Generators – Diesel, petrol, AVRs, ATS and AMF
Generators – what to do and what to get?
With most of the first and third world energy problems raising it’s ugly head more just a few times over the last few years it’s not surprising that many are turning to alternative means of energy: PV cells, batteries, inverters, grid and non-grid tied systems and the more popular petrol and diesel generators. The first thing one needs to know is that a licenced electrician is the only person allowed to touch the distribution board of your house, whether it be for modification or repair. Your family’s life depends on this, insurance companies will want to know this and if someone’s life is lost through negligence, the police will want to know this. Never, ever, connect and/or run a generator onto your home supply by yourself unless you are a licenced electrician. Always ensure on a certificate upon completion.
ATS and AMF installed on generators
The ATS or auto transfer switch is similar to the AMF or auto mains failure system where the genset can start itself and switch-over power when there is deep sagging, over-voltage or just plain blackout of utility power. Sound simple? It isn’t.
Utility power and grid
Your utility power runs through a distribution system or network (grid) and there are many parameters which determine where the power goes to, what the voltage should be, how many phases and what current the conductors should be able to carry. This includes therefore switchgear, contactors and huge transformers for stepping up or down the voltages. Because this forms part of a network, the supply will always be live if the network is supplied from the utility generators and the switches are closed. Although certain parts of the world only run on 100 or 110V, this supply is still lethal. High voltage linesmen get paid a lot of money for this very reason. They put their lives at risk each and every time they need to repair the lines.
The home generator is an extremely handy tool in the right hands, not so in the hands of anyone unskilled. An ATS system for your home generator is ideal if it can run supervised but never unsupervised. Large industrial generators have over-speed, under-speed, current trips, over-voltage, under-voltage, temperature and often synchro panels – they are installed by very skilled personnel and therefore need to be repaired by skilled personnel. The motor or prime mover delivers often hundreds of kilowatts at full power and needs to be extremely reliable. Utility power is often driven by nuclear, hydro, coal, wind and solar. They can deliver hundreds of megawatts. Coal firing supplies deliver upwards of 1 000MW at one site alone. Highly qualified electrical and mechanical engineers design and build these supplies to make it safe for everyone but still things go wrong. Hydro electric plants are still the biggest in the world, coal being pretty low down on the list at nearly 6 000MW. These powers stations comprise of multiples of alternators to get this combined power output.
Home generators are mostly single phase and are usually found from 1kW up to 10kW, enough for the necessities and 20kW to 30kW to supply the entire household comfortably. The larger generator sets have more safety features built in, as well as supplying three phase power – for the DIYer it would therefore seem feasible to build their own auto shut-off and fail-safe systems for the engine. Although in most cases the DIYer will build his own AMF or ATS panel it may not be legal. In fact this is highly unrecommended. Many companies sell interlocked contactors which by design prohibit backfeed onto the utility lines – these may prove to be illegal as well. Motorised rotary switches are becoming more commonplace (and possibly the only legal entrant) as this isolates the chances of relay contacts welding themselves closed. Backfeed is when the generator feeds power back onto the grid, an extremely dangerous practice, especially for any linesmen con ducting any repair suddenly finding an isolated line is now live.
Generator Voltage Regulation
The generator AVR (automatic voltage regulator) unfortunately is not very sophisticated in smaller gensets. This is made worse in smaller petrol engine prime movers – torque is low at lower RPM usually 3000 or 3600 RPM for 50 or 60Hz. When running the generator, fast acting transient loads cause the generator to hunt, easily heard by the generator changing speed – up and down. Over rating the generator for bare necessities by 100% or switching to diesel generators which have higher toque at lower RPM is often the best way to go. Petrol generators are cheaper to over-rate than diesel, they are also easier to move around. Diesel generators are supposedly more reliable, definitely much heavier but use less fuel for a given kW rating. Gensets should have surge protection and GFCI Ground Fault Circuit Interrupter) built in – if not, this is your first stop. Earthing and safety should always be on the top of the to do list. Surge protection is a simple enough modification, not so with the GFCI. Some generator sets have bonded earth and neutral which does not allow imbalance detection – the current through the live must equal that in the return path, the neutral. An imbalance often causes problems for the electrician – the home owner may just disconnect the earthing entirely. A very dangerous step to take. A qualified electrician has to ensure that the wiring is correct and legal. Which includes your family’s safety.
