Transformer with protection against direct current magnetization caused by zero sequence current

The present invention relates to a transformer being protected against direct current induced by geomagnetic flux changes, so called zero sequence current, whereby it comprises at least one compensation winding for direct current on the transformer core to compensate for undesired magnetization, by adding a current opposite to the direction of the magnetization caused by the zero sequence current carried by the alternating current to be transformed to reduce high magnetization saturation levels.

TECHNICAL FIELD

The present invention relates to a transformer with protection against direct current magnetization caused by zero sequence current, in a power generation, transmission or distribution system with a rated power ranging from a few kVA up to more than 1000 MVA and with a rated voltage ranging from 3-4 kV and up to very high transmission voltages, 400 kV to 800 kV or higher.

BACKGROUND OF THE INVENTION

The primary task of a power transformer is to act as an electric “gear box” and sometimes to create a galvanic isolation, allowing electric energy to flow from one electrical system to another. The electrical systems interconnected with a transformer usually have different voltages but always the same frequency. The power transformer, in its simplest form, comprises generally at least two windings, a primary winding and a secondary winding. The transformation ratio is defined by the winding turns in the primary and secondary winding and the connection of the windings, e.g., In “delta” or “Y”-connection.

In the transferring of large powers at high voltages over large distances, the geomagnetic field at changes thereof imposes an often quite large quasi-direct current, (DC) in the power line(-s), so called zero sequence current, which direct current accompanies the alternating current phase (AC-phase). The phase lines can be regarded as one line over long distances as the distance between each line becomes relatively small, which causes the induction of the DC current, the zero sequence current, to be equal in all phases, when the geomagnetic field is subjected to changes.

The direct current gives rise to unilateral magnetization levels of any transformer in the system, which may cause the core of the transformer to enter magnetic saturation. This leads to the transformer consuming high magnetizing currents, thus being disconnected, normally by means of a protecting system, which releases the transformer from the system. When a transformer is disconnected, released, from the system, this will of course lead to disturbances In the transmission and distribution of electrical energy.

SUMMARY OF THE INVENTION

It has turned out possible to introduce a passive compensation system of direct current, zero sequence current, induced by geomagnetic field changes in transformers eliminating high magnetization saturation levels, which is characterized in that a first impedance (Z1) is arranged from the neutral point to ground in parallel to the compensation winding, which Impedance provides a high impedance for low or zero frequencies, and any preferably, a low impedance for higher frequencies.

In one preferred embodiment of the invention the compensation windings are further connected to earth via a second impedance (Z2) being able to short circuit any DC voltage, and having any impedance for all other frequencies.

In a further preferred embodiment of the invention the first impedance is tuned for 3rdtone series or higher.

In one preferred embodiment the transformer is selected from the group of 1-phase or 3-phase transformers.

In another preferred embodiment the transformer is selected from the group of two-legged, three-legged, four-legged and five-legged transformers.

In further preferred embodiment the four and five-legged transformers comprises at least one magnetic return conductor leg, as well as three phase legs.

In another further preferred embodiment a compensation winding is applied to each phase-leg, which compensation windings are substantially Identical with regard to magnetizing ability.

In one preferred embodiment a compensation winding is applied to any magnetic return conductor leg present, whereby any two such compensation windings are substantially identical with regard to magnetizing ability.

In another preferred embodiment a counteracting current is arranged to be driven through the compensation winding(-s).

In further preferred embodiment the operation of the counteracting direct current Is made power electronically.

In further preferred embodiment the compensation winding(-s) is/are connected to ground.

