New electric generator takes a giant step back to basics

Electric generators have remained essentially unchanged for nearly a century – until today that is. Progress has obviously been made in areas such as insulation materials and production technology, but the basic generator concept has remained the same. Now ABB has developed a radically new electrical machine which rests principally on the fundamentals of nature, as described in the Maxwell equations, rather than on traditions emanating from the turn of the century, when insulation technology was still in its infancy. The new concept is based on circular conductors for the stator winding and it is implemented by using proven high-voltage cable technology.*

The immediate consequence of the new solution is the possibility of increasing the output voltage of the generator from today's maximum of about 25 kV to the level of the rating of the available power cables, which presently is of the order of 400 kV. The effects of this include the possibility of directly connecting the generator to the power grid without a step-up transformer and thereby eliminating the associated losses and costs. This new machine has been named Powerformer since it incorporates the functionality of both a generator and a transformer.

An issue that has permeated the development process of Powerformer is the importance of keeping safely within the limits of the ratings of the cables used and using standard components, for example for the cable joints, in order to provide assurance to the user the system will be reliable.

The new concept is suitable both for hydropower generators and for thermal power generators. The first generator to feature the new concept is a hydropower generator that has already been successfully factory tested, and will be commissioned during the spring of 1998 at the Porjus Hydro Power Centre in northern Sweden.

Fundamentals

In the mid 19th century, J. C. Maxwell managed to describe fully the phenomena of electricity and magnetism in a single system of equations, commonly known as the Maxwell equations. These equations contain the basis for rotating electric machines, for example Faraday's induction law.

For a rectangular conductor geometry, solutions to the Maxwell equations for the electric as well as the magnetic field have infinitely high field strengths at the corners of the conductor. A cylindrical conductor geometry, however, yields a smooth electric and magnetic field distribution, which is a prerequisite for a high voltage electric machine. The practical consequence of having a rectangular conductor in an electric machine is that the insulation material and the magnetic material of the machine are highly stressed and not uniformly loaded, which leads to uneconomical use of the materials involved. Failures in the machine related to the high stress of the materials are also very likely to occur. Such severe drawbacks are avoided when using circular conductors as these allow optimal use of insulation material due to the associated smooth field distribution.

So from a physics point of view, round conductors are the natural choice for the stator windings of an electric machine. However, engineering problems have until now led the development in another direction. To raise the output power of an electric machine, either the level of the output voltage or the current in the stator windings must be increased. When more output power was required from generators in the beginning of this century engineers, limited by the insulation technology at that time, had to raise the current in the machine instead of increasing the output voltage.

Maximising current loading in the machine without regard to the insulation aspects favours the use of rectangular conductors in order to obtain maximum copper packing density for the stator winding. However, as already noted, the geometry of rectangular conductors limits the maximum attainable voltage because of the uneven field distribution, with high field concentrations in the corners of the conductors. Such field concentrations have prevented the output voltage of generators from reaching levels higher than 36 kV even though insulation technology has developed considerably during this century.

What is required as a stator winding for a high voltage electric machine is a round conductor with a modern insulation system. This, coupled with the requirement to stick with proven technology for reliability, points to using something similar to modern high-voltage cables for the windings of electrical machines.

A typical standard power cable comprises a conductor composed of uninsulated strands around which there is a layer of semiconducting material for smoothing the electric field close to the strands (note: in this context the term semiconductor is used for describing a material with relatively high resistivity, but which more accurately should be denoted a resistive conductor). On the outside of the semiconducting layer there is solid insulation typically consisting of cross-linked polyethylene (XLPE), which in turn is surrounded by a second semiconducting layer. The outermost layers of a standard power cable consist of a metallic screen and a sheath which are finally surrounded by a plastic sheath.

A cable adapted for use as a winding in an electric machine, i.e. there are both uninsulated and insulated strands for minimising eddy-current losses and the semiconducting layer surrounding the insulation forms the outermost layer. For such a cable, it can be shown that the electric field is fully confined to the cable and that the thickness of insulation can be optimised easily using simple calculations. As already mentioned, an electric machine constructed with such a cable-based winding can provide extremely high output voltages and insulation failures and accidents can be avoided.

An analogue analysis of the magnetic circuit shows that, by using circular conductors in the stator winding of an electric machine, losses can be reduced, temperatures lowered, and the distortion of the output voltage can be reduced.

Design of rotating machines

In the design of an electromagnetic system the four main areas are:

Previously, due to limitations in the insulation systems, it was not possible to consider these elements independently of each other. With the new winding, however, these aspects may be treated separately, giving designers far better possibilities for optimising the performance of the machine. The independent treatment of, for example, the electrical design is made possible, on the one hand by the round conductors, which result in a smooth field distribution thus minimising the stress on the insulation material, and, on the other hand, by the semiconducting layers on each side of the solid insulation which confine the electric field to the winding. The thermal design is facilitated since cooling may be accomplished at earth potential. The four traditional design areas are discussed briefly below.

Electric design: The insulated conductors used for the stator windings of Powerformer are, as referred to earlier, based on proven high voltage cable technology. Currently, XLPE insulated cables are available for voltages up to 400 kV. This means that it is possible to design rotating electric machines, using the new cable concept for the stator winding, with output voltages of up to 400 kV. Therefore, Powerformer may be connected directly to the power grid without a step-up transformer. As mentioned above, an increase in the output voltage corresponds to a decrease in the loading current in the machine for a given power rating and a lower current density results in lower resistive losses in the machine.

