Over the past couple of decades, changes have taken place in high-voltage insulation systems that have produced safer and more reliable operations. The.
ver the past couple of decades, changes have taken place in high-voltage insulation systems that have produced safer and more reliable operations. The mica/epoxy insulation, which has been used in rotating machines for over 100 years, is now being replaced by a new concept using high-voltage cross-linked polyethylene (XLPE) cables. Three new products have been recently launched including the Powerformer, a new generator that can be directly connected to the transmission network without the need for a step-up transformer. The other products include the Dryformer, an oil-free power transformer, and the Windformer, a new wind power generating system. Due to the Powerformer’s ability to generate electricity at transmission voltage levels, it offers considerable gains with respect to reactive power production and plant efficiency. Hence, a Powerformer both facilitates network stability and decreases
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the exploitation of natural resources. The upper limit for the output voltage from the Powerformer is set by state-of-the-art XLPE power-cable technology. Therefore, Powerformers revolutionize the age of old power generation technology and signal a quantum leap in electrical engineering.
High-voltage vision Conventional high-voltage generators are constructed in a way that limits their output voltage to a maximum of 30 kV. The power grids with voltages up to 1,100 kV cannot be directly supplied by these generators, which is a reason large power plants use power step-up transformers in order to transform their generated voltage to a higher voltage level suitable for the interface with the transmission grid. Step-up transformers impose significant drawbacks on the power plant as a whole, starting from reduction in efficiency, high maintenance costs, more space, less availability, and an increased environmental impact. During the last century, a number of attempts were made at developing a high-voltage generator Powerformer that could be connected directly to the power grid, without going via the step-up transformer (see Fig. 1). Recently, a Powerformer IBRAHIM A. was developed with innovMETWALLY, R.M. ative features that enable it RADWAN, AND to connect directly to the A.M. ABOU-ELYAZIED transmission grid. Fig. 2 shows how the rated voltage of the Powerformer increased during the late 1990s. It is expected in the coming decade that its output voltage will reach up to 420 kV. High-voltage cables with XLPE insulation are available today for voltages of up to 500 kV. When XLPE-insulated cables were introduced in the 1960s there were some initial problems with their reliability, caused by poor control of the manufacturing processes. These problems have since been overcome, and today’s high-voltage XLPE-insulated cables have an impressive track record. Therefore, the development of the Powerformer is inherently linked to the reliability and the development of the XLPEinsulated cables. With the new technology, future transformerless power plants can be constructed,
Powerformers: A breakthrough of high-voltage power generators
© ARTVILLE
MAY/JUNE 2008
Digital Object Identifier 10.1109/MPOT.2008.915315
0278-6648/08/$25.00 © 2008 IEEE
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leading to a new concept of energy systems. The new machine Powerformer has high efficiency, lower maintenance costs, reduced environmental impact, and better availability, reliability, and robustness.
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Going against convention The Powerformer, although a new machine, is a three-phase synchronous generator with a rotor of conventional design. The basic difference, compared to the conventional generator, lies in the stator windings. With Powerformers the stator winding consists of high-voltage cables instead of the conventional rectangular cross-section windings. In order 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. Insulation technology limited the output generation voltage, so the solution was to increase the current in the machine instead of the output voltage. However, in Powerformers, the output power is increased by increasing the output voltage using XLPE cables in the stator winding. Figure 3 shows the quantities of steel and copper in the Powerformer and the reference (conventional) systems. The weights are stated as net weights per megawatt-hour of electricity produced. More copper is used in the conventional system than in the Powerformer, chiefly due to the copper content in the transformer. The Powerformer system has more steel, mainly because the Powerformer stator has 2.5 times more electrical sheeting than the conventional machine. There is considerable poten-
3 (b) Fig. 1 Schematic diagram of (a) a conventional plant with step-up transformer, and (b) the same plant with a Powerformer connected directly to the grid. 1) Generator, 2) generator circuit breaker, 3) surge arrester, 4) step-up transformer, and 5) circuit breaker.
tial for technological advances leading to lighter and smaller machines. The reference (conventional) system requires more consumption of carbon, oil, and gas than that of the Powerformer. The highest emissions originate from losses during the utilization phase. In addition, the emissions of carbon dioxide, nitric oxide, and sulfur dioxide for the conventional system are higher than that of the Powerformer.
