Digital Medium Voltage Switchgear

Author: Benton Vandiver III, ABB, USA

The electric power industry has been developing and implementing the concept of Smart Grids for some time based on the advances in computer and communications technologies in order to meet the requirements for improved reliability, security and efficiency of the electric power system. However, meeting these goals is not possible by just using microprocessor based multifunctional Intelligent Electronic Devices (IEDs) and communications. Improving return on investment includes reducing capital costs in equipment, as well as the cost of transportation, installation and maintenance.

Another important cost factor is safety, improving personnel safety working in proximity to the substation equipment is a key cost driver.

Medium voltage (MV) switchgear needs to meet many different requirements due to the wide range of its applications. While it is predominantly used in distribution substations, it is also used in industrial installations, commercial facilities, university campuses, as well as for the interconnection of distributed energy resources. This means that the switchgear needs to be flexible and accurate over a wide range of currents and voltages, while at the same time being safe and reliable, with minimum maintenance requirements.

Combining microprocessor technology, advanced communications, IEC 61850 and non-conventional instrument transformers allows moving medium voltage switchgear into the era of the Smart Grid.

Digital MV Switchgear

Considering how conservative the electric power industry can be, it is important to develop new and innovative solutions based on the combination of well-established and market-accepted products with the latest available technologies and standards in order to respond to new applications and challenges. The result is a new innovative digital solution to Medium Voltage (MV) switchgear that offers significant improvements to the customers by providing them with responses to the latest demanding requirements.

It is based on an optimized integration of MV sensors for current and voltage measurement into the MV switchgear combined with latest design of protection relays as well as with efficient utilization of IEC 61850 communication means for signal distribution within the switchgear.

Medium voltage switchgear used for the distribution of electrical energy is a very important element of electrical networks the function of which is to ensure uninterrupted power supply to the whole network.

With the new requirements and challenges the distribution networks are to face the MV switchgear assumes an increasingly important role as a part of the grid than ever before.

Evolution in distribution networks asks for a MV switchgear that is more efficient, safe, smart, reliable, environmentally friendly, and easy to engineer, install and operate.

The Digital MV Switchgear can be used to distribute electric power in a variety of demanding applications such as on off-shore platforms, in container or cruise ships, in mines as well as in utility substations, power plants or industrial plants.

It is accomplished by using well-proven components such as current and voltage sensors, multifunctional protection IEDs and IEC 61850 digital communication.

The current sensors used are of highly compact design, optimized for the use in Digital MV Switchgear. Each compartment can accommodate two sets of current sensors.

The voltage sensors are very compact as well. They are integrated as part of support insulators housed in the cable compartment or built directly in the busbar compartment.

The current and voltage sensors are of very high accuracy (accuracy class 0.5), however revenue metering might require yet higher accuracy classes or the installation of instrument current and voltage transformers for separation purposes. The transformers can optionally be added to sensor-equipped panels.  (Figure 2).

Sensors based on advanced measurement principles have been developed as successors to conventional instrument transformers in order to achieve significant reduction in dimensions, increase of safety and to provide greater rating standardization with a wider functionality range.

Conventional instrument transformers with magnetic cores are based on well-known principles that have been utilized with all their advantages as well as limitations for more than 120 years. However, the connected equipment (protection relays) has significantly changed during the last 20 years.

New protection relays place different requirements on primary measurement equipment (instrument transformers) compared to classic electromechanical relays.

These new requirements also open up the opportunity for the utilization of advanced measurement principles that offer a wide range of additional benefits.

Sensors open up a way for current and voltage signals needed for the protection and monitoring of medium voltage power systems. These advantages can be fully used in connection with modern protection relays. (Figure1).

The current and voltage sensors for the Digital MV Switchgear are designed without the use of a ferromagnetic core. The current sensor is based on the principle of Rogowski coil, while the voltage sensor uses the principle of resistive voltage divider. This kind of sensor technology brings several important benefits for the user and the application.

The main benefit is that the behavior of the sensor is not influenced by a magnetizing curve which results in a highly accurate and linear response across a wide dynamic range of measured quantities. (Figure 3).

The linear and highly accurate characteristic curve of the sensor across its full operating range enables several metering and protection classes to be combined in one particular device.

