HYDROELECTRIC POWER STATIONS - STELR

HYDROELECTRIC POWER STATIONS - STELR DEVILS GATE HYDROELECTRIC POWER STATION Figure 1, Outside Devils Gate Power Station. Devils Gate Power Station is situated near the foot of the Great Western Tiers, the mountain range in northern Tasmania that includes the well-known Cradle Mountain. It is nestled in a steep, picturesque valley, not very far from Devonport, a major town in north-west Tasmania. When you stand out the front of the power station, it is hard to imagine that the building houses so much inside. Before we look inside, it is important to know what actually happens at a hydroelectric power station. How hydroelectric power stations work Figure 2 shows how most hydroelectric power stations work. Generally water pours down from a dam through special wide pipes, known as penstocks, into the power station. The fast-running water sets the blades of a turbine spinning very, very fast. The water is then discharged into the river below the dam. The spinning turbine causes magnets that are surrounded by wire coils inside a generator to spin at very high speed. This induces (causes) an electrical current in the wires. The process of inducing an electrical current in wires is called induction. The power station may not be right up against the dam wall, as shown in Figure 2. The position of the power station, and the arrangement for carrying the water into the power station, depends on the terrain. The water may first run through canals (giant underground tunnels), or down penstocks that are located over a steep slope, before entering the power station. Figure 2. A simplified diagram of a hydroelectric power station. An animation of how hydroelectric power stations work For an excellent animation on how hydroelectric power stations work, go to the following website. Scroll over the diagram for more information. Before we go inside Devils Gate Power Station, let us view the dam that holds its water supply. Devils Gate Dam Figure 3 shows a photograph of Devils Gate Dam. You can see that the dam wall, which is 84 m high, is not the same shape as the one depicted in Figure 2. Instead, it is ‘double-curved’ to give it the strength to hold back the huge volume of water behind it without needing to be too thick. (This is known as a concrete arch dam.) Although they cannot be seen in the photograph, the penstock inlets are located a few metres down from the top of the dam wall. The dam wall also has a spillway, which will release excess water in the case of a flood, so that the pressure of water against the dam wall is never too great. Figure 3. Devils Gate Dam. If you view the water from above the dam, you can see that there are lines strung across the water, held up by flotation devices, to prevent any logs or tree branches that have floated down the river from entering the penstocks. These can be seen more clearly in Figure 4. To further prevent objects from entering the penstocks, there is a huge metal grid across their underwater opening in the dam. This grid, which is similar to a vertical cattle grid, is called a trash rack. In addition, a huge device known as a trash rack rake can be lowered from above the dam wall to the bottom of the rocks. It scoops up debris that has collected around the trash rack and pulls it up out of the water so that it can be disposed of. As well, there is an intake gate that is closed during any emergency to stop any water entering the penstock. These precautions are really important because they prevent blockages in the penstocks and protect the turbine from damage. Figure 4. The line of barriers that prevent floating objects from entering the penstocks. Notice the boat in the background. Where does the water come from? The water that flows into Devils Gate Dam starts its journey high up in the Great Western Tiers. It the Mersey, Forth, Fisher and Wilmot rivers. This flows down their slopes in four different rivers – region is known as the Mersey Forth Catchment. Devils Gate Dam is at the lower end of Lake Barrington. By the time it reaches Lake Barrington, the water has already passed through giant tunnels and penstocks into six other hydroelectric power stations. This system will be discussed later. Lake Barrington is renowned across the world as a rowing course of international standard. It was the venue for the 1990 World Rowing Championships and the rowing events for the 2000 Sydney Olympics, and is still used today for major events. The lanes are shown in the photograph of the lake, (see Figure 5). Figure 5. Lake Barrington. Inside Devils Gate Power Station One of the first things you notice when you enter the power station is that no-one works there! It is operated from a control centre in Hobart, which is hundreds of kilometres away. The control centre controls the power output of the power station by controlling how much water flows from the lake down into the giant turbine in each second. In the matter of a minute this power station can go from generating no electricity at all to generating up to 63 MW (megawatts). This is 63 million watts! The amount of power that is ‘commanded’ from the power station depends on how much the power supply companies in Tasmania and the Australian mainland are demanding at that