Evans Water Engineers

EVANS ENGINEERING

TRECARRELL MILL · TREBULLETT · LAUNCESTON · CORNWALL · UK · PL15 9QE
+44 1566 782285 · www.evans-engineering.co.uk · email sales@evans-engineering.co.uk

Power systems & Control

 

Selecting the system that is most suited for your site and needs, involves several areas of design. These include overall site layout, capacity and type equipment to be installed.

The site layout may be compact and the power may be distributed as heat, electricity or mechanically. Alternatively the end use may be several miles from the powerhouse and this will necessitate the use of high voltage transmission and there may be one or several end users.

Governing is the system that is required to ensure that the plant rotates at the correct R.P.M. to give the best efficiency and to maintain the correct voltage and frequency if the plant is used for electricity generation. The governing system can be based on the water going into the turbine or the power coming out of it.

Flow control is the system that is required to match the power demand and the available water. A number of separate water jets in the same machine or separate machines may be turned on and off in combinations and permutations to match the power or water requirements, or the geometry within the turbine is altered to change the water flowing through the turbine and hence the power it produces.

The end-use to which the power is going to be put is most important and many of the other systems including the governing and flow control system will have to be selected with the nature of the load in mind. Rapid changes in load such as the starting of electric motors may have to be catered for by shedding other loads, providing a flywheel or signalling the impending load change to the power-house prior to the application of the load.

Mechanical systems are used for driving simple agricultural machinery, low voltage generators, pumps, heat pumps and direct heating devices. The output may be non-governed and the turbine starts and stops when the water is turned on and off by means of a sluice or valve. Alternatively the speed of the turbine is governed, either by controlling the water flow or by using a mechanical device in the transmission from the turbine.

Electrical systems can be divided into four main areas, very basic, small synchronous, small induction and commercial scale. The voltage, frequency and whether a single or three-phase system will need to be decided.

Our basic system uses a simple alternator to provide constant voltage at variable speed. This can be used for battery charging, low voltage lighting or any application where variable frequency presents no problem. If a permanent magnet alternator is used, there needs to be a governing system that keeps the speed substantially constant, or the output needs to be ‘conditioned’ to connect to the load.

Small synchronous systems are built using brushless single-phase alternators up to about 50kW. These are generally fitted with our ‘Electronic Load Governor’ to give a controlled 50 or 60 Hz supply. The plant has the same limitations as any petrol or diesel plant in that it cannot be overloaded without bringing down the voltage and frequency, which may cause damage to the plant or connected appliances. Change over systems can be supplied but it is not generally worth the expense of paralleling to the mains system.

Small induction systems of both single and three-phase design can be used in parallel with the mains supply to carry your base load. With such a system it is not necessary to have dual wiring or changeover switches. The generator is simply switched in as you would an electric motor and as the turbine power is increased it pushes more power into your system.

G59 or G82 Protection will be required in the UK if any connection is to be made to the ‘Grid System’. Relays are required to disconnect the generator in the event of a mains failure, over or under voltage, over or under frequency, and to protect against ‘islanding’ (a situation where a section of the grid becomes isolated from the rest of the grid and is supplied from the hydro plant).

An automatic shutdown system should be fitted on all but the most basic plants. This system should be fail-safe and operated by gravity, spring or pressurised fluid, so that in the event of an electrical fault the plant will still come to a stop. Electrically driven actuators cannot be relied upon because they will require a battery system.

If your requirements are less than what you are generating then the surplus will be pushed back into the ‘grid’ system. It is seldom worth installing metering equipment for the sale of small amounts of power, so it is easier to monitor the flow of power and to switch on non-essential loads automatically.

Induction or Asynchronous generators are very similar if not identical to induction motors. The latter run at slightly less than synchronous speed (e.g.: 1450 R.P.M. for a four pole 50hz machine where the synchronous speed would be 1500 R.P.M.) As a generator it would have to be driven faster with ‘Positive Slip’ to about 1550 R.P.M. The frequency is determined by the incoming mains supply and not the speed of the generator. The characteristics of the generator, any capacitors and the network, will determine exactly what happens to the voltage and power factor when the generator is connected and loaded. There are a number of ways that these generators can be connected to the network, ranging from a simple contactor to a full ‘Soft Start ‘unit. The main objective is to connect to the ‘grid’ with as little disturbance as possible.

