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COMPENSATION OF COMMON MODE CURRENTS

IP.com Disclosure Number: IPCOM000240171D
Publication Date: 2015-Jan-08
Document File: 8 page(s) / 125K

Publishing Venue

The IP.com Prior Art Database

Abstract

The present invention proposes a technique to reject common mode current from the current measurement and also detects earth fault. Common mode currents are the sum of all phase currents in a multiple phase system. Insulation between electrical potentials is replaced by a small capacity capacitor. The capacities are charged and discharged by the common mode currents up to the level of common mode voltage. The common mode currents are separated from the load currents by an additional transducer and then the common mode current is passed in reverse direction through the transducer for load current measurement. As a result, the common mode current is directly compensated. High pass filter for the compensation of common mode current enables the earth fault detection.

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COMPENSATION OF COMMON MODE CURRENTS

BACKGROUND

The present invention relates generally to quality of drive control and earth fault detection and more particularly to compensation of common mode currents to control quality of the drive and earth fault detection.

Generally, earth fault is detected by summation of every phase current. Sum of all currents is zero in case there is no fault. The earth fault is detected when the sum of currents increases up to a pre-defined level.

Generally, common mode voltages exist in non-linear systems. A converter drives common mode voltage in a range of pulse frequencies. Every capacity of ground potential is charged and discharged up to a level of common mode voltage. The charging currents are common mode currents. The charging currents distort load current measurement and have similar effect as that from earth fault current. As a result, the earth fault detection is also distorted.

In a conventional technique, current measurements are neglected in case of transitions in common mode voltage. The technique is applicable in instances where common mode voltage is known. For example, common mode voltage is known in one single drive pulse width modulation (PWM) converter and in low path filter for current measurements. However, the conventional technique slows down over-current and short circuit detection.

It would be desirable to have an efficient technique to compensate common mode current and earth fault detection.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 depicts a technique wherein insulation is replaced by a small capacity capacitor.

Figure 2 depicts a technique having an addition of a transformer with one core.

Figure 3 is a graphical representation of assumption of time behavior of the common mode current and phase load current i1.


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Figure 4 is a graphical representation of current measurement signal im1 of current transducer T1 of Figure 1.

Figure 5 is a graphical representation of current measurement signal im1 of current transducer T1 with compensation of common mode current of Figure 2.

Figure 6 depicts earth fault currents detected by a current measurement transformer.

Figure 7 depicts an alternative way to separate earth fault current from higher frequency common mode current.

DETAILED DESCRIPTION

The present invention proposes a technique to reject common mode current from current measurement. The technique described herein also detects earth fault detection. Quality of drive control and detection of earth fault depend on accuracy of current measurement.

Common mode current is the sum of all phase currents in a multiple phase system. Usually, in electrical power systems designed topology does not include a path for such currents. Unsymmetrical systems, non-linear loads and converters initiate common mode voltage which is measured across insulation barriers.

Figure 1 depicts a technique wherein insulation is replaced by small capacity capacitors.


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