EP3786986B1 - Circuit assembly for the reduction of a unidirectional flux component in the soft magnetic core of a transformer - Google Patents

Description

Technical area

The invention relates to a circuit arrangement for reducing a direct flux component in the soft magnetic core of a transformer.

State of the art

In electrical transformers, such as those used in power distribution networks, an undesirable direct current can be fed into the primary winding or secondary winding. Such a direct current feed, also referred to as a DC component, can arise, for example, from electronic components such as those used today to control electric drives or for reactive power compensation. Another cause can be so-called geomagnetically induced currents (GIC).

Due to solar winds, the earth's magnetic field is changed and very low-frequency voltages are induced in conductor loops on the earth's surface. In long electrical power transmission lines, the induced voltage can cause relatively large low-frequency currents (quasi-direct currents). Geomagnetically induced currents occur in approximately ten-year cycles. They are distributed evenly across all (three) phases, can reach up to 30 A per phase and flow via the star point of a transformer. This results in heavy saturation of the core of the transformer in one half cycle and therefore high excitation current in one half cycle. This additional excitation has a strong harmonic component and is therefore caused by the stray field with harmonic content causes eddy current losses in windings and iron parts of the transformer. This can lead to local overheating in the transformer. Furthermore, the strong need for excitation leads to high reactive power consumption and voltage drops. Together, this can lead to instability of the energy transmission network. To put it very simply, the transformer behaves like a choke in a half wave.

Some power transmission companies therefore require 100 A direct current for the neutral point of the transformer in their transformer specifications.

According to the WO 2012/041368 A1 and the WO 2015/086048 A1 An electrical voltage induced in a compensation winding is used and used to compensate for the disturbing magnetic direct flux component by connecting a thyristor switch in series with a current-limiting choke in order to introduce the compensation current into the compensation winding. This solution works well for balanced direct currents in a range that are an order of magnitude smaller than geomagnetically induced currents, i.e. in the range below 10 A. For geomagnetically induced currents you would have to go to the medium voltage level, i.e. in the range of around 5 or 8 kV, and use powerful thyristors. Due to the high power loss of such thyristors, separate cooling would have to be provided for the thyristors, so that this solution would then not be economical.

Another solution for geomagnetically induced currents is the so-called DC blocker, in which a capacitor is in principle connected to the star point of the transformer. This solution is problematic because charging the capacitor creates a zero displacement voltage. In addition, the residual voltage on the capacitor is limited, so that not all of the direct current can usually be blocked. This solution is also problematic if it leads to one Short circuit in the transmission network and therefore zero currents occur.

Further state of the art is in the EP 3 196 902 A1 , the DE 40 21 860 A1 and the DE 27 23 767 A1 described. Presentation of the invention

It is an object of the present invention to further develop the prior art.

This task is solved by a device with the features of patent claim 1. Advantageous refinements result from the subclaims.

Short description of the character

The invention is explained in more detail using figures.

• Fig. 1 ein Ausführungsbeispiel der erfindungsgemäßen Lösung mit Transraktor und
• Fig.2 ein Ausführungsbeispiel mit steuerbarem Widerstand.

They show, for example:

• Fig. 1 an embodiment of the solution according to the invention with transractor and
• Fig.2 an embodiment with controllable resistance.

With so-called direct current compensation, direct current is specifically introduced into a compensation winding CW in order to cancel the direct current magnetization of the transformer core. To introduce the necessary magnetic flux (the so-called direct current ampere turns) into the compensation winding CW, the alternating voltage induced in the compensation winding CW is used; the compensation winding 1 acts like an ideal alternating voltage source. A diode D1 is connected to the compensation winding CW in series with a current-limiting choke L, by means of which the voltage induced in the compensation voltage CW is rectified.

The direct current obtained in this way is indirectly proportional to the effective inductance I _DC ~ 1/L of the current limiting choke. According to the invention, this is constructed from partial inductances L1, L2, L3, L4 that can be switched using semiconductor switches S ₁ , S2,... Sn and therefore has an adjustable inductance. By switching N partial inductances on and off, for example, the direct current can be changed in N equal partial steps (increments).

Through a transractor T connected in parallel to the current limiting choke L, as shown, for example, in the AT 519 338 A1 is known, a stepless direct current setting can take place.

According to this document, a transractor is understood to mean the combination of transductor and choke.

The transractor only has to provide the Nth part of the direct current and can be made much more compact than with the solution according to AT 519 338 A1 the case is.

Another significant advantage of the solution according to the invention is that the current pulses generated by means of the current limiting choke L and diode D1 only have the fundamental harmonic and a direct current mean value.

No further harmonics arise that could significantly increase the eddy current losses. Only the small residual transractor current for the continuous control of the direct current value has higher harmonic components, which, however, do not play a significant role because of the comparatively low current intensity.

The partial inductances L1, L2, L3, L4 can be implemented, for example, by taps in the winding of a choke or by separate partial chokes.

Since the construction output of the transractor is comparatively low, it can be implemented with inexpensive standard components. In addition, the size of the converter can also be reduced to act on the secondary side of the transractor. It is therefore also possible to do without a control transformer for voltage compensation on the secondary side, which further reduces the space requirement of the solution according to the invention.

As in Fig.2 shown, a preferred embodiment of the invention comprises a controllable resistor ES, which is connected in parallel to the current-limiting choke L instead of a transractor for continuous adjustment of the direct current.

The advantage of this solution is that it is easy to implement, for example using a field effect transistor (FET), and that it requires relatively little space.

The power loss occurring at the controllable resistor ES can be neglected since only the Nth part of the direct current has to be provided.

Reference symbol list

CW

Compensating winding

L

Current limiting choke

D1

diode

T

Transractor

S1, S2, S3, S4

Semiconductor switch

L1, L2, L3, L4

Partial inductors

IT

Controllable resistance

Claims (4)

1. Circuit arrangement for reducing a DC flux component in the soft-magnetic core of a transformer, comprising a compensation winding (CW) that is magnetically coupled to the core of the transformer, wherein a diode (D1), and in series therewith a current-limiting reactor (L), are connected to the compensation winding, by means of which the voltage induced in the compensation winding (CW) is rectified, characterized in that the current-limiting reactor (L) is constructed from partial inductances that are able to be switched by means of semiconductor switches (S1, S2, S3, ... SN) and therefore has an adjustable inductance, such that the DC current can be changed by connecting in and disconnecting partial inductances (L1, L2, L3, L4), wherein the switchable partial inductances (L1, L2, L3, L4) are arranged so as to be able to be connected in series.
2. Circuit arrangement according to Claim 1, characterized in that a transactor (T) is connected in parallel with the current-limiting reactor (L).
3. Circuit arrangement according to Claim 1, characterized in that a controllable resistor (ES) is connected in parallel with the current-limiting reactor (L).
4. Circuit arrangement according to one of Claims 1 to 3, characterized in that the winding of the current-limiting reactor (L) is provided with a plurality of taps in order to form the partial inductances, wherein the semiconductor switches are arranged as synchronized tap changers (S1, S2, S3, S4) for actuating the taps.

Images