In another further preferred embodiment the compensation winding(-s) is/are wound in the opposite direction of the winding carrying the current to be compensated for.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the invention shows (FIG. 2) a three-phase transformer1comprising three phase windings2,3,4one for each phase on each their leg, whereby the phase windings are separated in a primary winding and a secondary winding on each leg. The construction of primary versus secondary winding does not matter with the regard to the present invention, and these have been separated one over the other in the drawings, whereby alternatives are the one outside the other depending on the parameters to be chosen for the specific use of the transformer. Three compensation windings5,6,7connected in series (one for each phase leg) are present on the transformer1, whereby the transformer has no magnetic return conductor. The compensation windings5,6,7are all, preferably wound in a direction opposite the ones of the main phase windings2,3,4, however, the direction of their winding turns, in this embodiment is not critical due to the control of this transformer embodiment.

FIG. 3shows a second embodiment wherein a fourth leg9is present. This type of transformer is a transformer with magnetic return conductor. In this embodiment a counteracting compensation winding10is applied around the fourth legFIG. 3B. The compensation winding10is wound in a direction opposite to the phase windings2,3,4. In the description there is thus a question of transformer legs having different purposes.

Within the context of the present application a phase leg is a leg carrying a primary and/or secondary phase winding, and a return leg is a leg functioning as a magnetic return conductor being free from any phase winding.

The compensation winding arrangement ofFIG. 2can also be applied to a 4-legged transformer, i.e., a part winding is arranged to each of the legs carrying the AC-phase windings (FIG. 3).

FIG. 1shows a 1-phase transformer, which can be used as such dividing off an ingoing phase line, or can be used in series with two identical transformers each handling their ingoing phase. In the 1-phase transformer, a compensation winding5is wound around the magnetic return conductor, or depending on the design of the transformer is split between the different legs. An 1-phase transformer can be said being a two-legged transformer, where the primary winding may be present around one leg and the secondary winding around the other leg, or the primary winding is split into two, each part being placed around each leg, and carrying the secondary windings, within or around the primary ones.

FIG. 4shows a third embodiment of a transformer having five legs, where compensation windings25,26have been applied around the two non-phase legs,FIG. 4B. The number of turns of the compensation windings is preferably the same to simplify control of the operation of the compensation current from the DC source.

The compensation winding arrangement ofFIG. 3can also be applied to a 5-legged transformer, i.e., a part winding is arranged to each of the legs carrying the AC-phase windingsFIG. 4A.

FIG. 2shows an embodiment with three-legged transformer1having its transformer windings2,3,4, which is provided with a middle point. The middle point22is connected to ground via a compensation winding5,6,7applied on each leg. In this case the transformer compensates itself. There might be an impact on the impedance on other zero sequence current components, which impedance may change at compensation. This problem is substantially eliminated or at least reduced to a major extent by having an impedance (Z1)31connected to ground in parallel to the compensation windings5,6,7, preferably tuned for the 3rdtone series or higher. The impedance Z1shall have a high. impedance at less than 10 Hz, but provide any Impedance for all other frequencies.

In a preferred embodiment a further impedance (Z2)32Is applied between the compensation windings and earth at22, which impedance will be low or zero at less than <1 Hz and will provide any impedance for all other frequencies.

FIG. 3shows a further embodiment showing a four-legged transformer with its windings2,3,4having its middle point22, whereby a compensation winding20is applied to the fourth leg9. The middle point22Is connected in series to the compensation winding10, which in turn is connected to ground. In series herewith a series resonance link31is arranged, which link31is tuned in 3rdtone or higher, such as the 9thtone. The compensation winding arrangement ofFIG. 3Acan also be applied to a 4-legged transformer ofFIG. 4A.

A five-legged transformer can be construed for self-compensation in the same way as the four-legged one, whereby the compensation winding10has to be distributed to both the fourth and the fifth legs, as in the embodiment ofFIG. 4.

If it is supposed that the magnetizing current is only some percentage of the rated current and that the resistive losses at rated current are some percentage of the rated power a winding of the same size as the phase winding request a resistive loss in the order of 10−5to 10−4times the rated power, if it should only transfer a current of the same order as the magnetization current. This means that reasonable powers are involved even if a compensation winding is made substantially smaller than the real phase winding.