The winding of Powerformer is a multilayer concentric winding where the potential along the winding increases with each turn starting from the inside of the stator and increasing towards the periphery. Accordingly, the demands made on the insulation thickness increase towards the periphery. It is therefore possible to use thin insulation for the first turns and then increasingly thick insulation for the subsequent turns. This arrangement makes it possible to optimise the use of the stator core volume.

The electric design is further facilitated by the practically zero electric field outside the cable. For example, there is no need to control the electric field in the coil-end region as is the case with conventional machine technology. Personnel safety is also increased since the entire outer semiconducting layer of the cable can be held at earth potential.

For jointing and terminating the cables in the winding of Powerformer, readily available standard components are used.

Magnetic design: The stator of Powerformer consists of a laminated core, built up from sheet steel. The stator slots are radially distributed cylindrical bores running axially through the stator and joined by narrow sub-slots. The round shape of these cable ducts, corresponding to the shape of the cable, is also preferable with respect to the Maxwell equations since the roundness results in an even distribution of the magnetic field.

As a consequence, the loss in the generator is reduced and the output voltage contains less harmonics. Since, as already mentioned, the cable diameter decreases towards the rotor due to the decreasing requirements on the insulation thickness, the cross-section of the cable ducts decreases. This results in a practically constant radial width of the stator teeth (the area between the stator slots) which in turn reduces losses in the teeth.

In a conventional winding coil the conductor laminations have to be transposed along their length in order to reduce the eddy-current losses. For the new round conductor arrangement, minimisation of the eddy-current losses is achieved by using insulated and twisted strands in the cable forming the winding.

It is possible to install additional windings in the stator of Powerformer which can, for example, be used for supplying the plant with auxiliary power. Moreover, Powerformer can be used as a rotating transformer with possibilities of simultaneous connection to several grid voltages using winding taps.

Thermal design: Inherent to Powerformer is that the loading current for a given output power is considerably lower than in a conventional generator. Thus, the main part of the losses are iron losses in the core and not resistive copper losses in the winding. Consequently, the cooling requirement of Powerformer is concentrated on the core and not on the winding. Since, as already mentioned, the electric field is confined to the winding, the cooling of the stator core is performed at earth potential. Both liquid and gaseous media can be used for cooling.

Mechanical design: Powerformer is fitted with a conventional rotor. Therefore, only the stator related aspects of mechanical design will be covered here.

When designing a generator, current forces exerted onto the end windings during normal operation as well as during short circuits have to be considered in terms of the support of the windings. Due to the lower currents and current densities, the current forces in Powerformer are considerably smaller than in a conventional rotating machine. As a consequence, the support for the end windings can be made simpler in Powerformer.

In a conventional generator the stator winding is typically arranged in two layers, resulting in short stator teeth. The multilayer winding of Powerformer requires deep stator slots and thus the stator teeth are long. Mechanically this leads to lower bending stiffness in the teeth and lower resonance frequency, which are unfavourable features. By using a new type of stiff, pre-stressed slot wedge in Powerformer, problems related to the long stator teeth have been eliminated.

Construction advantages

The most prominent feature of Powerformer is its capability for generation at high output voltage: Powerformer can be connected directly to the power grid without an intermediate step-up transformer. When dispensing with the transformer, a number of advantages can be identified for the power plant and the grid.

For example, the consumption of reactive power by the transformer is avoided and the reactive power produced in the generator may be used elsewhere in the network. Additionally, the cable winding technology stretches the limits of the short term overload capability of the generator such that more reactive power can be produced in Powerformer than in a conventional generator. Other advantages resulting from the omission of the step-up transformer include a reduction of the number of components in the plant, such as medium-voltage busbars and generator breakers. Thus, maintenance costs are expected to be reduced and availability as well as reliability increased. Moreover, the layout can be made more compact for a plant equipped with Powerformer than for a conventional generator due to the exclusion of the step-up transformer and its auxiliaries.

Environmental aspects

Using life-cycle assessment (LCA), it is possible to get an overall picture of the environmental impact from a product or a system, from raw material extraction to disposal via production and use. A comparison of the environmental impact, over a lifetime of 30 years, has been carried out between two hydropower plants, rated at about 150 MVA and connected to a 130 kV grid, one with a conventional generator (including a step-up transformer) and the other with Power-

former. The results show that the system with Powerformer has a lower impact on the environment than the system with the conventional generator, due to the lower energy losses. The manufacturing process of the system with Powerformer has a lower environmental impact than that of the conventional system.

First use installation

The first generator based on the new technology, a hydropower machine rated at 11 MVA, 45 kV, and 600 r/min, has already been successfully tested in the factory. This unit is due to be commissioned in the spring of 1998 at the Porjus Hydro Power Centre in northern Sweden. The unit will be inaugurated on 10 June 1998. The Porjus Hydro Power Centre will then feature two hydropower units: Powerformer and a conventional machine, rated at 11 MVA, 10 kV, and 429 r/min. The two units are intended for research and development and for hydropower training, respectively. Both machines will deliver power to the Swedish national grid.

Giant step

Powerformer represents a giant step in electrical engineering. With the large development potential of this new technology we can look forward to an exciting future with new electrical machines and possibilities far beyond those offered by machines based on conventional technology.