kv2c
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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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