The present invention relates to a method for starting electrical machines driven from a power source via one or more transformers between the electrical machine and the power source, according to the preamble of claim 1.

The present invention also relates to a system for starting electrical machines driven from a power source via one or more transformers between the electrical machine and the power source, according to the preamble of claim 13. Background

In many applications, e.g. in subsea oil and gas processing, electrical machines a re d riven from power sources located at a considerable distance from the driven equipment and the machines - such systems are often called "systems with long step-outs". In the long step-out systems, high voltage (for example 20-30 kV) is preferable for transmitting the power between the source and the machine with low losses. At the same time, the power source often includes static frequency converters, to control speed of the d riven electrical machines, and such converters have limited voltage ratings (in most cases 0.4-11 kV). Also insulation design of the electrical machines is often limited to the level of 11 kV. Thus, the voltage ratings of the static frequency converter and the driven machine are considerably lower than the desirable transmission voltage. The typical solution is to use transformers for stepping up the voltage for transmission over the long cable and then stepping it down nea r the electrica l mach ine to match its voltage rating. Similar systems are described in, for example, W09318566A1 or in WO2013039404A1. If voltage rating of the converter is lower than that of electrical machine there can be used only step-up transformer. In such a case transmission voltage is the same as the machine voltage.

The long step-out systems with at least one transformer between the converter and the machine has a ma in challenge during sta rt-up, when low frequency and low voltage is formed by the frequency converter for the electrical machine, - the challenge is conversion of power at very low frequencies (≡ 0...2 Hz) in the transformer which may cause saturation in the transformer. To prevent this transformers have in some cases been oversized. The lower the frequency is, the la rger the transformer must be.

Usually induction motors can be started from frequencies of 1.5 Hz or even higher. For such frequency the transformer can have double size compared to the one designed for rated frequency. The requirement to apply very low frequency (≡ 0...1 Hz) is especially important when starting permanent magnet synchronous machines. If starting at higher frequencies the machine may not catch up the rotating magnetic field. Transformer for such system would be especially large - several times larger than the one designed for rated frequency.

Some loads require high torque right at the start-up - at low frequency. For example, in subsea pumps there is so-called "stiction torque" (see Figure 1.) which must be overcome at start-up. That means that enough current should be provided for the machine at start-up, which in turn means there should be enough voltage at the transformer secondary winding, i.e. the transformer should not be saturated.

WO 2007/058782 and WO 2008/130411 describe bypass solutions for soft starters.

To summarize, start-up of an electrical machine via transformer at very low frequency requires oversizing the transformer. In the case of starting an induction machine, transformer size may be reasonable, though larger than the one designed for the rated frequency. In the case of starting permanent magnet machine transformer size may be unacceptably large.

Object

The main object of present invention is to solve the problem of prior art for the systems with transformers between power source and electrical machine and ensure reliable start and control of electrical machines without oversizing the transformers.

Another object is to solve the problem of prior art for the systems with long step-outs, i.e. long cables and at least one transformer between power source and electrical machine.

Final object is to enable reliable start and control of permanent magnet machines in the systems with at least one transformer between power source and permanent magnet machine. The invention

A method for ensuring reliable start and control of electrical machines driven through transformers without oversizing the transformer according to the present invention is described in claim 1. Preferable features of the method are described in the claims 2-12.

A system for ensuring reliable start and control of electrical machines driven through transformers without oversizing the transformer according to the present invention is described in claim 13. Preferable features of the system are described in the claims 14-23. The present invention is related to starting electrical machines (motors) driven from frequency converters via long cables and where transformers are arranged between the electrical machines and a power source.

In the present invention it is proposed to by-pass at least one of the transformers during start-up of the electrical machine. Without transformer between the power source (e.g. frequency converter) and the electrical machine it is possible to transfer power at very low frequencies or even at DC. Starting an AC machine (motor) requires both low frequency (usually 0-10 % of rated frequency) and low voltage (usually 0-20 % of rated voltage), therefore, voltage rating of the power source will be enough and stepping the voltage up with the help of the transformer is not necessary. According to the present invention, after the successful start-up, when the electrical machine has started rotating (or started moving in linear machine case) and the frequency is high enough, i.e. stiction torque is overcome (this can be seen by considerable load reduction on the power source), the transformer can be connected again to provide higher voltage. The transformer can be pre-magnetized before connecting it to limit rush -in current.

According to an embodiment of the present invention both step-up and step-down transformers are by-passed during start up.

According to another embodiment of the present invention only the step-up or step-down transformer is by-passed during start up. For example, if it is desirable to avoid using subsea switchgear then subsea transformer should be oversized to ensure start-up without by-passing. In such a case only step-up transformer can be by-passed at start-up.

It can be also an advantage to warm up the electrical machine before starting it by supplying the machine with current of very low frequency (e.g. below 1 Hz), pulsing current or direct current (DC). It is possible when there is no transformer between the power source and the electrical machine, i.e. when the transformer is by-passed.