Conventional generators The stator windings of the conventional generators consist of rectangular conductors that lie in the stator slot. The main goals of rectangular conductor shape selection are to maximize both the current loading and the filling factor. According to Maxwell’s equations, the shape of these conductors results in an uneven electric field as well as a
Expected in a Decade
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Rated Voltage, kV
350 300 250 200
420 kV 345 kV
245 kV
155 kV 136 kV
150 100
45 kV
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magnetic field distribution with values, inevitably created at each of the slot four corners, as shown in Fig. 4. This intensification of corner field dictates the use of insulation materials with very high dielectric strength (e.g., sheet mica set in epoxy resin). The practical consequence for a rectangular conductor in an electric machine is that the insulation and the magnetic materials of the machine are highly stressed and not uniformly loaded, which leads to an uneconomical use of the involved materials. Failures in the machine related to the high electric stresses on the insulating materials are also very likely to occur. Therefore, intricate measures have to be taken in the end-winding region to control the electric field so as to avoid partial discharges and corona. To minimize the eddy current losses in the stator coils, the copper laminations constituting the conductors must be transposed along the winding according to an elaborate scheme.
Powerformers Contrary to conventional generators, the windings of this new high-voltage generator have cylindrical conductors. As can be inferred from Maxwell’s equations, a cylindrical conductor yields an even electric and magnetic field distribution, which is a prerequisite for a highvoltage electric machine (see Fig. 4). As mentioned earlier, the stator winding of the Powerformer consists of highvoltage cables. Consequently, the output voltage of Powerformers is only limited by the state-of-the-art high-voltage cable technology. Recently, insulation materials and production techniques offer reliable cables at operating gradients in the order of 10 kV/mm and even more. Such high electric field is not accepted for the conventional mica/epoxy-based coil insulation. The cable circular cross section solves the two basic problems arising from the use of conventional rectangular stator windings: • First, within the stator slots, the uniform electric field in the insulator maximizes insulation performance and the voltage rating of the cable. • Second, bending a cable of circular cross section does not result in the kinks and sharp edges that arise with a rectangular cable. Thus, even in the end regions where the cable is bent to make the transition from one slot to the next, the electric field within the insulator
IEEE POTENTIALS
remains free of singularities. At the end regions of the Powerformer, the electric field remains confined within the cable. Consequently, the need to control an external electric field, as in a conventional machine, is eliminated.
MAY/JUNE 2008
0 Copper
Steel
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Fig. 3 Net weight consumption of copper and steel in Powerformer and the reference system.
the strands in the outermost layer may be non-insulated. The induced voltage in a Powerformer generator stator winding will gradually increase from the neutral point to the line terminal. Therefore, the cable used for the stator winding is accordingly exposed for different electrical stresses along the length of the winding. It is therefore feasible,
Conventional Generator
in a Powerformer, to use a thinner insulation for the first turns of the winding and thereafter increase the insulation thickness; it is called “stepped insulation” (Fig. 4). One way of obtaining this is to use a predefined number of different cable dimensions per phase (i.e., a stepwise increase in the insulation thickness). This type of graded insulation facilitates
Powerformer Cable-Winding
Stator-Bar
Ground Wall Insulation
Ground Wall/Turn Insulation
Turn Insulation Strand Insulation
Conductor
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6 kV/mm or Even Much Higher
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As previously indicated, the winding of the Powerformer consists of insulated high-voltage power cables similar to the standard and commercial power cables used in power system distribution. However, the cables in the Powerformer have neither a metallic screen nor a sheath. Figure 5 illustrates the construction of the Powerformer winding cable. It consists of a stranded conductor, an inner semiconductive layer, a solid dielectric (normally an XLPE), and finally, an outer semiconductive layer. The purpose of the inner semiconductive layer is to create a uniform electric field at the inner surface of the insulation layer, while the outer semiconductive layer acts to confine the electric field within the insulator. The word “semiconductor” describes a material with relatively high resistivity, in this case XLPE doped with carbon. Such a semiconductor is, more accurately, a resistive conductor. In general, in the stranded conductor there is a center wire surrounded by concentric layers of 12, 18, 24, 30, 36, and 42 wires. This is commonly known as a “concentric-lay” conductor. Each layer is applied with alternate direction of lay. The conductor cross section will be dimensioned with respect to the prevailing system voltage and the maximum power of the generating unit. A conductor used in an electrical machine is exposed for a higher magnetic leakage flux than a conductor used in transmission or distribution systems. In order to minimize the additional losses due to the magnetic leakage flux in the Powerformer conductors, it is necessary to subdivide the conductor into mutually insulated strands. The majority of the strands may be insulated, but to ensure an equal electrical potential of the strands and the inner semiconducting layer, one or more of
Stator Rotor Transformer Other Total
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Cable design
Reference
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The concept of Powerformers features innovations that include commercial high-voltage power cables, lie in circular bores, and are accommodated in the stator slots.
