Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Reexamination Certificate
1998-03-02
2001-05-01
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
C361S056000, C361S115000
Reexamination Certificate
active
06226163
ABSTRACT:
FILED OF THE INVENTION AND PRIOR ART
This invention is related to a device in an electric power plant for protection of an object connected to an electric power network or another equipment in the electric power plant from fault-related over-currents, the device comprising a switching device in a line between the object and the network/equipment. In addition, the invention includes a method for protecting the object from over-currents.
The electric object in question is preferably formed by a rotating electric machine having a magnetic circuit, for instance a generator, motor (both synchronous and asynchronous motors are included) or synchronous compensator requiring protection against fault-related over-currents, i.e. in practice short-circuit currents. As will be discussed in more detail hereunder, the structure of the rotating electric machine may be based upon conventional as well as non-conventional technique.
The present invention is intended to be applied in connection with medium or high voltage. According to IEC norm, medium voltage refers to 1-72,5 kV whereas high voltage is >72,5 kV. Thus, transmission, sub-transmission and distribution levels are included.
In prior power plants of this nature one has resorted to, for protection of the object in question, a conventional circuit-breaker (switching device) of such a design that it provides galvanic separation on breaking. Since this circuit breaker must be designed to be able to break very high currents and voltages, it will obtain a comparatively bulky design with large inertia, which reflects itself in a comparatively long break-time. It is pointed out that the over-current primarily intended is the short-circuit current occurring in connection with the protected object, for instance as a consequence of faults in the electric insulation system of the protected object. Such faults means that the fault current (short-circuit current) of the external network/equipment will tend to flow through the arc created in the object. The result may be a very large breakdown. It may be mentioned that for the Swedish power network, the dimensioning short-circuit current/fault-current is 63 kA. In reality, the short-circuit current may amount to 40-50 kA.
A problem with said circuit-breaker is the long-break time thereof. The dimensioning break-time (IEC-norm) for completely accomplished breaking is 150 milliseconds (ms). It is associated to difficulties to reduce this break-time to less than 50-130 ms depending upon the actual case. The consequence thereof is that when there is a fault in the protected object, a very high current will flow through the same during the entire time required for actuating the circuit-breaker to break. During this time the full fault current of the external power network involves a considerable load on the protected object. In order to avoid damage and complete breakdown with respect to the protected object, one has, according to the prior art, constructed the object so that it manages, without appreciable damage, to be subjected to the short-circuit current/fault current during the break-time of the circuit breaker. It is pointed out that a short-circuit current (fault current) in the protected object may be composed of the own contribution of the object to the fault current and the current addition emanating from the network/equipment. The own contribution of the object to the fault current is not influenced by the functioning of the circuit-breaker but the contribution to the fault current from the network/equipment depends upon the operation of the circuit breaker. The requirement for constructing the protected object so that it may withstand a high short-circuit current/fault current during a considerable time period means substantial disadvantages in the form of more expensive design and reduced performance.
The rotating electric machines intended here comprise synchronous machines mainly used as generators for connection to distribution and transmission networks collectively denoted power networks hereunder. The synchronous machines are also used as motors and for phase compensation and voltage regulation, then as mechanically idling machines. The technical field also comprises double-fed machines, asynchronous converter cascades, external pole machines and synchronous flux machines.
The magnetic circuit referred to in this context may be air-wound but may also comprise a magnetic core of laminated, normal or oriented, sheet or other, for example amorphous or powder based, material, or any other action for the purpose of allowing an alternating flux, a winding, a cooling system etc., and may be disposed in the stator or the rotor of the machine, or in both.
It is, according to the invention, primarily the intention to protect a non-conventional rotating electric machine for direct connection to all kinds of high voltage power networks. Such a machine has its magnetic circuit designed with a threaded conductor, which is insulated with a solid insulation and in which earth has been incorporated.
In order to be able to explain and describe the non-conventional machine, a brief description of a rotating electric machine will first be given, exemplified on the basis of a synchronous machine. The first part of the description substantially relates to the magnetic circuit of such a machine and how it is constructed according to classical technique. Since the magnetic circuit referred to in most cases is located in the stator, the magnetic circuit below will normally be described as a stator with a laminated sheet metal core, the winding of which will be referred to as a stator winding and slots arranged for the winding in the laminated core will be referred to as stator slots or simply slots.
Most synchronous machines have a field winding in the rotor, where the main flux is generated by direct current, and an AC winding in the stator. The synchronous machines are normally of three-phase design and the invention mainly relates to such machines. Sometimes the synchronous machines are designed with salient poles. However, cylindrical rotors are used for two- or four-pole turbo generators and for double-fed machines. The latter have an AC winding in the rotor and this may be designed for the voltage levels of the power network.
The stator body for large synchronous machines are often made of sheet steel with a welded construction. The laminated core is normally made from varnished 0.35 or 0.5 mm electric sheet. For radial ventilation and cooling, the laminated core is, at least for medium size and large machines divided into packages with radial or axial ventilation channels. For larger machines, the sheet is punched into segments, which are attached to the stator body by means of wedges/dovetails. The laminated core is retained by pressure fingers and pressure plates. The stator winding is located in slots in the laminated core and the slots have, as a rule, a cross section as a rectangle or as a trapetzoid.
Polyphase AC windings are designed either as single layer or two-layer windings. In the case of single-layer windings, there is only one coil side per slot, and in the case of two-layer windings there are two coil sides per slot. By coil side is meant one or more conductors brought together in height and/or width and provided with a common coil insulation, i.e. an insulation intended to withstand the rated voltage of the machine relative to earth. Two layer windings are usually designed as diamond windings, whereas the single-layer windings, which are relevant in this connection may be designed as diamond windings or as a flat winding. In the case of a diamond winding, only one coil span (or possibly two coil spans) occurs, whereas flat windings are designed as concentric windings, i.e. with a greatly varying coil span. By coil span is meant the distance in circular measure between two coil sides belonging to the same coil, either in relation to the relevant pole pitch or in the number of intermediate slot pitches. Usually different variants of chording are used, for example fractional pitch, to
Berggren Bertil
Bergkvist Mikael
Bernhoff Hans
Ekberg Mats
Isberg Jan
Asea Brown Boveri AB
Dykema Gossett PLLC
Jackson Stephen W.
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