology that makes small plants more and more reliable and economically sustainable.

Most of the cogenerations operating in Europe are powered by low-cost fuels:

·        CHP plants using natural gas

·        Biogas CHP plants

·        Vegetable oil-fuelled CHP (palm oil, rapeseed oil..)

·        Syngas CHP (gas produced by pyrolysis processes)

·        Diesel CHP plants

In Europe the installations’ trend follows that of incentives, so the choice of which fuel to use is based on economic factors.

Companies that choose to produce energy with a CHP plant are those that consume large amounts of thermal and / or electrical energy, better if both.  Think of a hotel, shopping centres, sports facilities, dairies, these are just some of the major energy consumers who choose a CHP cogeneration.

The advantage of a CHP plant is not only in the cost of self-producing energy, but also in the energy independence that is achieved. The most virtuous and careful structures that we follow have chosen a triple power source: CHP plant + Emergency Generator + Electricity Grid. In this way, not only do they have a low energy cost, but they do not risk staying in the dark in the event of a CHP plant malfunction or during maintenance stops.

When is a CHP plant worthwhile?

The entrepreneur who chooses to install a CHP plant, wants to make profits by reducing costs, and has made one or all of the following evaluations.

He assessed the energy demand of his structure by doing an energy audit. In the event that electrical consumption prevails over the thermal consumption, assess whether there is a physical proximity between the CHP plant and any thermal user, bearing in mind that this must go in unison with the electrical one, while maintaining a certain flexibility, as the demand heat and electricity can be disproportionate to each other in some periods.

The actual convenience is finally assessed on the basis of the cost of energy by the supplier and the sale of the same with the introduction into the network. The advantage is certainly that of putting all the electricity that is not consumed into the grid, earning a profit.

Finally, the assessment is objectively influenced by the cost of the fuel and its purchase conditions, on which the market price fluctuates (the fuel, whatever it is, is subject to price variations) and to the possibility of deducting costs and excise taxes. .

Business Plan is not a "Fable".

When designing a cogeneration plant, the conditions that allow a return on investment within the terms set in the financial plan are to be considered.

Unfortunately,  the conditions resulting from the experimental calculations are not reflected in the real operating conditions and in general during the whole life of the CHP plant.

Even the best CHP plants stay off many hours a year for small or big problems. From direct experience in the operation of cogeneration plants we can say that a small component, even a simple temperature probe, or a sleeve is enough to force a generator to stop.

The business plan is not a fable, it is a tool to validate the financing of the bank, but it is above all the project that every entrepreneur should check and use to make further assessments on the investment time and duration of exposure that this involves. . With this we must not imply that a CHP plant is a nut to track, but only that the best of conditions is not said to be the most realistic. All this because the income statement is based on the incentives provided for the plant, therefore only costs are accumulated when the plant is stationary.

How does a CHP plant work?

Simplifying to the extreme, we can state that the CHP plant consists of two main elements, the Generator, in turn composed of engine and alternator, and one or more heat exchangers.

At a theoretical level, the exhaust gases emitted by the generator and the coolant used to cool the engine and keep the temperature constant during operation, are already potentially usable because they are available at high temperatures. In reality, obviously, it is not possible to use these fluids directly, as being contaminated they are harmful.

The exchanger is installed in such a way as to intercept the exhaust fumes, these give heat to another fluid (be it air or water or other fluids in the liquid state).

The most common exchangers are plate and tube bundle exchangers. In the first, the fluids at different temperatures exchange their thermal content through the appropriately arranged surfaces of the exchanger, the plates in fact.

How is a generator tested?

The term "test" defines a series of activities necessary to determine if a product is suitable for operation. Testing is often mistakenly misunderstood as the commissioning and the start-up of the product.

Let’s start by making a few clarifications: not all manufacturers of generators in the world have an equal and repeatable product testing process. Even those who adopt an ISO9001 quality management system do not have a standard test.

Some choose to carry out tests  on batches of products, while others choose to individually test each generator set, but with procedures that allow testing the functioning and not the performance of the generator. Then there are those who have chosen to adopt a complete test procedure that allows to test the functioning at 100% of the nominal power of the genset.

