Power transformer with integrated tertiary winding for earthfault current compensation

Abstract

Power transformer (2) for three phase systems including three galvanically separated windings of which two windings are the Y-connected main windings and the third winding is connected as open delta for connection of an earthfault current compensation equipment (4, 6), and the method of grounding the neutral on either of the main windings decides on which side of the transformer the earth fault current compensation works.

Description

[0001] Power transformer with integrated tertiary winding for earthfault current compensation

[0002] Technical field of the invention

[0003] The invention comprises a power transformer for three-phase systems with three galvanically separated windings, of which two are the Y-connected main windings and the third winding is an opendelta winding for connection of known equipment for earth fault current compensation. The power transformer is preferably used to separate grid sections with increased fire risks. These sections require special earth fault protection to prevent fires, started from line fracture and other high impedance earth faults.

[0004] Background of the invention and prior art

[0005] Today, transfer of electric power is mainly accomplished by means of cyclo-symmetrical three phase voltage systems. The basic requirements of the system are that production and consumption of power must be in balance at all times. In order to facilitate these requirements large national transmission networks have been created, where all producers and consumers of electric power are interconnected. The connection of these national grids to transnational networks provides further advantages with respect to the basic balance requirement. For example, one such transnational network is the Scandinavian NORDEL network.

[0006] To reduce transfer losses in the transmission networks, the long distance transport of energy is accomplished at high voltage, preferably 400kV. The energy output to the consumers from the transmission network is carried out via grid exit transformers, which in turn, supply a limited geographical area. The distribution is mainly effected at 10-30kV voltage levels. Large industry customers are connected directly to the distribution network, while smaller consumers and households are supplied over yet another transformation to 400 / 230V. From the beginning distribution networks have developed after two main principles: The “European” three phase system, where payload is only connected between the phases, and the “Anglo-American” multigrounded 4-wire system, where payload also can be connected between one of the phases and a multigrounded neutral wire following the line.

[0007] This difference has led to different protection concepts for earth fault protection. While earth fault protection in the European network has developed solutions to reduce and finally eliminate the fault current, the earth fault current in the multgrounded 4-wire system is systematically increased by solidly grounding the neutral of the network, to finally clear the faulty section by ordinary overcurrent protection.

[0008] In the later protection concept it has shown to be impossible to detect high impedance line fracture faults, which under stormy weather conditions often start devastating wildfires. The mixing of earth fault currents with single phase load currents of varying amplitude and phase in the current summation (zero sequence) makes the separation practically impossible.

[0009] In order to stop the increased number of devastating wildfires caused by line fracture faults during severe weather conditions, authorities in several countries demand improved earthfault protection. For the 4- wire system this means all single-phase loads must be reconnected to two-phase, before the earthfault protection can be made sensitive enough.

[0010] The ongoing climate change put some pressure on the question. In order to speed up reduction of fire risks in particularly exposed areas, the demand for sensitive earthfault protection will be prioritised there. To change the neutral treatment for particular areas, a specific transformer with gavanically separated windings is necessary. The suggested 3-winding transformer provides a more compact and cost-effective solution, without separate high-voltage equipment for the neutral treatment.

[0011] Summary of the invention

[0012] The objective of the invention is to solve the above problems and offer a simpler and more cost-efficient solution for earthfault current compensation in restricted areas. A further objective is to compensate the earth fault current in these areas completely. Another objective is to improve the reliability of the compensation device and thereby the reliability of the whole power transmission. Yet another objective is to lower the costs for the compensation of the earthfault current.

[0013] These objectives are achieved by means of a device, defined in the preamble of claim 1 , which is characterized by a 3-winding power transformer provided with a primary side that is connected to the three phases (A, B, C) of the supplying network and provided with a secondary side (a, b, c) is connected to the earthfault compensated secondary side (see figure 1 ).

