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Current and voltage sensors as an alternative to traditional CTs and VTs
Current and voltage sensors as an alternative to traditional CTs and VTs (on photo: The current snsors type KECA 80 Cxxx are intended for use in current measurement in medium voltage air insulated switchgear type UniGear ZS1 12/17.5kV)

Sensors as alternative

As an alternative for traditional primary current and voltage measurement techniques, the use of sensor technique is gaining field. This technique is typically applied to current and voltage measurement in medium-voltage metal-enclosed indoor switchgears.

There are many undeniable advantages with sensors when compared to the traditional solutions:

  • Non-saturable
  • High degree of accuracy
  • Personnel safety
  • Extensive dynamic range
  • Small physical size and weight
  • Possibility to combine current and voltage measurement into one physical device with compact dimensions
  • Environmental friendliness (less raw material needed)

The above statements are discussed in more detail in the following paragraphs while introducing the sensor techniques and the actual related apparatus. Let’s say a word about each type of sensor and some conclusion at the end:

  1. Current sensors
  2. Voltage sensors
  3. Combined sensors
  4. Conclusion and answer on why sensors are not 100% alternative

Current Sensors

The measurement of current is based on the Rogowski coil principle. The Rogowski coil is a toroidal coil without an iron core. The coil is placed around the current-carrying primary conductor. The output from the coil is a voltage signal, proportional to the derivative of the primary current.

The signal is then integrated in the secondary device to produce a signal proportional to the primary current wave form.

Since no iron core is employed, no saturating occurs, unlike with traditional current transformers.

The open-circuited traditional current transformer produces dangerous voltages to the secondary side and lead to a serious overloading of the transformer. Since the output from the current sensor is a voltage signal, the open-circuited secondary conditions do not lead to a dangerous situation, neither to human beings nor apparatus.

Principle of current measurement based on Rogowski coil
Figure 1 – Principle of current measurement based on Rogowski coil

The transmitted signal is a voltage:

Uout = M · dip / dt

For a sinusoidal current under steady state conditions the voltage is:

Uout = M · j · ω · Ip

With traditional current transformers, the ratio of the CT is fixed to one value, or in case of multi-ratio CTs, to several values. These values are chosen according to the specific application needs and load currents.

As a result, one, for example medium-voltage primary switchgear, installation usually requires several CT types.

With a current sensor, the situation is simpler, since one type of sensor covers a range of primary currents and in optimum case the whole installation can be covered with one type only.

To give an idea of the secondary-voltage signal level, one fixed point (ratio) inside the rated current range could be 400 A primary value, typically corresponding to 150 mV secondary signal level.

Example on current sensor’s rated current range
Figure 2 – Example on current sensor’s rated current range

The problems related to saturating iron core in conventional current transformers can be overcome with the sensor technology. The below figure demonstrates the difference between the secondary-signal performance for both traditional current transformer and current sensor.

Principle comparison of current sensor and current transformer secondary-signal performance as a function of combined error (ε) and primary current (IP)
Figure 3 – Principle comparison of current sensor and current transformer secondary-signal performance as a function of combined error (ε) and primary current (IP)

Due to the compact size of a current sensor (no iron core), there are better possibilities to integrate the measurement devices inside other constructional parts of a metal-enclosed switchgear.

An example of this possibility would be the integration of a sensor inside plug-in-type medium-voltage cable terminations.
On the left a current sensor inside cable plug-in termination and on the right a current sensor inside conventional housing
Figure 4 – On the left a current sensor inside cable plug-in termination and on the right a current sensor inside conventional housing

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Voltage Sensors

The measurement of voltage is based on voltage divider. Two main types are available, namely the capacitive one and the resistive one. The output in both cases is a low-level voltage signal. The output is linear throughout the whole rated measurement range.

The considerations and protection methods against the ferroresonance phenomena, discussed with traditional voltage transformers, are not applicable with voltage sensors.

Two main principles for voltage sensor implementation
Figure 5 – Two main principles for voltage sensor implementation

As with current sensors, also with voltage sensors it is possible to cover certain voltage range with one sensor type. To give an idea of the secondary voltage signal level, one fixed point (ratio) inside the rated voltage range could be 20000/√3V primary value, typically corresponding to 2/√3V secondary-signal level.

Voltage sensor implementations. On the left a dedicated voltage sensor and on the right a sensor located inside a support insulator
Figure 6 – Voltage sensor implementations. On the left a dedicated voltage sensor and on the right a sensor located inside a support insulator

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Combined Sensors

The sensor solution being quite compact and space saving, it is possible to combine both current and voltage sensors in one physical device. This device can be part of the switchgear’s mechanical basic construction, having other functions beside the measurement, like being a part of medium-voltage cable termination or busbar support construction.

These features give new possibilities to design switchgear constructions that are built according to specific customer needs and on the other hand they help the standardization work for the bulk type of switchgears.

A combined current and voltage sensor acting also as a busbar tube support insulator
Figure 7 – A combined current and voltage sensor acting also as a busbar tube support insulator

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Conclusion and comparison

The features of the sensor measurement technique compared to the traditional approach are shortly summarized in the figure below.

It could also be asked why the sensor approach has not totally taken over the traditional approach, at least when it comes to medium-voltage indoor switchgear. This is a very valid question and several answers could be given, depending on the viewpoint of the person answering.

FeatureCT/VTSensors
Signal1/5 A / 100/110 V150 mV / 2V
Secondary cablesTo be addedIncluded and tested
LinearityNoYes
SaturationYesNo
Ferro-resonanceYes (VT)No
Temperature coefficientNoIncl. in accuracy
EMCNoShielded
Short-circuited secondaryDestructive (VT)Safe
Open secondaryDestructive (CT)Safe
Weight40-60 kg (CT+VT)2-25 kg (combined)
Standardisation possibleLimitedWider possibilities

Without going into this discussion any deeper, one valid argument is the limited selection of sensor-connectable secondary devices other than protection relays (IEDs).

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Reference // Distribution Automation Handbook (prototype) – ABB

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

9 Comments


  1. Tom Neilson
    Apr 21, 2016

    Hi Edvard,
    Good article, but your last point nails the problem – there are hardly any IEDs/relays that are compatible with these sensors. Do you know of any third-party amplifiers which would transform the 2-3V output of the resistive divider voltage sensors into typical 110V VT output levels so that we could use sensors in an off-the-shelf relay designed for VTs?
    Regards,
    Tom


  2. AMandaric
    Dec 21, 2015

    perfect


  3. Suresh Deshpande
    Jul 17, 2015

    interesting article although thinking why this is not possible! But design and practical was not feasible in the available resources!


  4. Karol
    Jun 03, 2015

    Hi,
    article is great, if you want to read more about Sensors and their application in Medium Voltage switchgear, ABB has its UniGear Digital solution. For more details go here : http://new.abb.com/medium-voltage/switchgear/air-insulated/iec-and-other-standards/unigear-digital


  5. PRASAD RAO
    May 28, 2015

    great


  6. Anupam
    May 27, 2015

    Happy about that these are so efficient than ct vt..but does not it have any drawbacks?? Eager to know…..


  7. Habu
    May 25, 2015

    Informative


  8. nitin
    May 25, 2015

    Great

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