WO2010066842A2

WO2010066842A2 - Component for power supply system

Info

Publication number: WO2010066842A2
Application number: PCT/EP2009/066844
Authority: WO
Inventors: Egil Hystad; Svein Magne Djuve
Original assignee: Wartsila Norway As
Priority date: 2008-12-10
Filing date: 2009-12-10
Publication date: 2010-06-17
Also published as: WO2010066842A3; NO330369B1; NO20085145L

Abstract

This invention relates to a power supply system comprising a rectifying bridge supplying current to a power consumer, e.g. a motor, wherein the rectifying bridge including two AC power feed inputs adapted to be coupled to a power network as well as at least one output for supplying power to said consumer, and wherein a transformer having two identical primary windings coupled between said inputs, the windings having a relative phase shift of 30°.

Description

COMPONENT FOR POWER SUPPLY SYSTEM

This invention relates to a low loss power supply system comprising a rectifying bridge and inverter supplying current to a power consumer, e.g. a motor, the rectifying bridge including two AC power feed inputs as well as at least one output for supplying power to said motor.

In supplying power to electric motors, e.g. in diesel electric propulsion systems or pipe heating systems in offshore installations, it is usual to use a rectifying bridge coupling coupled to a three phase power supply through a transformer, said transformer providing two AC supplies to the inputs of the rectifying bridge. This solution does, however have a problem related to the generation of noise comprising over harmonic voltages which combined with the full load current through the transformer windings, represents a strain factor on the existing circuitry in the rectifying bridge, the remaining network and an increased power loss through the transformer. The problem is especially related to the 5th and 7th over harmonic.

In order to cancel out the 5th and 7th over harmonic currents the output of the transformers have been adapted to provide signals to the rectifying bridge having a phase shift of about 30°s. It is an object of this invention to provide an improvement over this system. Through a special design of the low loss transformer, combined with a small filter, the transformer losses will be greatly reduced, and the total harmonic distortion on the total power system will decrease. US 5,446,643 and WO 2005/119892 show examples of filtering of 5 and 7 harmonic currents through a transformer coupling.

WO 2005/119892 also describes a propulsion system illustrated in figure 1 wherein an transformer is coupled between said inputs, said transformer, comprising (but not limited to) a star and delta winding, and being adapted to provide an approximately 30° phase shift relative to the chosen fundamental frequency between said inputs. This way the 5th and 7th harmonics are cancelled or at least suppressed a considerable degree. The power feeds at the transformer inputs are symmetric, so that no current at the fundamental frequency, passes through the transformer. The transformer thus acts a filter for over harmonics. Further the system transformer in WO 2005/119892 also comprises a second star winding at a 150° angle from the first star winding so as to suppress 1 lth and 13th harmonics of the signal.

A problem related to the know solutions in related to the lack of balance between the voltages at the two main windings. This gives raise to problems in the load on the two rectifier bridges as well as currents through the LLC transformer and there is a challenge to obtain an exact relationship between the primary windings in a delta-star configuration.

It is also an object for the invention to improve the filtering of over harmonic currents, e.g. the 11^th and 13^th harmonics as well as making it possible to apply the LLC transformer in systems using larger voltages than 1000V, while at the same time maintaining a standard construction with filter components made for low voltage applications so as to increase the personnel safety of the LLC unit.

The objects stated above are obtained with a power supply assembly being characterized as stated in the accompanying claims.

The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of example.

Figure 1 illustrates the prior art according to WO 2005/119892

Figure 2 illustrates a preferred embodiment of the invention for low voltage applications.

Figure 3 illustrates an alternative embodiment of the invention for high voltage applications.

