# Current and voltage sensors as an alternative to traditional CTs and VTs

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## 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:

### 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.

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.

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.

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.

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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.

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.

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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.

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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.

 Feature CT/VT Sensors Signal 1/5 A / 100/110 V 150 mV / 2V Secondary cables To be added Included and tested Linearity No Yes Saturation Yes No Ferro-resonance Yes (VT) No Temperature coefficient No Incl. in accuracy EMC No Shielded Short-circuited secondary Destructive (VT) Safe Open secondary Destructive (CT) Safe Weight 40-60 kg (CT+VT) 2-25 kg (combined) Standardisation possible Limited Wider 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

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### About Author

#### Edvard Csanyi

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

### 13 Comments

1. Achmad Zainuri Rajasa S.P
Dec 28, 2017

Great Article Edvard. Share more useful.

2. Jibu Varghese
Dec 28, 2017

Nice and informative article

3. Hamed Rezaei
Dec 02, 2017

There is actually an amplifier interface from a German company Kries which converts the low power of sensors to the typical 100/150V needed for conventional IEDs.

4. Noor uzzaman
Dec 22, 2016

Nice presentation.

5. 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

6. AMandaric
Dec 21, 2015

perfect

7. 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!

8. 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

9. PRASAD RAO
May 28, 2015

great

10. Anupam
May 27, 2015

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

11. Habu
May 25, 2015

Informative

12. nitin
May 25, 2015

Great

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