Powerformer
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Fig. 4 Stator-bar of the conventional generator and stator cable-winding of the Powerformer.
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a better optimization of the volume of the laminated stator core. Also, stepped insulation has the effect of ensuring that the tooth width is effectively constant along its length, irrespective of the radial spread, keeping the flux density constant.
Stator design
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The shape of the cross section of the rubber hose is designed to allow for elastic deformation necessary to keep the fixation forces within certain limits. The maximum force must be limited to reduce the visco-elastic deformation of the cable cross section. A minimum force has to be maintained at low temperatures to avoid loss of contact between the cable and slot wall. However, to avoid local deformation on the cable at the end-winding region due to vibration and bracing forces, the cables are separated from each other by a rubber distance element.
Fig. 5 Powerformer winding-cable: 1) The Powerformer is fitted with a conconductor, 2) inner semiconducting ventional rotor. Therefore, only the statorlayer, 3) XLPE insulation, and 4) outer related design aspects will be covered semiconducting layer. here. The Powerformer stator consists of a laminated core, built up from electrical outside the outer semiconducting layer is sheets. Teeth in the outer section point close to zero in the coil-end region. inward toward the rotor (at the center). Consequently, there is no need to conThe winding is located in the slots formed Cooling system trol the electric field in the coil-end by the teeth. The cross-section of the slots The cooling system of the region as in the conventional generator. decreases toward the rotor because each Powerformer stator core is also based In the conventional generators, the winding turn requires less cable insulation on a new concept. This is due to the field has to be controlled at several closer to the rotor. The cross section of low current in the cables of the stator locations per turn. This eliminates field the winding cable is taken into account by winding and the lower ratio between concentrations in the core, the coil-end the stator slot design. Each slot has circuohmic and iron losses than that for a regions, and the transition between lar bores at intervals, forming narrow conventional generator. Accordingly, them. There is no risk for either partial waists between the winding layers. most of the heat is generated in the stadischarges or corona in any region of The slots should enclose the casing tor core, which is grounded. This fact the winding. Moreover, personal safety of the coil as closely as possible. At the greatly simplifies the cooling system. is increased substantially as the endsame time, the teeth should be as broad The new cooling system is an indirect winding region is at ground potential. as possible at each radial level. This system that cools the stator core by axiDue to the lower currents and curreduces the losses in the machine and ally inserted water pipes made of high rent densities, the current forces in also the excitation needed. The stator density XLPE. Thus, the stator has no Powerformers are considerably smaller teeth can also be designed such that the radial air cooling ducts, and this leads than those in conventional generators. radial width of the slot is largely conto a homogeneous stator core. This As a consequence, the support for the stant over its entire length. This equalmakes the gross length of the stator end windings can be made simpler in izes the loading on the stator tooth. shorter, the efficiency improves, and the Powerformer. Another important The winding can be described as a the stator assembly is more convenient, aspect when designing a Powerformer multilayer concentric winding, which especially with respect to the cable is the minimization of the cable vibrameans that the number of coil ends installation through the slots. tion. To achieve this goal and to ensure crossing each other is minimized. This As the water cooling is carried out at good electrical contact between the feature allows simpler and faster threadground potential, there is no need for cable and the laminated core, the cable ing of the stator winding. Figure 6(a) de-ionized water as in the conventional is fixed in the slot. It is based on a triand (b) shows a sectional view of the water-cooled stator windings. Ordinary angular silicon rubber hose that is Powerformer stator and the temperature tap water may be used for the cooling inserted between cables and slot wall distribution around a stator slot. of the Powerformer stator core. The use as shown in Fig. 7. As a result of using a high-voltage of plastic tubing also eliminates the risk cable in the stator of a short circuit winding, an increase between the tubes in the output voltage and the core and 6 5 4 corresponds to a problems with eddy decrease in the currents in joints 3 loading current in and pipes. On the 3 the machine for a other hand, the given power rating. rotor and the end Therefore, a lower windings are air 2 current density cooled. results in lower resistive losses in Faulty the machine. The behavior 1 outer semiconductThe performance (a) (b) ing layer cable is of the Powerformer connected to earth Fig. 6 (a) Sectional view of the Powerformer stator: 1) rotor, 2) section of stator, 3) under fault condipotential. Hence, teeth, 4) slots, 5) main winding cable, and 6) auxiliary winding and tions, whether the the electric field fault is internal or (b) temperature distribution around a stator slot.
