Patent Application
20170169940
1. Field of the Invention
The invention generally relates to electrical transformers as used in power systems for the generation, transmission and distribution of electrical energy and, more particularly, to a circuit arrangement for compensation of a DC component in a transformer, where the transformer includes a winding arrangement that is connected via connecting lines to the power system and includes a neutral point connected to earth.
2. Description of the Related Art
In power systems for the generation, transmission and distribution of electrical energy, for a variety of reasons, a DC current that is proportionately overlaid on the alternating current can form. A direct current component of this type, hereinafter known as a DC component can be created, for example, by a power converter connected to the power system, or can be a “geomagnetically induced current” (GIC).
A GIC is caused by solar wind in that the Earth's magnetic field changes so that in a conductor loop constituting network lines and earth return conductors, a flux change occurs and the electrical voltage induced therein generates the GIC. The GIC is time and direction-dependent. However, the speed at which the GIC changes is so relatively slow that it can be regarded as a DC component in the power system.
Regardless of its cause, a DC component in a transformer is always undesirable because a magnetic unidirectional flux portion is associated with it, which becomes overlaid on the alternating flux in the core of the transformer, so that the output of the transformer core is no longer symmetrical. A displacement of the operating point of the magnetic material occurs. Depending on the design of the transformer, even a very small DC component of a few 100 mA can increase the emission of operating noise by 10 to 20 dB. The displacement of the operating point of the magnetic material can lead to a significant increase in the losses in the region of 20-30%. Locally, increased heating can arise in the transformer, so that the lifespan of the insulation of the electrical winding can be impaired. In the case of a large GIC, hotspots can form on metallic parts and lead to the degradation of the insulating liquid, which can result in the formation of decomposition gases.
Various methods and devices are known for reducing a unidirectional flux portion in the core of a transformer. For example, in EP 2 622 614 B1, it is proposed to provide the transformer core with a compensation winding and, via a semiconductor switching device, to feed thereinto a compensation current the effect of which is directed contrary to the disruptive unidirectional flux portion. The advantage herein is that this compensation winding is galvanically separated from the energy network, i.e., has no connection to the primary or secondary winding system of the transformer.
A compensation winding of this type can now be realized at a reasonable cost if it is provided during the production of the transformer. Depending on the size of the DC component to be compensated, a correspondingly dimensioned installation space is to be provided, so that the dimensions of the transformer core change.
Compared with this, a retrofitting is far more effort and barely of any practical importance. If a transformer that is already in use is to be retrofitted with a DC compensation, then the transformer must be removed from the power system and the compensation winding must be installed in the interior of the transformer tank. A retrospective installation of a compensation winding is often not at all possible. In any event, the costs of a subsequent installation are very high so that, to date, retrofitting has hardly ever been performed.
Restrictions also result from the limit load of available semiconductor components. In transformers that are used as part of a high voltage DC transmission system, an induced voltage of several 1000 V can be induced in a compensation winding. A GIC can also reach a current strength of more than 50 A, as a result of which the technical implementation of the circuit is difficult.
The arrangement of semiconductor components with a limited lifespan in the interior of a power transformer or a distribution transformer that is configured for fault-free operation over several decades is also problematic.
It is an object of the present invention to provide as simple and robust an approach as possible for the compensation of a DC component, which can also be realized economically with transformers already in operation and which also leads to expectations of a long service life and a high level of reliability.
This and other objects and advantages are achieved in accordance with the invention by a circuit arrangement in which the compensation of a DC component is not brought about by components in the interior of the transformer, but rather—similarly to a protective device—outside the transformer housing, where a separate compensation winding is not necessary.
Starting from a transformer, the winding arrangement of which is connected via connecting lines on the supply side to a power system for transporting electrical energy and having a neutral point connected to earth. The circuit arrangement comprises a transductor circuit arranged in a current path that connects a connection point situated on a node-free portion of the connection line to earth, and a control and regulation device that controls the transductor circuit via a control signal, to which is fed, on the input side, a signal provided by a detection device concerning the size and direction of the DC component to be compensated.
The transductor circuit functions as a magnetic switch. The function of a transductor is known per se, and need not be described further here. It is advantageous that the transductor circuit has no actively functioning components, such as controlled semiconductor valves, but only passively functioning components. These components are arranged in a current path that connects to earth a node-free portion of each feed line to the transformer. The transductor circuit is controlled by a control unit to which an item of information concerning the size and direction of the DC component to be compensated is fed. Such an information item can originate, for example, from a sensor in the interior of the transformer, for example, a magnetic field sensor that measures the disruptive unidirectional flux portion in the core of the transformer, or is alternatively obtained from the power system, for example, by measuring the DC component in a network line (although this is technically complex due to the greatly differing sizes of the supply current and the DC component). Also conceivable are other measuring devices installed in the power system that detect or predict a GIC.