[0014] The earth fault current compensation in the secondary side of the transformer is achieved by known arrangements for earthfault compensation, i.e. a Petersen coil and / or known arrangements for residual current compensation, connected to the tertiary winding, which is shaped as an open delta winding (see figures 1 and 2). Dependent of its design, the earth fault current and its damage at the fault site will thereby be reduced or practically completely eliminated.

[0015] A known arrangement for residual current compensation was already developed in the eighties at the Royal Institute of Technology “Elkraftcentrum” in Stockholm. The purpose of the arrangement was to improve localisation of high impedance earth faults and included among other the application of a new protection algorithm. Details are described in a paper contribution to the PAC World Conference, June 2024, Athens / Greece by Klaus Winter. This protection algorithm is based on two subsequent measurements of the zero-sequence impedance, one measurement before, and one measurement right after the earthfault has been detected. The difference between the two measurements represents the current at the fault site. By differential measurement on the same sensor, the error of the sensor itself is eliminated.

[0016] Beside the improved detection of high impedance earthfaults, the difference between the two measurements can be used to compensate the earthfault current completely, by injecting an equal but opposite current at a more controlled place in the zero sequent system, for instance in the neutral of distribution network. In the majority of the installed systems, this is done with an inverter in parallel to the arc suppression coil.

[0017] Arrangements for improved detection of high impedance faults and fast compensation of the earthfault current have become more relevant in recent days. The increasing number of devastating bush- / wildfires as a consequence of undetected line fracture faults, makes the introduction of these protection a concern.

[0018] Another advantage of the invention is that the number of power supply interruptions will be reduced, or the interruption can be avoided completely in the resonance grounded section of the grid. Thereby the new grounding also improves the reliability of the power supply. The new equipment can be manufactured at low costs and is relatively simple to install in existing distribution networks.

[0019] An equipment according to the invention can be used together with other known control- and measurement equipment for earthfault current compensation.

[0020] Brief description of the drawings

[0021] Figure 1 shows a connection diagram for a 3-winding transformer according to the invention. Figure 2 shows the equipment for earthfault compensation, connected to the tertiary winding of the power transformer.

[0022] Detailed description of the invention

[0023] Figure 1 shows the detailed connection diagram for the transformer 2, according to the invention. According to the diagram, one of the known equipment for earth fault current compensations is directly connected to the open delta winding of the transformer. In the shown 3-winding power transformer each winding is represented by a bold line. The transformer’s primary side is connected to the three phases (A, B, C) of the supplying power system and the transformer’s secondary side (a, b, c) is connected to the earth fault protected grid section.

[0024] Figure 2 shows an equipment for earthfault current compensation, composed by an adjustable inductance (Petersen coil) 4 and a compensator 6 for the residual current compensation, to be connected to the tertiary winding. For the residual current compensator 6 current / voltage output and phase can be adjusted.

[0025] Because the voltage ratio between primary, secondary and tertiary winding is free, the voltage level for the open delta winding can be selected for direct connection of cost-efficient low voltage equipment. The vector group works both for three-legged and four / five-legged transformer cores 8.

[0026] The invention is not limited to the above shown arrangement, but can be varied in many ways within the scope of the claims.

Claims

Claims1 . Power transformer (2) for three-phase power system with at least three galvanically separated windings, of which two are Y-connected main windings, characterized in that a third winding, a tertiary winding, is connected as an open delta winding where an arrangement (4, 6) for earth fault current compensation is connected, and the method of grounding the neutral on either of the Y-connected main windings determinates on which side of the transformer the earthfault current compensation works.

2. Power transformer (2) according to claim 1 , where the magnetic core is made up of 4 or 5 legs.

3. Power transformer (2) according to claim 1 or 2, where the rated voltage of tertiary winding is selected with respect to the rated voltage of the earth fault current compensation equipment, to avoid costly intermediate transformers.

4. Method to use a power transformer according to claim 1 , 2 or 3, characterized in that the method is used for earth fault current compensation in power networks.

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