It is clear that the difference between these three test procedures affects the final result and the list price of the generating set.

With the sample test, usually chosen by large companies that produce large quantities of generator sets every day and in particular by all those companies that produce generators of small power, the risk of defective product is not insignificant. In fact, if the average incidence of the non-conforming product is 1%, in a company that has adopted this testing system it cannot be ruled out that this incidence is concentrated entirely in a few lots or even in the same lot.

Those who choose to test 100% of the production, but are limited to a functional test, can "guarantee" the conformity of the product in functional terms, but cannot guarantee the performance. For example, a 10kVA generator set is tested with a switch to the main parameters, but it is not subjected to an electrical load that can simulate the generator's behaviour in all its functions, so you cannot know if it will actually deliver the 10kVA for which it is sold on the market, nor can you be sure that when reaching the maximum power there are no technical problems.

In the end there are the manufacturers who choose instead to carry out a scrupulous test, which allows to verify the operation and performance of the generator, subjecting it to an electrical load to simulate the operation in the most severe condition of the generator.

The latter type of testing is indeed the most appropriate. the term "test" comes from the Latin cum-laude which means "work of art". So testing is definitely something that serves to establish if a product has been done well, to the letter.

Is it better to test the generator set directly at the installation site?

Some, explicitly ask for the factory test, while others take for granted that the test is performed at the site where the generator is installed. The testing is the responsibility of the manufacturer, who after a positive test issues a declaration of conformity. The ideal testing conditions can be reproduced only with suitable instruments present in the testing rooms and in particular with instruments equipped with calibration, however, even if  these equipment are available at the installation site, the operating conditions are hardly ideal for testing, especially the electric load connected to the generator.

A professional manufacturer, sets up the generator and tests it with a standardized and repeatable procedure. Here's how (synthetically) a general test is performed in a few steps:

• Check electrical connections, hydraulic connections and tightening.

• Starting with blank test.

• Try loading steps. (x% of nominal power up to 100% divided into different and increasing load ramps).

• 100% power test.

• 110% power test.

• Alarm simulation, instrument reading test, emergency stops ... etc.

During a test, the behaviour of the user, i.e. the dummy load used to simulate the user to feed, (usually a resistive load) is not comparable to a system with different loads and subject to anomalous voltages, unbalanced absorption, eddy currents, and so on and so forth.

From the point of view of the end user, there is no difference between factory testing and on-site testing, but in light of what is written above and considering the fact that the constructor of the generating set is not responsible for what is connected to the generator, the difference exists and is relevant.

Factory testing is a protection for the manufacturer and for the customer. If the generating set reaches the nominal power in the factory, while it presents anomalies of operation at the site of installation, it is possible to make evaluations to compare between the measurements taken during testing and those measured in service to the user.

Many companies allow to assist the testing of the generator, others are available to add specific test sequences required by the customer.

The test report must be requested during the signing of the supply contract, when ordering. In fact, many companies have a digital system for archiving tests and do not issue a paper certificate if it is not explicitly requested.

Testing is not just for the new generators.

I have a generator for a few years, which has been repaired, how can I be sure that it works like before...?"

The test is not exclusive for the "new" ones, in fact any generator can be tested with the same procedures adopted for a new generator. It is the only way to be sure of the functioning of a generator.

Let's take an example: Two years ago, I assisted as a counterpart, at the Expertise of  a Technical Consultant for the court. The engineer was in charge of determining whether a disputed Generating Set was in default or functioning normally.

Being also a Consultant for the Court of Cagliari, I immediately had doubts about the procedure adopted to determine if the generator set was working, in fact, the engineer with the help of an assistant, started the generator engine and seeing it turned on, he related to the judge that the generator was tested and was well functioning.

Now, to make a comparison, it is as if to check the functioning of a car for which you are complaining of a fault, you limit yourself to turning the starter key and turning on the engine to say that it works. A minimally qualified technician will take care of a test run and test the greatest possible number of instruments and accessories, reaching the maximum speed allowed to evaluate the effective efficiency.