The sensor technology means no transfer of power from the primary to the secondary side, which means negligible power losses. Therefore, the sensors exhibit extremely low energy consumption the value of which is just a fraction of what is converted into heat in a conventional instrument transformer.

This fact contributes to significant energy savings during the system's entire operating life, supporting world-wide efforts to reduce energy consumption.

Since the sensor elements are particularly small and the same elements are used for both measurement and protection, the current and voltage sensors can easily be integrated in the switchgear.

The current sensors based on the Rogowski Coil principle provide some additional benefits, such as:

  • Due to their low inductance, they can respond to fast-changing currents
  • Due to their linearity high-current Rogowski coils can be calibrated or tested using much smaller reference currents
  • They are much safer than conventional current transformers, because there is no danger of opening the secondary winding
  • The have lower construction costs
  • Temperature compensation is simplified

These facts enable the sensors to be designed in a highly optimal way, which contributes to a high level of switchgear simplification and therefore standardization, while at the same time reduces the size of the switchgear.

The sensors are connected to the protection relay via RJ-45 connected cables. In case both current and voltage sensors are connected to a protection relay, a coupler adapter is used. The coupler adapter can be a one phase or three phase adapter. When the protection IEDs used in the Digital MV switchgear have combined sensor inputs. Each current and voltage sensor has a separate cable with one RJ-45 connector. The cable is a separable part of each sensor which requires the use of a coupler adapter to combine two RJ-45 connectors from current and voltage sensors into a combined sensor input for each phase on a protection IED as shown in Figure 4.

IEC 61850 Communications

The IEC 61850 standard was released in 2004 as a global international standard representing the architecture for communication networks and systems for power utility automation. It also includes the related system requirements and data model of protection and control functions. Standardized data modelling of substation functions including the communication interfaces pave the way to openness and interoperability of devices.

The IEC 61850 standard defines the Station bus and Process bus, where the Station bus IEC 61850-8-1 defines the vertical and horizontal GOOSE communication (real time communication between protection relays) and the Process bus IEC 61850-9-2 defines the transmission of Sampled Measured Values (SMV) obtained by measurements. The GOOSE and SMV profiles make it possible for the MV substation communication to be designed in a novelized and flexible way to make the protection relay process data available to all other protection relays in the local network in a real-time manner.

Protection and control IEDs generate signals for interlocking, blocking and tripping between panels via horizontal GOOSE communication in Digital MV switchgear. Today, GOOSE communication usage is increasing in substation applications and it offers additional advantages like simplicity, functional flexibility, easy scalability, improved diagnostics and faster performance compared to conventional hard wired inter-panel wires.

Process interfaces to MV apparatus (e.g. voltage sensors) are on the process level. Besides conventional signal wiring between the process interface and protection relays, IEC 61850 introduces a concept where the exchange of process signals can take place across the process bus, as per IEC 61850-9-2.

In MV switchgear applications, the station and the process bus can be combined together with proper configurations to reduce cost.

When using conventional voltage instrument transformers (VTs) in a MV switchgear these usually are installed in the incoming feeders on the cable side, with busbar voltage measured in any of the outgoing feeders or in a dedicated metering panel. The sharing of the busbar voltage is done by interconnection wires between busbar VTs and the protection relays in all outgoing feeders.

Usage of sensors and IEC 61850-9-2 has a significant effect on the design of the switchgear. The signal from the voltage sensor measuring the busbar voltage in one of the protection IEDs is digitized by an integrated Merging Unit function element into sampled values stream shared over Ethernet network published 80 times per cycle based on the implementation agreement IEC 61850 9-2 LE. The interconnection wiring in switchgear becomes simplified as fewer galvanic signal wires are needed. Transmitting voltage measurements over process bus enables greater error detection because the signal transmission is continuously supervised. And even higher availability is possible by using redundant Ethernet network over which the GOOSE and SMV signals are transmitted. (Figure 5).

Ethernet Redundancy

IEC 61850 specifies network redundancy that improves the system availability for substation communication. It is based on two complementary protocols defined in the IEC 62439-3 standard: the parallel redundancy protocol (PRP) and high availability seamless redundancy (HSR) protocol.