Figure 1 illustrates AC power feeds A and B coupled to a transformer 1 for providing two inputs to a rectifying bridge 3, e.g. for providing power to a motor M. A transformer device is provided between these inputs having a Star/Delta (YD) winding structure, preferably a DQY_H structure, providing a 30° phase shift. A specific transformer winding connection according to the IEC vector group DoYn will always have a 30° phase shift between the two windings, as is documented in IEC norm IEC 76. This 30° phase shift is related to the fundamental frequency, normally 50 or 60 Hz. A 6 pulse rectifier connected to a 3-phase voltage source will introduce harmonic currents into the system when loaded. The harmonic current in this distortion will be I_thd=6n±l (n= 1,2,3...). In addition a third winding is introduced to the system for suppressing the 1 lth and 13th harmonics. The transformer thus providing a D0/Y11/Y5 vector group and a phase shift of 150° or 180° for the third winding in relation to the main windings. The theory for this is elaborated in the abovementioned WO 2005/119892 and will therefore not be discussed in detail here.

The present invention is illustrated in figures 2 and 3, where the transformer is constituted by two identical primary windings 1 ,2 having a phase shift relative to each other of approximately 30°. In this way the 5^th and 7^th harmonics are reduced, while the voltage characteristics of the windings are identical as they may be produced with identical number of windings and thus be perfectly balanced. The solutions according to the known art are based on Star-Delta windings, giving exactly 30° phase shift but slight differences in voltage over the windings as they are not identical and there are difficulties in adapting the number of turns in each delta or star winding to the chosen voltage. Practice has, however, shown that it is advantageous to have identical windings and thus balanced voltages while allowing for deviations in the phase shift.

Preferably the phase shift of the two identical primary windings relative to the the fundamental frequency between the inputs is in the range of -15° and +15° respectively, thus also providing a symmetric component which is easy to handle when designing the component into a larger system, and also being easier to produce.. The windings in the figures are of the Delta type, more specifically Do-windings, but Star- windings may also be used, as well as extended star and delta windings, the main point being that the windings are produced using the same method and thus being as similar as possible.

It is important for the optimal function of the invention that the voltage between the two main windings is equal. This is to obtain the best possible load distribution between the two rectifiers in the rectifier bridge, as well as to minimize reactive currents through the transformer, both in symmetric and asymmetric power production. This is difficult to obtain using in a Star-Delta configuration.

Figure 2 illustrates the preferred embodiment in the low voltage applications is shown where an additional winding 4 is applied to the transformer being coupled to one of the inputs and providing a cancellation of the 11^th and 13^th harmonics. In order to obtain the cancellation of the 11^th and 13^th harmonics the additional winding preferably provides a phase shift of 165° relative to the input. In this way a total of 180° phase shift is obtained relative to the fundamental phase of the power supply system In the illustrated example this is obtained using a Dβ-winding coupled to the pair of Do- windings. Even if the preferred phase shift is of 165° it is possible to use phase shifts in the range of ±30° from the of 150° shift, with acceptable effects on the results, but preferably within the range of ±15°. Thus an Y5 winding may also be used.

This solution has the advantage of being symmetric so as to make the coupling to the system symmetric and less complicated.

In order to improve the filtering the additional windings are coupled to the input through an LC filter adapted to maximize the attenuation of both the 1 lth and 13th over harmonic current components. This is obtained by adjusting the filter inductance and capacitance and the filter winding to the chosen resonance frequency. Usually this will be close to the 12^th harmonic frequency.

For realizing the transformer for medium and high voltage systems using voltages above 1000V it is advantageous that the filter components are constructed for low voltage systems. This is solved with the solution illustrates in figure 3, where a second additional filter 5 winding is added being in phase with one of the main windings. The secondary voltage of the two filter windings may then be a standard 690V but with the 150°- 180° phase shift depending on the chosen winding configuration, and may be provided with a LC-filter with chosen values adapted to provide the corresponding attenuation in the chosen phase shift range..

The additional windings may be chosen depending on the intended phase shift and filtering, and in order to provide an attenuation maximum e.g. at 150° or within the range of ±30° from 150°, preferably within the range of ±15°. They may thus be constituted by pairs, such as Yi+Dβ, D0+Y5, D0+Y7 or Do+Dβ with suitable LC-filters between them.

The main functionality of the system for providing power to a motor may thus for example be

• Normal rating of the LLC transformer is approx. 50% of the motor rating.

• Vector Group based on extended windings: De/De/D6 or Ye/Ye/D0

• Approx. 70 - 80% of the 5th , 7th., 1 lth & 13th.Harmonic currents will be cancelled through the LLC transformer & filter. • At symmetric power feed no current of the fundamental frequency (f.ex. 60 Hz) will flow through the LLC transformer main windings.

