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GE Type kV2c Meter 

Current Sensing

July 2003

A new version of the GE type kV2 meter, kV2c, incorporates a different currentsensing technique. Novel, high performance current transformers are used in this design.

Characteristics of the new sensor design

The new CT used in the GE type kV2c meter has the following characteristics:

• Advanced core material and construction

• Similar structure for transformer rated and self contained meters

• Embedded burden resistors

• One piece primary conductor • Molded body and sleeved leads

The current transformer designed for the type kV2c meter uses an advanced core material providing high stability with mechanical stress and temperature variation, enhanced

linearity in ratio and phase over a wide range of operating conditions, and excellent

magnetic properties including low losses. These characteristics provide the basis for excellent metering performance.

The physical construction of the family of current sensors is structured for rationalized

 production, high quality and long life. Similar structures are used for all ratings of self 

contained and transformer rated devices. Burden resistors unique to each rating are

embedded into the CT further simplifying the product family structure. A robust one piece primary conductor is used. Finally, a high level of insulation integrity is provided

 by the molded enclosure and sleeved leads.

CT sensors in base CT assembly with primary

conductor

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Previous type sensors – Why change? Previous versions of GE type kV and kV2

meters have used a GE proprietary current sensor 

design. This sensor was based on a resistive current

divider and active feedback current comparator. Itis referred to as a “null flux sensor”. This

technically sophisticated sensor structure provides

excellent accuracy over a wide range of operatingconditions, including the presence of half wave

rectified loads which can create high levels of DC

current in the load. This immunity to the presenceof DC current, while an excellent characteristic in

 principle, has not been a general requirement in

commercial application. From a productionstandpoint, the sensor does require management of 

multiple suppliers and processes for its component  parts. Further, advances in current transformer 

core technology have enabled development of current transformers with enhanced characteristics.

The evaluation of these combined evolving trends

has led GE to decide that a transition to high performance CTs is the best course for this revision

of the kV2 meter.

GE null flux sensor

 

Detecting the presence of DC current High performance current transformers have the characteristic of saturation when

the primary current includes a high level of DC. This could be induced by the presence

of half wave rectified loads, but is not generally found in commercial meteringapplications. The kV2c meter provides a mechanism to detect this condition, should it

occur, and alert the user.

Typical meter loads do not contain waveforms with significant even harmonics.

Rectified loads however do contain large amounts of second harmonic. The relativemagnitude of these harmonics is further enlarged should saturation occur. So, it is

 possible to infer likely DC presence by measurement of the second harmonic content in

the measured current waveform. Detection of this condition causes the meter diagnosticcondition, d5, distortion, to be set. D5 is set when the total current level and second

harmonic level are high enough for saturation to be possible, and the ratio of second

harmonic current to total current exceeds 20%.The chart below shows the high percentage of second harmonic to total current in

a half wave rectified waveform. Also shown is the phase shift in the CT associated with

the progressive saturation condition at higher rectified currents. Due to this shift, these

loads do not necessarily mean under registration, as this positive phase shift can causeover registration of lagging loads.

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Rectified load - phase and 2nd Harm

200 amp CT meter 

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Fund phase shift

2nd Harm %