Here is how UPS (Uninterruptible Power Supply) and Diesel Generator can work togheter.

When genset and UPS do not get along

In modern backup systems, it is not difficult to find systems that show the generating set and the Uninterruptible Power Supply in service on the same utility.

The configurations may be different. In many cases the generating set powers the Uninterruptible Power Supply which in turn supplies the user, while in other cases the UPS is independent of the Generator since it supplies a the non-connected part.

In the case in which a genset must supply a user through a UPS the installer finds himself dealing with the possibility that the UPS has tolerances relative to the electrical values that are incompatible with the generator.

The generator in operating mode has more or less stable values, specifically the voltage and frequency. The level of stability is not the same for all the generators, in fact, it can never be stressed enough, the generators are not all the same and are not distinguished only by electric power.

Usually the Uninterruptible Power Supply require a particularly rigid frequency and voltage stability, the reason is that the online double conversion UPS rebuild the voltage with its inverter. Therefore, the greater the instability of incoming values, the greater the work that the rectifier and the inverter will have to perform.

What happens when the power generator and the Uninterruptible Power Supply do not get along?

"When the generator is running the UPS issues an alarm ..."

Everyone is in crisis while trying to reason with the continuous "Beep" that haunts and prevents to focus on the solution.

When a UPS measures input voltage and / or frequency values outside the allowed limits (or rather, set by the manufacturer) it goes into alarm status and often an automatic bypass occurs.

For reasons still shrouded in mystery, the Customer and the Installer often prefer to make changes to the generator by intervening on the voltage regulator, rather than seeking a balance between the two products. This problem is nothing more than the result of incorrect sizing in the design phase, or an overly precise and rigid setup on the UPS.

It is also possible that the engineer did not consider the Harmonic component present in the system.

How to make peace between UPS and Generator.

Now that the problem is understood  we can move on to the solutions.

Mainly the problem is connected to the frequency oscillations of the diesel generator, in particular the rectifier present in the UPS continually senses the variation in the value and prevents the insertion of the inverter.

a)      The oscillations of a diesel generator are mainly due to load variations. The major problems occur when one or more loads are inserted. Example: The generator supplies a preferential line protected by UPS but also a part of the system consisting of motors and reactors. In this case, when the motor is switched on, a significant variation in frequency and voltage that could not be tolerated by the UPS will be inevitable.

b)     Many UPS allow to modify the intervention thresholds, therefore they allow to open the fan and frequency tolerated without affecting the quality of the output voltage.

c)      The generator intended for applications such as those protected by the UPS should be sized taking into account the class of the engine speed regulator. To learn more about this topic you can read an article about this topic and download the free material that we provide at the end of the article.

d)     Some manufacturers suggest the application of harmonic filters to the Uninterruptible Power Supply in order to reduce the harmonics (THD) to 10%.

e)     Do not be rushed to blame the generator or UPS, it is true that the harmonic distortion depends on the conversion system of the UPS but it is also true that the generator set has an impedance that is not comparable to that of the network.

The right generator for your UPS

The correct selection of a Diesel Generator destined to supply a Uninterruptible Power Supply begins with the definition of the operating logic criteria of the plant.

It is essential to know which loads in addition to the UPS will be powered by the emergency generator.

The main features involved in sizing are:

·        The overall power of the Generating Set.

·        The RPM regulator of the primary motor installed in the generator.

·        The power of the alternator.

The nominal power of the Diesel Generator

The nominal power of the genset is to be considered PRP, that is prime power of the ISO 8528 standard.

By definition of the standard, the PRP is the maximum power that the generator can provide during a variable power cycle.

When defining the power of the Generating Set it is important to consider the absorption values of the UPS during the recharging phases of the accumulator battery.

If you want more advices about that, please send an email to us:

If this article was useful, don’t hesitate to like it and share it with your colleagues ...!

Written from Andrea Orefice

Even Airplanes needs Power Generators

Have you ever heard about APU? It’s an auxiliary generator commonly found on large aircrafts to run their electrical systems even with engines are switched off.

