Current and voltage measurements using Rogowski coil in MV air-insulated switchgear

Measurements and communications

This study focuses on measuring currents and voltages by Rogowski coil and voltage dividers and communication bus using IEC 61850-9-2 in MV switchgear. Both technologies as such, have been independently subject of the investigations and explorations in many papers and publications. This thesis will propose and verify the simultaneous combination of both technologies implemented and practically verified in MV Switchgear.

It will offer a clear view, why both technologies deployed hand in hand will create new opportunities how to protect and control MV switchgear and networks in a different manner.

A further target is to explain why Rogowski coil and voltage dividers are a better alternative to well-known inductive transformers for current and voltage measurements in MV switchgear. An additional target is to offer technical arguments of what is the next step is the architecture of MV switchgear.

The thesis offers a system view on MV switchgear, it will explain how to connect loose well-known components: sensors – IEDs – digital bus to one seamless architecture and it will demonstrate how this integration will impact MV switchgear engineering and design approach.

First Part – The first part of the thesis will be focused on a detailed investigation of possible measurement accuracy influence by external factors in an indoor substation environment. Based on the experimental measurements thesis will investigate how good is the immunity of the Rogowski coil and voltage dividers against external factors that influence the accuracy of the measurement.

Second Part – The second part will propose an alternative approach in residual current measurement by Rogowski coil and IED processing in comparison to the existing practice of the residual current measurement by core balance transformer. It will analyze the applicability of the alternative measurement and processing of residual current in insulated and directly grounded networks.

Third Part – The third part will define the new system architecture of the MV air-insulated switchgear with seamless integration of the sensors, IEDs, and digital bus in the switchgear and substation.

Important aspects of communication network reliability and availability will be verified for the proposed architecture. The final part will demonstrate the practical application of the new proposed architecture deployed in real products and applied in a real substation.

The architecture of the MV air-insulated switchgear with sensors and process bus. Most of the air-insulated switchgear are single bus bar systems with two sections where each section includes one incoming feeder and several outgoing feeders. Both sections are horizontally separated with a bus coupler element which is typically operated with a normally open circuit breaker.

A typical single line diagram of such a substation configuration is in Figure 1.

Each circuit breaker feeder includes an IED. The most elementary task of the IED is to protect a particular feeder and load against fault situation and by breaking of the energy by tripping of the circuit breaker limit potential mechanical or thermal damages as a consequence of the potential fault preferably to zero level.

Measurements of currents and voltages are essential input data for the protection algorithms of IEDs. The majority of the protection functions requires the following measurements for their principles of work:

• Line to ground currents
• Incoming feeders line to ground voltages
• Bus bar line to ground voltages
• Residual current
• Residual voltage

The architecture of the substation where all above-mentioned measurements are available to any IED at any time would be very beneficial to use any protection function independently on the substation hardware configuration.

Assumption of availability of all needed measuring data can decouple configuration and engineering of the measuring hardware from protection application in particular IEDs.

The basic precondition to developing such architecture is to use Rogowski coil and voltage divider for current and voltage measurements. This decoupling can create many advantages what comes to engineering activities through the entire life cycle of the MV air-insulated switchgear.

• Linearity and accuracy of the current measurement by Rogowski coil does not require engineering of its parameters as a result of the short circuit selectivity study
• Decision on protection scheme to be finally used in the particular feeder can be done any time during project execution
• Additional protection function can be included in the protection scheme once switchgear is under operation without the need to re-engineer the measuring apparatus and changes in substation wiring
• Change of the rated current level during the life cycle does not require changes of the current measuring apparatus

A bottom-up approach was selected to define the new architecture of the MV air-insulated switchgear application which would meet the above idea of decoupling measuring apparatus specification and engineering from protection application.

1. Definition of the standard feeders
2. Overall example of the single line diagram and communication bus
3. Calculation of communication bandwidth
4. Selection of communication architecture and redundancy principle

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