• Fundamental frequency current through filter winding between 5 and 10% of nominal rating.

• Losses are reduced to 5 - 20 % of nominal losses, (only losses caused by harmonic currents)

• Voltage distortion from 5th and 7th. Harmonic currents reduced to less then 3%. Total Harmonic Distortion (THD) less then 5%.

• If one of the power feeds are lost, approx than 50% of the load will flow through the LLC transformer. • At asymmetric powerfeed, only a small increase in THD will be observed.

Below is a table of possible values of the filter components L (μH) and C (μF) with different ratings of the transformer, are calculated based on resonance at the 12^th harmonic in a 60Hz system as will be understood by a person skilled in the art:

To summarize invention relates to a power supply system comprising a rectifying bridge supplying current to a power consumer, e.g. a motor, wherein the rectifying bridge including two AC power feed inputs adapted to be coupled to a power network as well as at least one output for supplying power to said consumer, and wherein a transformer having two identical first windings is coupled between said inputs, said windings being positioned with a relative phase shift relative of 30° , and relative to the the fundamental frequency between the inputs preferably in the range of approximately -15° and +15° respectively. The windings being e.g. delta windings, star windings, extended delta windings or extended star windings and thus provide an improved balance in the voltage over the two primary windings and also a simplified production of the system.

Preferably the system includes at least one additional winding coupled to one of said inputs providing a chosen phase shift, e.g. in the range of ±30° from 165°, to one of said first inputs, the additional winding being coupled to one of said inputs through a chosen LC assembly having chosen values, the additional winding being a suitable delta or star winding cancelling the 11^th and 13^th harmonics and being coupled to one of the inputs of the transformer through a chosen LC-filter.

The power supply system according to the invention may also include includes two additional windings instead of one, in which case the two are coupled to one another through a chosen LC assembly having chosen values and proving a relative phase shift in the range of 150 °. Preferably they are chosen from the list of Yi+Dβ, D0+Y5, D0+Y7 or D₀+D₆.

The system may be used in both low voltage, e.g. 690V, and high voltage, above 1000V applications, where the symmetry in the first embodiment illustrated in figure 2 may be practical in low voltage applications as the system is not affect whether the filter is coupled to the first supply A and the additional winding coupled the winding coupled to the second supply B, or vice versa. As stated above the second embodiment using two additional windings are preferred in high voltage applications as it allows for the use of low voltage filters. The system will work with both symmetric and asymmetric power supplies, but with higher loss in the latter case.

Claims

C l a i m s

1. Power supply system comprising a rectifying bridge supplying current to a power consumer, e.g. a motor, wherein the rectifying bridge including two AC power feed inputs adapted to be coupled to a power network as well as at least one output for supplying power to said consumer, and wherein a transformer having two identical primary windings coupled between said inputs, the windings having a relative phase shift of 30°.

2. Power supply system according to claim 1 wherein said primary windings being positioned with a relative phase shift relative to the fundamental frequency between the inputs in the range of -15° and +15° respectively.

3. Power supply system according to claim 1, wherein the primary windings are Delta-windings.

4. Power supply system according to claim 1, wherein the primary windings are star-windings.

5. Power supply system according to claim 1, wherein the transformer also includes an additional winding coupled to one of said inputs providing a chosen phase shift, e.g. in the range of ±30° from 150°, preferably within ±15°, to one of said first inputs, the additional winding being coupled to one of said inputs through a chosen LC assembly having chosen values.

6. Power supply system according to claim 5, wherein said additional windings is Delta-winding.

7. Power supply system according to claim 5, wherein said additional windings is a star-winding.

8. Power supply system according to claim 1, wherein the transformer also includes two additional windings, being coupled to one another through a chosen LC assembly having chosen values and proving a relative phase shift in the range of 150° ±30°, preferably within ±15°.

9. Power supply system according to claim 8, wherein said two additional windings are constituted by pairs chosen from a list of Yi+D₆, D0+Y5, D0+Y7 or D₀+D₆.