Having been trained in the Aeronautical Construction sector, I personally found it interesting to expand the application and the use of two devices frequently found in the aeronautical sphere which are APU and GPU

APU

Auxiliary Power Unit.

The use of APU  has been banned in many airports due to the noise they generate once activated. Imagine all the operators approaching the aircraft for the various activities, using ramps, unloading luggage, assistance services, and for many hours they must be near an APU in operation!

GPU, Ground Power Unit is in effect an electric generator. . It is used in airports to power an Airplane or a Helicopter when it is stationed on the ground with the engines turned off.

It is characterized by a range of voltages, currents and frequencies that are not used in other applications 28VDC o 115VAC, 400Hz depending on the type of aircraft

Why using the GPU

Keeping the engine running on an airplane has very important costs even when it does not produce the necessary energy for take-off.

To understand the difference between the consumption of a diesel-powered GPU and the engine of an airplane, a Boeing 757 for instance, powered by Kerosene, we consider that the latter will consume in 1 hour what the GPU will consume in 24 hours. Approximately 270 liters

I'm starting the APU. When a generator set saves lives

US Airways Flight 1549 - 15: 27: 21.3 HOT-1 “I'm starting the APU”

An APU produced by Honeywell has contributed with Captain Sullenberger's intrepidity in saving the lives of US Airways Flight 1549, which with both engines in failure, made an emergency landing on the Hudson River.

The 131-9A Auxiliary Power Unit (APU), friendly called U9A, is installed in the tail section of the Airbus A320.

After both engines got stuck due to the impact with a flock of Geese, U9A intervened by feeding the on board devices to temporarily hold in position the flight controls, displays and other sensitive and necessary equipment on the fly.

Had it not been for the APU, Captain Sullenberger could not have landed in the Hudson River at the lowest possible speed and saved all those lives.

KVA TO KW. THE GENERATOR POWER EASY AS DRINKING A BEER

Everyone who’s dealing with a generating set has for sure heard about kW and kVA and found it easier to take the kW as a unit of measurement.

Knowing the difference between watts (kW) and Va (kVA) is useful for more detailed determination of the scope of the generator set.Knowing the difference between Watt (kW) and Va (kWA) is useful to determine in greater detail the specific application of the generating set.

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Last year during class I used a classical example but obvious to the audience to explain to my student the difference between kVA and kW: Comparing the electric power of a generator to a glass of beer.

Pouring some beer in a glass, two separate parts are formed: the beer itself and the foamy head. We can consider the glass as the maximum power, the foam as the reactive power and the beer as the real power.

“Apparently” the glass is full but what will quench our thirst is only the part of the beer not the foam. However, to drink the beer you should also drink the foam which, even though it tastes a bit bland, helps to fill out the glass first than the belly.

Just like the foam in the beer, the reactive power is to be considered even when it has no need and it is connected to the power factor which is 0,8 for the alternator of a generator.

The power factor is the ratio between the real power vector module flowing to the electrical load and the apparent power in the circuit.

Therefore, to determine the real power of a generator we just need to multiply the kVA value by 0,8 and vice versa divide by 0,8 to switch from kW to kVA:

kW = kVA * 0,8

kVA = kW / 0,8

Example: To which nominal kW does a 20kVA generator correspond? You should multiply 20 x 0,8 to obtain the real power in kW which is 16kW.

The term “apparent” also gives the idea of something that looks like but it isn’t that.

If the reactive power is almost considered as waste why then the apparent power is taking into consideration when talking about diesel generators?

This is a common question that I have always heard because, in fact, it seems not to make sense taking into consideration the apparent power when looking for the real one.

The answer is simple: The apparent power, being connected to the current value present in the circuit, is useful. It can be considered as the maximum value of real power that we can obtain by reversing the discrepancy between voltage and current.

Remember the beer example? Well, imagine you are reducing the foam quantity in the glass and refill it with other beer!

The real and apparent power values get close until they become equal when the power factor tends to be 1. This happens when the generators powers a resistive load.

For the single phase generating sets with 230V voltage, there is no discrepancy therefore, considering a power factor 1, the real power will be the only useful power to be considered.

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