Protection Testing in Medium Voltage Systems with Voltage and Current Sensors

More and more medium voltage switchgears are equipped with voltage and current sensors. Alternative terms which are also used for sensors are LPIT (low power instrument transformer) or NCIT (non-conventional instrument transformer). In a medium voltage switchgear sensors are directly connected to IEDs like projection devices or fault locators, as the distance to the process is small. Where on the other hand, in high voltage switchgears sensors are connected to IEC 61850 merging units, to bridge the bigger distances between the process and the control cabinet. 

The reasons for the usage of such sensors are the advantages due to their design. Sensors are smaller and lightweight so they can fit into a normal cable connection cabinet of a compact ring-main-unit. Sensors ensure high system and personal protection because they have only a small output power. So arcs when opening conventional secondary current transformer terminal or exploding voltage transformers, when short circuit secondary wirings are not happening anymore. As the current sensors have a high bandwidth fewer types are needed. Also, sensors don’t show ferroresonance effects and safe energy as there are lower heating losses.

Medium voltage switchgear equipped with sensors consists of three parts. The IED, the sensors, and of course the switchgear itself. It needs to be verified that all components fit each other. Manufacture and system integrators will assist on that topic.

The three most common sensors are:

There are many advantages, there are also some drawbacks or differences to conventional instrument transformers. The connection is meanwhile mostly RJ45 connectors, but there are different pin assignments for different vendors. The output signal of sensors is mostly a low voltage signal (also for Rogowski current sensors). There are various nominal voltages like 22,5 mV, 150 mV, or 3,25 V (L-L). 

But there is also good news: 

• From the manufacturer's side, there is a trend towards standardization.
• You can perform secondary test on medium voltage systems with voltage and current sensors. 

To perform such protection or meter test OMICRON offers a wide range of accessories for their CMC test sets to simulate secondary signals of sensors. 

For further please contact OMICRONs technical support or have a look at the product websites: 

• LLX1 Low Level Output Accessory - Testing devices with sensor inputs
• CMLIB 7Sx8 - Interface adapter for SIEMENS protection relays with sensor inputs
• CMLIB REF6xx - Interface adapter for ABB protection relays with sensor inputs
• CMLIB A - Connection box for low level signals

These accessories can be used with small adaption existing OMICRON Control Center Templates. Here is an example of the configuration in Test Universe.

Adapt the instrument transformer ratio in the test object to 1/1 and the limits of the primary values:

In the hardware configuration add a voltage and current sensor. Enter the voltage (e.g. 80 A / 150 mV) and current sensor (e.g. 1/10 > 80 A / 15 mV) ratios and route the outputs to your needs.

Know you can start protection testing like you are used with CMC test sets. Keep in mind to work only with primary values, as the secondary values are not in the conventional format and can be very confusing.