The by-pass is achieved by means of a by-pass arrangement including a by-pass circuit and switches, e.g. three switches, where e.g. one switch is arranged in the by-pass circuit and one switch is arranged at each side of the transformer in a power line of a system. By means of the switches the transformer can be connected or by-passed as desired.

The switches of the by-pass arrangement are controlled by a control unit provided with means and/or software for this. The control unit can be an integral part of the frequency converter or can be a separate unit. Further preferable features and advantageous details of the present invention will appear from the following example description.

Example

The present invention will below be described in further detail with references to the attached drawings, where:

Figure 1 shows a torque curve of subsea pump-motor unit,

Figure 2 shows a prior art system with one step-up transformer,

Figure 3 shows a prior art system with two transformers; step-up and step-down,

Figure 4 shows a first embodiment of a system according to the present invention,

Figure 5 shows a second embodiment of a system according to the present invention, and

Figure 6 shows a third embodiment of a system according to the present invention.

Reference is now made to Figure 1 which shows a torque curve of subsea pump-motor unit. As the figure shows, stiction torque ST is quite high at start-up, while main torque curve MT is "quadratic", i.e. torque is proportional to speed squared.

Reference is now made to Figure 2 which shows a prior art system with one step-up transformer 14. The system contains main switch 17, grid side transformer 16 with three windings, power source 15 in the form of a static frequency converter, cable 13, as well as an electrical machine (motor) 12 and a driven mechanism 11. The system with only one transformer 14 (step-up) can be used when voltage rating of the frequency converter 15 is lower than voltage rating of the electrical machine 12 and at the same time transmission distance is not long, usually below 5 km.

Reference is now made to Figure 3 which shows a prior art system with two transformers; step-up 14 and step-down 18. The system with two transformers is used, for example, when distance between the power source and the driven mechanism is long, usually over 5-10 km. Then power transmission should be done at high voltage to minimize losses.

Reference is now made to Figure 4 which shows a system according to a first embodiment of the present invention provided with a by-pass arrangement for by-passing the step-up transformer 14 of the system in Figure 2 in the operation of starting the electrical motor 12. The by-pass arrangement according to an embodiment of the present invention includes three switches 19, 20 and 21, wherein switches 20 and 21 are arranged on each side of the step-up-transformer 14, respectively, and the third switch 19 is arranged in a by-pass circuit 30. The switches 19-21 can be "mechanical", "solid state" or of other suitable types. Example of starting of the electrical machine 12 is described in the table below.

Table 1. Switching states at start (variant).

When switching to normal run (connecting the transformer 14) it is important to have the same voltage phase angle at both sides of the transformer 14. A preferable way to ensure this is by using a transformer without phase shift (e.g. YyO, DdO), i.e. use a transformer that has a vector group without phase shift. Another way is to change phase angle of the voltage at the moment of switching with the help of the frequency converter 15. The phase angle shift should take place at the moment of switching the transformer 14 between the power source 15 and the electrical machine 12.

The transient processes (oscillations etc.) due to the switching can affect stability of operation of the electrical machine 12. This is especially relevant for permanent magnet synchronous machines (PMSM), which can fall out of synchronism. To ensure stable operation a damper winding (not shown) can be arranged at a rotor of the PMSM.

Reference is now made to Figure 5 which shows a system according to a second embodiment of the present invention provided with a by-pass arrangement for both step-up 14 and step-down 18 transformers for the system in Figure 3 in the operation of starting the electric motor 12. The by-pass arrangement in this embodiment consists of six switches 19-24, i.e. two times the by-pass

arrangement 30 in Figure 4, one associated with each transformer 14 and 18, respectively. In the case of subsea application, the transformer 18 as well as the switches 22-24 will need to have subsea design. It will be an advantage to use transformers without phase shift or to have phase shifts compensating each other (e.g. step-up transformer Yy6 and step-down transformer Yy6 as well). Example of starting is described in the table below.

Table 2. Switching states at start (variant).

Reference is now made to Figure 6 which shows a system according to a third embodiment of the present invention provided with by-pass arrangement for both step-up 14 and step-down 18 transformers, used for starting subsea rotary frequency converter, comprising several electrical machines (motors) 12 connected in parallel driving subsea pumps or compressors 11.

The electrical machine 12 can in other embodiments be part of a cascade, i.e. there can be other electrical machines between the electrical machine 12 and the driven equipment 11.

Other similar embodiments of the system should be apparent to a person skilled in the art.

List of numerals:

11. Driven mechanism (pump, compressor, etc.)

12. Electric machine (motor)

13. Cable

14. Step-up transformer

15. Frequency converter

16. Grid side transformer

17. Main switch

18. Step-down transformer

19. Switch by-passing step-up transformer

20. Switch 1 connecting step-up transformer

21. Switch 2 connecting step-up transformer

22. Switch by-passing step-down transformer

23. Switch 1 connecting step-down transformer

24. Switch 2 connecting step-down transformer 30. by-pass circuit