Synchronous generators are capable of generating electricity whether they are connected to the network or not. They are therefore potentially more dangerous should they stay connected to the network during a fault, since the generator side of the fault could stay live unknown to engineers working on the line. In addition to the relays required for an asynchronous generator the electricity supply-company will probably require a relay that detects symmetry and the rate of change of the monitored parameters. Synchronous machines are much more flexible in their use and can provide a useful function apart from the power produced. Long rural distribution lines, particularly in Developing Countries, a prone to very low power factors and synchronous generators of suitable capacity can be used to export VArs and correct the power factor, which in turn reduces the system losses. Connecting a synchronous generator to a network is more complicated than connecting an asynchronous machine since both sides are live. We have several methods depending on the type of turbine and how easy it is to control. Impulse turbines with jet deflectors can be treated much like diesel plants, in that the power can be changed quickly without any consideration to the water flow. Electronic load control can be used in a similar way but the cost becomes significant on larger powers. A combination of ‘Soft Connection’ and ‘Soft Drive’ gives the most flexible and economic approach for machines between 50kW and 1MW.

The end users skills, requirements and resources all have to be considered. Manual control systems can sometimes be used if the load on the plant is substantially constant or if the operator works or lives adjacent to the plant. Many small waterpower plants work with a fixed load or one that is adjusted to the water available. A voltage regulator that reduces the excitation as the speed rises above the rated speed will overcome the danger of over voltage. If there are several end users and it is necessary to keep the frequency constant then a governing system will be required. Most plants operate unattended but it is still necessary to know if a fault is developing. If it is not easy to reach the powerhouse because it is remote or buried under snow, communications can be sent via the power cables or a separate communications cable. This and other more sophisticated load and plant management systems are usually confined to plants over about 100kw. Long periods of un-attended operation will require very simple robust engineering that is not adversely effected by such things as damp or overheating. Alternatively a sophisticated monitoring systems will be required that will stop and then restart the plant when the fault has cleared.

The power produced at the turbine shaft will be about 80% of the theoretical power in the water. If the turbine is used to belt drive a generator 5% will be lost in the drive and about 15% in a small generator, giving a typical overall efficiency of 65%. Higher efficiencies are possible with direct drive and careful design but it may not be cost effective with very small installations.

If electricity is generated it is necessary to keep the voltage and frequency within reasonable limits. A suitable ‘automatic voltage regulator’ or AVR can be supplied to keep the voltage substantially constant regardless of the generator speed. The turbine speed will change according to the electrical load applied unless a governor is used. For many application ‘Constant Voltage Variable Frequency’ is cheap, safe and perfectly adequate for domestic and small industrial uses. The use of permanent magnet generators is more difficult because the field cannot be regulated.

Electronic Load Governing was developed and patented by us in 1976 and is now used throughout the world to control water turbines. It works by matching the power of the turbine to the electrical load on the generator so that the speed remains constant. We manufacture many systems, each matched to the site and application.

Grid connection using a simple induction generator can be very convenient where loads are constantly changing and the switch-gear and wiring is to be minimised. Such installations can provide the base load, only importing for short times when load is high. Exporting surplus when demand is low can be economic with larger plants over about 50 kW but the extra cost of metering and administration can reduce the returns substantially. It may be an important consideration that power can be maintained to the end user during ‘power cuts’, and this will require a synchronous generator.

Direct use of the power for drive machinery and pumps is feasible but less common today, but producing heat with a heat generator or a heat pump can be very attractive. The latter is roughly twice as expensive to build and an electricity producing plant but delivers at least three times the energy in the form of heat. We have over ten years of experience in this field and in 1994 we completed a plant with an output of over 60 kW.

Making use of the power 24 hours a day is essential if a small plant is to be economic. Lighting and domestic appliances will typically account for only 10 % of the power produced. Off-peak water heating, space heating and heat storage cooking can be used to level the load and make better use of the output of the plant. It may not matter if you cannot produce all of your power all of the time so long as what you do produce is economic. Reducing your total fuel and electricity bill to 30% may be a realistic target.

The economics of an installation can be looked at in many ways depending on the source of the finance and an individuals tax situation. The return is usually taken as the value of the energy produced or replaced whether it is electricity or fuel oil compared with the capital cost. Since electricity is the most expensive energy source you should aim at replacing it first. Most domestic installations pay for themselves in less than ten years, equating to return of 12 to 15%.

The quality of the installation will reflect the amount of maintenance and life expectancy of the equipment. A design life of thirty years and a maintenance interval of ten years is a good yard stick to work from. We offer long guarantees and maintenance contracts if required, though most customers after a little training are happy to carry out their own.