time. A submarine (undersea) cable that lies across Bass Strait enables electricity to be sent and received between Tasmania and Victoria. This cable is 290 km long, and is the second longest submarine cable in the world! The cable, together with the system of overhead and underground power lines in Tasmania and Victoria that transmit the electricity to and from the cable, is known as Basslink. It connects Tasmania to the National Electricity Market (NEM). Remote control of the power station is only possible because the power station is set up and programmed to detect any problems and take suitable action immediately, and to respond to instructions from the control centre. However, that does not mean to say no-one is ever there. A specially trained hydroelectric engineer visits it regularly to inspect it and check its systems. The turbine A turbine is a machine that consists of a set of blades, ‘scoops’ or rotors that spin very fast when pushed by fast-moving air, water or steam. There are different designs for turbines. The kind of turbine used in a particular hydroelectric power station depends on the pressure of the water that will be flowing into the turbine. This pressure principally depends on the height of the water above the turbine. Since water flows into the turbine in Devils Gate Power Station at a medium pressure, a Francis turbine is used. A schematic diagram of this kind of turbine is shown in in Figure 6. The Francis turbine used in Devils Gate Power Station is huge. It has 16 blades and weighs about 24 tonnes! Figure 6. A schematic diagram of a Francis turbine. As stated earlier, Devils Gate Power Station is deceptively small from the outside. Inside it goes deep down over many levels. To view the turbine from an inspection opening, shown in Figure 7, the hydroelectric engineer or technician has to climb down many stairs and then a ladder to one of the lowest levels. Of course, many strict safety precautions must be taken in this area. As shown in Figure 2, the turbine is located in the centre of the working part of the power station. Water rushes into a huge snail-like channel, known as a spiral casing. This channel is 4.2 m in diameter where the water enters, but gets narrower and narrower, ending up with a diameter of 2.1 m. The water then pours from the channel into the turbine from every direction, giving the blades a huge push. When the maximum power of 63 MW is being generated, 102 cubic metres of water are flowing through the turbine every second! The fast-flowing water makes the turbine blades spin. However, the rate at which they spin is carefully controlled. Their normal running speed is 16 revolutions per minute. (This is classified as a slow- revving turbine.) As you can imagine, when the turbine is in operation, the roar of the water is absolutely deafening! Figure 7. The walkway to the inspection opening for the turbine. The opening is just to the left of the short ladder in the centre of the photograph. When you look inside this opening, the turbine is below and the driving shaft that connects the turbine to the generator above it is right in front of you. This photograph was taken when the turbine was not operating. More about turbines in hydroelectric power stations If the pressure of the water flowing through the turbine is high, a Pelton turbine is used. The orange and yellow scoops in the foreground of Figure 8 are part of a Pelton turbine. (The outer casing of the turbine has been removed to show its design.) Figure 8. Pelton turbines at the Waddamana Power Station in Tasmania. This power station is no longer in operation. It has been converted to a museum. The generator The generator is situated directly above the turbine, as shown in Figure 2, and is connected to it by a huge vertical solid cylinder of metal known as a driving shaft. The driving shaft is surrounded by a protective solid metal ‘cage’. The shaft can only be seen through the inspection opening to the turbine in the ‘cage’. (See Figure 7.) The generator consists of a central part that rotates, called the rotor, and a stationary part around the outside of this, called the stator. The rotor is 6.5 m across, weighs about 26 tonnes and contains 36 electromagnets (known in the industry as poles). The stator weighs about 120 tonnes and contains huge coils of copper wire. The total length of wire in the coils is almost 3 km! The spinning turbine makes the driving shaft spin as well. This makes the magnets spin around, which induces an electrical current in the wires. When working at capacity, the electrical current ‘produced’ is 3 935 amps at 11 000 volts. When the generator is working, it too makes a huge noise. The transformers Transformers are devices that change the voltage of an electrical current. An alternating current, or AC, keeps switching the direction in which it flows. This is the most efficient kind of current for carrying electricity along power lines. It is the type of electrical current supplied to the power points used in buildings. (People inside the buildings can use a small transformer to transform it into the type of current and the voltage suitable for purposes such as charging their mobile phones.) Well-protected cables carry the 11 000 V AC current from the generator