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IEEE POTENTIALS
external, and the comparison between the fault currents of a plant equipped with a Powerformer with the fault currents in a generating station equipped with a conventional generator and a step-up transformer have revealed many positive points in favor of the Powerformer. The internal fault refers to a fault that occurs at the terminals of the generator, and an external fault is a fault that occurs at the high-voltage side of the step-up transformer. In the case of a Powerformer, internal and external faults are basically the same as a Powerformer that is connected directly to the high-voltage bus-bar in the generating station. Here is a summary of different faults. • For external faults, the fault currents from a Powerformer at external three-phase short circuits will be of the same magnitude as the fault current from the conventional unit. • The fault current from a Powerformer at external single-phase ground faults will be lower than that from the conventional unit. The reason is that the neutral point of a Powerformer is isolated from ground while the neutral point of the step-up transformer of the conventional generator is solidly grounded. Therefore, the introduction of a Powerformer decreases the fault current at an external singlephase ground-fault because the elimination of the step-up transformer increases the resulting zero-sequence reactance. • The internal three-phase shortcircuit current of a Powerformer is less than that of the conventional generator due to its higher output voltage. • In the case of the internal twophase-to-ground and the internal phaseto-phase faults, the fault current in a conventional generating unit will be substantially higher than the fault current in the Powerformer. The currents at internal faults, for the studied specific case, are summarized in Fig. 8. • For the internal single phase-toground internal fault, the fault current in a conventional generating unit is much lower than that of the Powerformer due to the high impedance grounding of the neutral of the conventional generator (Fig. 8). In service, it is indispensable to eliminate discharges in the interstices between the main insulation of the conductor/cable and the walls of the slot. The damage to the insulation is produced when the partially conducting coating on the bar surface becomes
MAY/JUNE 2008
electrically isolated from the slot walls. A voltage is developed on the surface coating, which may be sufficient to break down the airgap and produce arcing between the surface coating and the slot wall. This results in the fusing of areas of the insulation surface. This slot-discharge activity highlights the need to earth the surface coating to the stator core at some points along its length. In the Powerformer, the original design of the cable fixation in the slots was based on the principle of grounding the outer semiconducting layer of the cable in the slot by pressing the cable against the stator laminations via fixation hoses (Fig. 7).
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Fig. 7 Fixation of the winding cables in the Powerformer slots: 1) laminated stator core, 2) XLPE insulation, 3) conductor, and 4) fixation hose.
Differential protection The Powerformer winding cables can be considered as a capacitor with charges on the electrodes that are, in this case, the inner and the outer semi-conducting layers. The electrical charge on a phase winding of a Powerformer at voltage maximum is 30 times larger than that
on a phase winding of a conventional generator with the same rated apparent power. The internal short circuit current should be investigated and simulated accurately for the purpose of the protection of Powerformers by taking into account the distributed capacitance. Large conventional generators and long transmission lines are confronted with the problem of increasing capacitive charging current. Therefore, the impact on the reliability of differential protection should no longer be negligible. In a Powerformer’s case, where the cable is considered as a capacitor with charges on its two electrodes as discussed earlier, the capacitance in the protection zone causes two problems. First, the operating value of the differential protection must be increased to avoid unwanted operation caused by the capacitive differential current. Second, the differential protection must suppress transients in the differential current to avoid unwanted operation caused by the capacitive inrush current. The differential protection for the conventional generators rarely considers the influence of capacitance in the protection zone because the value of the direct earth capacitance is quite low. Similar to the analysis of the capacitance of the transmission line, the equivalence of winding capacitance of the generator, in which the capacitance distribution along with the winding is represented by a lump capacitance with 50% at the phase terminal and 50% at the neutral point. This assumption is suitable for the cases of capacitance evenly distributed, such as the transmission line and the winding of a conventional generator. However, it will lead to errors in analyzing the stator winding of the Powerformer in that the winding capacitance does not
150 kV
15 kV
150 kV External Network
∼
External Network
∼
Δ (a) Short circuit currents 3-phase-to-ground: 53 kA 2-phase-to-ground: 45 kA 2-phase: 45 kA 1-phase to ground:
