Variable speed drive system, method for operating a variable speed drive system and method for refrigerating a hydrocarbon stream

Patent metadata

Field	Value
Publication number	US-9479103-B2
Application number	US-201314423940-A
Country	US
Kind code	B2
Filing date	Aug 29, 2013
Priority date	Aug 31, 2012
Publication date	Oct 25, 2016
Grant date	Oct 25, 2016

Abstract

A variable speed drive system employing an electric motor and a frequency converter arranged between an AC power source and the electric motor is operated. The frequency converter functions to convert AC power obtained from the AC power source at a source frequency to converted power at a variable drive frequency. A mechanical assembly can be coupled to the electric motor. Electric modulation circuitry is provided interacting with the frequency converter. It is arranged to impose fluctuations, independently from any torsional excitation in the mechanical assembly and the AC power source, in the inter-harmonic frequencies of inter-harmonic currents generated in the frequency converter.

First claim

The invention claimed is:

1. A method of operating a variable speed drive system comprising an electric motor and a frequency converter arranged between an AC power source and the electric motor, the frequency converter being coupled to the AC power source via a power line, and the electric motor being coupled to the frequency converter via an electric line, the method comprising the steps of:
employing the frequency converter to convert AC power obtained from the AC power source at a source frequency to converted power at a variable drive frequency that is unequal to the source frequency, wherein the converted power comprises inter-harmonic currents having additional frequency components at inter-harmonic frequencies that are unequal to integer multiples of the source frequency;
powering the electric motor with the converted power via the electric line;
driving a mechanical assembly comprising at least a drive shaft and a mechanical load with the electric motor via the drive shaft;
modulating the variable drive frequency to cause fluctuations in the inter-harmonic frequencies independently from any torsional excitation in the mechanical assembly and the AC power source,
wherein the electric motor rotates at a speed that is controlled by a speed controller that interacts with the frequency converter via a speed control signal, wherein said modulating the variable drive frequency to cause fluctuations in the inter-harmonic frequencies comprises periodically changing the speed control signal around an average speed control signal according to a time-dependent deviation from the average speed control signal, wherein the speed at which the electric motor rotates is fluctuated from the average speed by less than 100 rpm.

2. The method of claim 1 , wherein said fluctuations may be imposed in accordance with a pre-determined pattern of fluctuations.

3. The method of claim 1 , wherein said causing of fluctuations in the inter-harmonic frequencies consists of repeatedly and periodically changing the inter-harmonic frequencies.

4. The method of claim 1 , wherein the fluctuations in a selected inter-harmonic frequency stay within a maximum deviation of between 3% and 15% from the frequency of the selected inter-harmonic frequency in case it were unfluctuated.

5. The method of claim 1 , wherein said modulating the variable drive frequency to cause fluctuations in the inter-harmonic frequencies comprises changing the variable drive frequency around an average drive frequency according to a time-dependent deviation from the average drive frequency, the time-dependent deviation from the average drive frequency staying within a maximum deviation from the average drive frequency of between 3% and 15%.

6. The method of claim 1 , wherein the speed control signal is modified according to a square wave pattern.

7. The method of claim 1 , wherein the speed at which the electric motor rotates is fluctuated from the average speed by less than 20 rpm.

8. The method of claim 1 , wherein converting of said AC power to converted power comprises forming a rectified signal by rectifying said AC power and converting said rectified signal to said converted power using an inverter comprising switching components activated by firing pulses, wherein said causing of fluctuations in the inter-harmonic frequencies comprises periodically changing a firing angle of the firing pulses around an average firing angle according to a time-dependent deviation from the average firing angle, wherein the time-dependent deviation from the average firing angle stays within a maximum deviation from the average firing angle of between 0.5° and 10°.

9. The method of claim 1 , wherein said modulating the variable drive frequency to cause fluctuations in the inter-harmonic frequencies comprises modulating the inter-harmonic frequencies in accordance with a pre-determined wave form.

10. The method according to claim 9 , wherein the pre-determined wave form varies in a pre-determined way as a function of drive frequency.

11. A method of refrigerating a hydrocarbon stream, comprising the steps of:
operating a variable speed drive system in accordance with a method as defined in claim 1 , wherein the mechanical load comprises a compressor;
cycling a refrigerant fluid between compressing the refrigerant fluid with the compressor and cooling the hydrocarbon stream by heat exchanging the hydrocarbon stream against the refrigerant fluid.

12. A variable speed drive system, comprising:
an electric motor;
a frequency converter arranged between an AC power source and the electric motor, the frequency converter being coupled to the AC power source via a power line and the electric motor being coupled to the frequency converter via an electric line, to convert AC power obtained from the AC power source at a source frequency to converted power at a variable drive frequency that is unequal to the source frequency;
a mechanical assembly comprising at least a drive shaft and a mechanical load, the mechanical assembly being coupled to the electric motor via the drive shaft; and
electric modulation circuitry interacting with the frequency converter and arranged to modulate the variable drive frequency to cause fluctuations, independently from any torsional excitation in the mechanical assembly and the AC power source, in the inter-harmonic frequencies of interharmonic currents generated in the frequency converter, the electric modulation circuitry forming part of a speed controller which is arranged to interact with the frequency converter via a speed control signal,
the electric motor being arranged to rotate at a speed that is controlled by the speed controller via the speed control signal, said electric modulation circuitry being arranged to modulate the variable drive frequency to cause fluctuations in the inter-harmonic frequencies by periodically changing the speed control signal around an average speed control signal according to a time-dependent deviation from the average speed control signal, the electric circuitry being arranged to fluctuate the speed at which the electric motor rotates from the average speed by less than 100 rpm.

Assignees

• Shell Oil Co

Inventors

• Verhulst Michel Pieter Alfons

Classifications

• H02J3/0014

  for preventing or reducing power oscillations in networks · CPC title

• F25J1/0022

  Hydrocarbons, e.g. natural gas · CPC title

• H02P29/0033

  Electricity · mapped topic

• H02P29/0038

  Electricity · mapped topic

• F25J1/0298

  Safety aspects and control of the refrigerant compression system, e.g. anti-surge control · CPC title