14-02-2013, 12:21 PM
POWER FORMER -New trend in powersystem
POWER FORMER.doc (Size: 168 KB / Downloads: 63)
Abstract:
Power requirement is increasing day by day due to the maximum power utilization. In order to meet this demand a large power has to be generated in an efficient way. A new machine namely “Powerformer” can do this. This paper gives an over view about generation of power &transmission in an efficient manner. The conventional plant generated voltage is limited to 30KV. This voltage is stepped up to a high voltage level in the order of 800KV with the help of power step-up transformers. The step-up transformer imposes great drawbacks on the power plant as a whole, starting from reduction in efficiency, high maintenance costs, and more space, less availability and not to forget the increased environmental impact of the plant. During the last century, a number of attempts were made at developing a high-voltage generator that could be connected directly to the power grid, i.e. without going via the step-up transformer. The powerformer gives also less impact on environment.
The powerformer has opened a new chapter in the generation and transmission of electrical energy
Introduction
Today’s high-voltage generators are constructed in such a way that limits their output voltage to a maximum of 30 kV. Those generators cannot, directly supply the power grid with voltages up to 800 kV, a reason why large power plants nowadays are using power step-up transformers in order to transform their generated voltage to a higher voltage level suitable for the interface with the transmission grid.
The step-up transformer imposes great drawbacks on the power plant as a whole, starting from reduction in efficiency, high maintenance costs, and more space, less availability and not to forget the increased environmental impact of the plant.
During the last century, a number of attempts were made at developing a high-voltage generator that could be connected directly to the power grid, i.e. without going via the step-up transformer. However, although grid voltages can reach 800 kV or more, generators are presently constructed for voltages up to 30 kV only, as stated above.
ABB has developed in close co-operation with Vattenfall (the Swedish state-owned power utility) a new high-voltage generator with innovative features that enables it to be connected directly to the transmission grid; its output voltage can reach levels up to 400 kV. With the new technology, future transformer-less power plants can be constructed leading to a new concept of energy systems.
The new machine has been named Powerformer; its benefits such as higher efficiency, better availability, lower maintenance costs and reduced environmental impact are straightforward consequences of transformer-less power plants.
Powerformer Concept
Powerformer, although a new machine, it is a 3-phase AC generator with a rotor of conventional design. The difference compared with conventional generator lies in the stator windings. In Powerformer stator winding consists of high-voltage cables instead of today’s windings with a square cross-section. By using high-voltage cables as generator stator winding, it is possible to highly increase the generated voltage. The decisive difference between this design and present-day technology is that Powerformer allows direct connection to the high-voltage grid. This is illustrated in Figure (1)
Non conventional stator design
The design of the slots and teeth in the magnetic circuit plays a decisive role in the optimization of a rotating electrical machine. The slots should enclose the casing of the coil as closely as possible. At the same time, the teeth should be as broad as possible at each radial level. This reduces the losses in the machine and also the need for excitation.
The stator of Powerformer consists of a laminated core, built up from electrical sheet. Teeth in the outer section point inwards towards the rotor (at the center). The winding is located in the slots formed by the teeth, the cross section of the slots decreases towards the rotor since as already mentioned, each winding turn requires less cable insulation the closer it is to the rotor.
Each slot has circular bores at intervals, forming narrow waists between the winding layers, as depicted in Figure (4) below.
Efficiency
In general, a power plant with Powerformer has 0.5-1.5 % higher efficiency than a conventional power plant (Powerformer plants have 0.5-1.5 % less active losses than conventional plants). For a 120 MW plant the figure is about 1.5%. This means that a plant with Powerformer will produce 1.8 MW more power than a conventional plant. This extra power, obviously, improves the economy of the whole plant.
Reactive Power Capability
The generation of reactive power is needed to compensate for the reactive power losses in the transmission networks. With Powerformer the reactive power losses in the step-up transformer is eliminated. With more reactive power capability, Powerformer will become competitive alternative to traditional Reactive Power Compensators (RPC), because Powerformer, unlike traditional RPCs can be overloaded over rather long period of time, due to its robust design. This feature is desired during disturbances in high voltage transmission networks. For example neither the copper windings nor the laminated core in Powerformer are affected by any rapid temperatures, this considerably reduces the risk of damage to the generator insulation.
Conclusion
ABB’s new high-voltage generator (Powerformer) has been studied through out this seminar; the new concept provides the possibility to directly connect a rotating machine to the high-voltage power grid without going via a step-up transformer. Powerformer is a high-voltage generator; the limitation in its rated voltage is set solely by the AC
Power cable and the cable accessories (terminations and joints) used in its stator winding. This means that Powerformer of voltage ratings up to 400 kV can be realized, although has not been proved yet.