The technical effect achieved with the circuit arrangement in accordance with the invention resembles that of a set of “DC points” which acts as a protective device. The disruptive DC portion is “conducted away” to earth through targeted control of the transductor circuit arranged in the current path between the transformer feed line and earth. Thus, the DC portion is counteracted in advance so that it does not reach the transformer (at least not to its full extent). For this “compensation” that occurs as early as the feed line, a separate compensation winding magnetically coupled to the transformer core, as is otherwise usual for this purpose, is no longer necessary. With the present circuit arrangement, a DC portion even of more than 50 A, such as a large GIC, can be controlled in an easy manner. Both are highly advantageous. In contrast thereto, however, is the requirement that the circuit arrangement with all the components lying in the transductor current path must be configured according to the voltage level of the transformer supply line, such as 20 kV or 120 kV or more. However, the above-mentioned advantages outweigh this additional effort.
As previously stated, the components that conduct away a DC component are arranged outside the transformer housing. They function purely passively. Maintenance is readily possible. The reliability of the circuit arrangement is high.
In place of the power electronics components (e.g., thyristors) that are otherwise usual, for the generation of a compensation current, an “inductively acting switch”, a transductor circuit, is used. The components of a transductor circuit, magnetic coils and uncontrolled valves, lead to the expectation of a long lifespan and trouble-free operation over many years. On the part of the network operator, therefore, the level of acceptability of a transformer with a DC compensation device of this type is high.
In a preferred embodiment, the circuit arrangement is configured as a transductor circuit with two parallel current branches. In each of these current branches, therefore, a transductor load winding and, in series therewith, an uncontrolled valve are respectively provided. The current flow direction of the two valves is opposed. Each load winding is magnetically coupled via a saturatable transductor core to a transductor control winding, where a control signal is fed to the control winding. The saturation state of the transductor core and thus the switching state of the “magnetic valve” are pre-settable via the control signal. In conjunction with the opposed arrangement of the valves, it is achieved that, depending on the current direction, a mirror-inverted DC compensation is set against a DC component.
An embodiment that is more cost-efficient relative thereto can be constructed so that the transductor circuit has a single load winding that is arranged in series with a single valve and a switching device for reverse-poling of the current flow direction of the valve. Here, this load winding of the transductor circuit is also coupled via a soft magnetic transductor core to a control winding. A control signal, the information of which is provided by a detection device that detects the size and direction of the DC component to be compensated, is fed into the control winding. Herein, by reverse-poling this single valve, the compensation of a DC portion in both directions is possible.
In order to reduce the production costs, it may be favorable for both of these embodiments if the transductor core is configured as a slit strip core.
In order to keep the control power required for controlling the transductor circuit as low as possible, it can be advantageous if the slit strip core is made of sheet metal lamellae of a soft magnetic material which has an essentially narrow rectangular hysteresis loop. The control and regulation device can thus be constructed simpler.
A further reduction of the control power can be thereby achieved by arranging the transductor core in a magnetic circuit that has at least one air gap. This results in an inclination of the hysteresis loop so that the magnetic flux density is limited to less than/equal to 20% of the saturation flux density.
Particularly favorable can be a circuit arrangement in which the detection device that detects the size and direction of the disruptive DC portion is configured as a magnetic field measuring device. Such a magnetic field measuring device can be arranged in the interior of the tank lying on the transformer core. In this way, just one signal strand is fed out of the interior of the transformer tank (to the control and regulation device), but not a power strand. A retrofit can be realized at relatively little cost.
PCT/EP2010/054857 discloses a magnetic measuring device. Here, the measuring device has a C-shaped shunt component, the limbs of which are directed toward the core of the transformer so that a magnetic partial flux is diverted therefrom. The diverted partial flux induces an electrical voltage, which represents the unidirectional flux portion to be compensated in the core, as the signal in a sensor coil provided at the shunt component.
In other words, the present solution approach enables a transformer in operation in the power system to be equipped retrospectively in a simple and economical manner with the functionality of a DC protection system. Not a compensation winding, but only a measuring device must be installed in the transformer housing, provided the information concerning the DC portion to be compensated does not come from another detection device. Thus, in transformers already in operation, the installation of a DC compensation with relatively little effort is also possible retrospectively. Until now, a retrofitting of this type has been barely justifiable for cost reasons.
It appears favorable if each diode is configured as a high-blocking power diode that has a high blocking ability and a low forward resistance. This can be achieved with a construction type of a diode, where a thin low doped intermediate layer is formed between the highly doped pn zones.