inside the power station to the two transformers outside at the back of the power station. These ‘step-up’ the voltage from 11 000 volts to 110 000 volts. The current is then transmitted along high voltage power lines. The transformers are enclosed within a high security fence for safety. They are shown in Figure 9. Figure 9. The transformers at Devils Gate Power Station. Transmitting the electricity High voltage power lines transmit the electricity generated at the power station across the countryside to where it is needed. (See Figure 10.) Figure 10. A high voltage power line from Devils Gate Power Station. Batteries in a power station? At first it is surprising to see huge banks of batteries inside the Devils Gate Power Station. Why would you need them when the power station is connected to the state power grid? Couldn’t the power grid supply electrical power for all its control systems, computers, lighting, and so on, when the power station is not generating electricity? The reason for the batteries is that the power station must always have power. Should there be an interruption to its power supply, which might be caused by a bushfire, for example, the batteries can deliver the power needed to fully control the power station. The Mersey Forth Catchment Figure 11 shows a diagram of the Mersey Forth Catchment. As the diagram shows, the Devils Gate Power Station is the seventh of eight hydroelectric power stations along this river system. These include the ‘mini-hydro’ at Parangana. There is only one more power station after it before the water runs into Bass Strait. The series of power stations is designed to make the most of the power available from this river system. This is known as a run-of-river system, because the water keeps flowing along it. Water is not stored in the dams for long periods of time. . Figure 11. The Mersey Forth Catchment. For an enlarged view of Figure 11, click on the small copy of it at: OTHER HYDROELECTRIC POWER STATIONS IN TASMANIA Tasmania is a very mountainous island, so its terrain is ideally suited for building hydroelectric power stations. It has six major water catchments and thirty hydroelectric power stations with a total capacity of over 2600 MW. The first of Tasmania’s power stations was built at Waddamana (Figure 8), which started operating in 1916. Most of these power stations were developed by the Hydroelectric Commission. Hydro Tasmania, has developed and manages two large wind farms in northern Tasmania and integrates wind power with other energy sources on King Island.in Bass Strait. The energy supply on King Island Visit for more information about the integrated energy supply on King Island. Hydro Tasmania also provides expert advice and technical support through its consultancy business, Entura, to other companies that are establishing hydroelectric power stations in other Australian states and in other countries. THE ‘ELVER LADDER’ Once they are constructed, hydroelectric power stations are renewable energy resources that do not produce greenhouse gases or toxic gases. In contrast to fossil fuel power stations, they therefore bring a long term benefit to the environment and the ecosystems within that environment. However, constructing dams and power stations along a river system sets up barriers that change the natural flow of water down the system. This affects many species that live in the river. For example, the barriers can prevent the mass migration each spring of native fish species such as eels, which travel upstream from salt water to fresh water as part of their life cycle. One way in which Hydro Tasmania has acted to reduce the impact of its dams is its elver ladder at Lake Trevallyn. An elver is a juvenile (young) eel. Elvers are natural climbers. At Trevallyn Dam they are attracted to water coming from a valve at the toe (bottom edge) of the dam. Near the valve is a drain out of which water flows. Hydro Tasmania has constructed an aluminium ramp to enable the elvers reach the entrance to the drain. (See Figure 12.) Figure 12. Trevallyn Dam. The elver ladder is located just to the right of where the water is pouring out from the toe of the dam, which is on the far left of this photograph. Inside the dam’s ‘access gallery’, an elver ladder has been built along the gallery wall to further assist their migration (Figure 13). Figure 13. The elver ladder is along the wall on the left. It is a 120 m long, 150 mm wide metal channel. It climbs about 30 m in height over its length. Figure 14 shows the elvers moving along the ladder. It is lined with a special drainage material that has bumps on it. These bumps give the elvers the traction they need to climb the long ladder. Figure 14 Elvers climbing the ladder, aided by the bumps on its special lining. Once they have climbed the ladder, the elvers are temporarily contained in the trap shown in Figure 15. This is automatically emptied every 10 hours. The elvers then slide down a 45 m pipe into Lake Trevallyn, then continue their journey upstream. During the elver migration season, Hydro Tasmania monitors the effectiveness of the elver ladder and trap with a webcam. Figure 15. Elvers collecting in the trap before they are released through an exit pipe on the upstream side of the dam wall.