Safety circuits for the motor
Small generator engines should have the following safety circuits built in as a matter of course, if not, do the necessary changes if you have the skillset:
- Low oil pressure – shut off.
- Incorrect mains frequency – shut off.
- High and Low voltage – shut off.
- High temp – shut off.
Over and above this keep an approved fire extinguisher nearby for the designed purpose. Usually CO2.
If the generator is powering the house through a changeover switch ensure that the cabling to the main DB can carry the maximum load current and non-essential circuits are switched off automatically. This includes the stove, geyser and pumps. As this will be done by the electrician in any event, do request that the highest current cabling that you can afford be put in place for future upgrades.
Depending on where you live sometimes having a generator outside is impractical (theft) – know your region bylaws and legislation and ensure that exhaust emissions are outside. Carbon monoxide detection circuits should also be mandatory if the genset is positioned in the garage (legally).
Insurance companies often do not pay out when there is a line surge. Ensure that all equipment have surge protectors. Low line voltages can causes the fridge compressor to burn out. Although your plasma and LCD panels may have switched mode power supplies which can take large voltage swings, voltage surges and spikes will way exceed manufacturer ratings.
More UPS are damaged through line surges than anything else. A common myth is that online UPS are better than line-interactive UPS for mission critical power supplies – the online UPS or double conversion UPS can boost and buck a supply rail more effectively than a line interactive unit but a surge remains a surge. Because of this, more often than not the more expensive online UPS is connected to a generator which then fails because the generator or the power utility has a huge surge on start up or switchover. UPS input power must only be applied when the line voltage is stable.
Important note: The UPS has a modified sine wave which is not recommended to drive inductive loads e.g. transformers and drilling machines. Use a pure sine wave inverter for this purpose. This is a fairly comprehensive and technical subject which will be covered in another article.
Very tiny gensets, in the 800W to 100W range should be modified to feed through a mains transformer, stepped down to 12V, rectified, filtered to feed a deep cycle battery which in turn runs an inverter to power a TV set or low power appliance. These gensets are otherwise perfect for small drilling machines or non-sensitive equipment. Definitely not a plasma or any panel screen.
A 170kVA generator was installed at a premises where power was known to be unstable. The lighting inside the premises suddenly started glowing very brightly, the generator started and then shut down, restarted and then shut down again. This happened quite a few times before one of the staff pushed the kill-switch and isolated the building power. The mains voltage read 258V in a 230V area. A typical example of the designer not following the ‘what if’ scenario. The generator was designed to start up on high or low utility power. However the sense circuit detected mains incoming which included generator power. on switch-over. This same generator initially never started automatically because the battery always ran down. The alternator field was coupled directly to the battery, was not isolated through switching, neither had a charge lamp.
In most cases your best bet electrician for genset installations are ex-personnel from large industry, including mining and marine. Experience is king, not the text book theory.
As an alternative: DC to DC
An auto or car alternator, 12V or 24V can be a very versatile device. It is small and because of this can be mounted at any angle without taking too much space. Using any petrol or diesel motor of possibly around 1kW one can power an inverter from the alternator in cases of emergency whilst keeping the battery charged. Use a deep cycle battery, original AVR and lawnmower chassis of your Briggs and Stratton, a V-belt (same ratio as your car, for every 1000 RPM the alternator runs at 2x or 3x) and your tried and trusted B&S you have at least 50A to feed an inverter or directly to 12V LED lighting in your house. Recommended to use 12V for most small voltage appliances through a linear regulator if need be. When the generator is off one still has the battery to feed the alternator or 12V load. The 12V battery can also be fed from a solar panel or two. This may sound like a cheap and nasty solution to many problems but it is not – it is very effective and extremely reliable. Most parts can be obtained 2nd hand but always use a new battery. Useful post.
Generators are cheap but have high running and maintenance costs – they will never compete with the utility kW/hr rates but are ideal under blackout conditions and in critical circumstances. Solar energy is great when and where there is high absorption throughout the year but setup costs remain extremely high. The solution is to provide light first and then look at energy demand. The DIYer then comes into his own. Natural gas is not always a solution, especially when the gas is bottled and not piped. In most cases natural gas supply is low during an energy crises and natural catastrophes cause more mayhem . Battery power means lead acid secondary cells, tried and tested but never trusted. We may believe in an accumulative solution of all these forms of energy but right now the biggest money spinner is in the sale of portable petrol, gas and diesel generators.
- Read up on portable generator power here.
As the earth spins and there is a magnetic field one can only wonder….
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