In order to limit the electric current in the conductive state of the transductor, it can be favorable if a current-limiting reactor, arranged in series with the load winding of the transductor circuit, is arranged in the current path (power branch). This current limiting reactor can be configured like a “shunt reactor” for comparatively low current and low power. The control and regulation device can thus be constructed simpler because the effort for safety technology otherwise to be provided is lower.
In an alternative thereto, it can also be advantageous, however, if each of the parallel-connected load windings or the single load winding of the transductor circuit device is configured for current limitation in the current path. A separate current limiting reactor is then not required. Herein, the control and regulation device can also be constructed a simpler manner.
It is also an object of the invention to provide a method for compensating for a DC component in a winding arrangement of a transformer, where the winding arrangement is connected via connecting lines to a power system for transporting electrical energy and where a circuit arrangement described in detail above is used.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.
For further explanation of the invention, reference will be made in the following section of the description to drawings which illustrate further advantageous embodiments, details and developments of the invention, using a non-limiting exemplary embodiment, in which:
In the example in
The connection point 12 lies on a feed line portion 31 between a connection node point 14 and a transformer connection 13. The node point 14 is part of a power system 15 for the generation, transmission and distribution of electrical energy that also comprises further node points 14′, 14″, 14″'. No further network nodes are arranged between the connection network node 14 and the transformer connection 13, i.e., the portion 31 of the three-phase conductor system 3 is node-free.
It is assumed that a DC component (IDC) flows in the feed lines 3 (represented in
The transductor circuit 1 functions as a magnetic switch or “magnetic valve” and is controlled by a control and regulation unit 6. In the conductive state, the current limitation occurs via a choke 2 that is arranged in a series connection with the transductor circuit 1. The control and regulation unit 6 comprises a computer unit with an algorithm able to execute thereon. This generates the control signal 16, where the measurement signal 17 fed in on the input side is used. The measurement signal 17 is a representation of the DC component to be compensated and is provided by a magnetic field sensor 5. This magnetic field sensor 5 is arranged in the interior of the transformer 4, where it measures a unidirectional flux portion flowing in the core of the transformer and originating from the DC component. PCT/EP2010/054857 describes one type of a magnetic field sensor.
The transductor circuit 1 also enables the compensation of comparatively high GIC DC currents, which can amount to more than 50 A. In the embodiment shown in
Arranged in series with the load winding 9 is a switching device 18 that includes a first switch contact 18′ and a second switch contact 18″. Arranged between these switch contacts 18′, 18″ is a single diode 7. In the switching position shown, the first switch contact 18′ is connected to the anode of the diode 7, and the second switch contact 18″ to the cathode. Depending on the switching position of these two switch contacts 18′, 18″, the polarity of the diode 7 can be reversed. Thus, also in this circuit embodiment, where only a single uncontrolled valve 7 is used, a bidirectional compensation of a DC component is possible. (See double arrow in
The actuation of the switching device 18 can occur in different ways, such as through an actuator or a motor, and manual operation is also conceivable.
Again, for the sake of clarity, the reference numeral 3 in
In
In the two embodiments, the DC protective effect occurs according to the principle of DC points directly at the feed line, i.e. the compensation current IK need only have the mirror-inverted size of the disruptive DC current on the line 31.
It is herein particularly advantageous that with the present circuit arrangement, large currents of over 50 A, as can occur with GIC, can also be counteracted.
Both embodiments have the essential advantage that the installation of a compensation winding is not necessary either subsequently in the context of a retrofit or during the production of the transformer.
For a transformer already in operation, the substantial advantage results that for the first time, a DC protection/DC compensation is realizable at a reasonable cost.
During the production of a transformer, the installation space that would otherwise be required for the compensation winding is dispensed with. This results in a compact design. This is particularly advantageous if large GIC currents are to be compensated because, in this case, the compensation winding is relatively voluminous and a correspondingly large installation space has to be provided.
The circuit arrangement has no active power electronics, but only passively acting components. As a result, the circuit arrangement, can easily be dimensioned for large voltages. The inductive switch 1 is, in principle, a transformer in no-load operation, where the entire voltage (110 kV, 220 kV, 340 kV, etc.) drops to ground. It can be realized with relatively little cost. The other components are common in transformer design or are commercially available.
Although the invention has been illustrated and described in detail based on the two preferred exemplary embodiments, the invention is not restricted by the examples given and other variations can be derived therefrom by a person skilled in the art without departing from the protective scope of the invention.
Next, a control and regulation device (6) controls the transductor circuit (1) via a control signal (16), as indicated in step 320. Next, a signal (17) with respect to a size and direction of the DC component (IDC) to be compensated is fed on an input side of the control and regulation device (6) by a detection device (5), as indicate in step 330.
While there have been shown, described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15198567.8 | Dec 2015 | EP | regional |