Both protocols are capable of overcoming a failure of a link or switch with zero-switchover time. In both protocols, each node contains two identical Ethernet ports for one network connection. They rely on the duplication of all transmitted information and provide zero-switchover time in case of failure of a link or switch, thus fulfilling all the stringent real-time requirements of substation automation.

PRP and HSR redundancy is supported by our protection relays and the choice between these two protocols depends on the particular application and the required functionality.

Time Synchronization

Protection relays utilizing sampled measured values need to be synchronized between the publishing (protection relay sharing the analog value) and the receiving protection relays. The synchronization in the protection IEDs supports the Precision Time Protocol (as defined in IEEE 1588 standard), with microsecond accuracy.

A protection relay can act as a master clock for Best Master Clock algorithm in case no external master clock is available.

PTP synchronization method enables usage of existing Ethernet network to propagate synchronization messages across the network, which eliminates the need for extra cabling in the substation. (Figures 6 and 7).

Example Application

UniGear ZS1 Digital is suitable for indoor installations and applications with voltage level up to 24 kV, rated feeder current up to 4 000 A, and short-circuit current of up to 50 kA. UniGear ZS1 distributes energy in a variety of demanding applications such as off-shore platforms, on container or cruise ships, in mines as well as utility substations, power plants or chemical plants. (Figure 8).

Multifunctional Protection IEDs

The multifunctional protection IEDs are a compact and versatile solution for power distribution in utility and industrial applications. They provide standard configurations for specific applications, which allows the user to easily adapt and set them.

A dedicated feeder protection IED is designed for the protection, control, measurement and supervision of utility and industrial power distribution systems including radial, looped and meshed networks, also involving possible distributed power generation.

They support IEC 61850 based peer-to-peer communications and can publish or subscribe to both GOOSE messages, as well as sampled values.

Testing of Digital MV Switchgear

To ensure that the Digital MV Switchgear will operate successfully under challenging environmental conditions it is necessary to conduct EMC tests for both the current sensors and voltage sensors. The tests need to be performed in an accredited EMC test laboratory, in accordance with IEC 60044-8 (current sensors) and IEC 60044-7 (voltage sensors) standards.

With the purpose of proving the superior EMC performance of the Digital switchgear, specific EMC real live tests have been conducted at the HV laboratory of the University of Stuttgart. These tests are even more demanding compared to the above mentioned IEC standards.

Furthermore, testing simulating the arcing at the truck contacts during switching operations as well as the switching of a reactor load with re-ignitions have been conducted. Both of these tests passed without findings of arcing in the switchgear environment.

During all these tests the process bus traffic has been monitored with no negative effect recorded.

Also a specific robustness test of Ethernet traffic transmitted over the metallic Ethernet wires has been conducted during internal arc testing of the UniGear at CESI laboratories. No influence on the GOOSE Ethernet traffic was recorded.

Benefits of Digital MV Switchgear

Digital MV Switchgear represents an advanced switchgear solution addressing important requirements of the future:

  • Flexibility
  • Increased process efficiency
  • Lower cost of operation
  • Maximized integration
  • Reliability and safety

Thanks to such advanced technology the user does not need to face many of the common challenges in today‘s more complex applications, but can focus on meeting application requirements and create a reliable, efficient electrical network due to use of well-proven components: current and voltage sensors, protection and control relays with IEC 61850 digital communication.

The switchgear can be easily adapted to changes implemented in your application.

There is no need to replace the switchgear hardware, but only to update the parameters or logics of the protection relay. This is achieved thanks to digital communication running between panels instead of using traditional hard wired signals.

Changes are handled by modifying the software of the digital relays and using the IEC 61850 communication. Thanks to their linear characteristics the sensors can operate over a wide range of primary currents, so if the application’s load current changes the equipment does not.

Digital MV switchgear provides a reliable solution which minimizes the risk of outages and increases switchgear availability. The sensors are not using an iron core, therefore they are immune against grid disturbances such as ferroresonance. Thanks to their smaller dimensions the sensors contain less insulation material which reduces the risk of isolation degradation in a switchgear. Moreover, it increases the safety of operating personnel thanks to error free low voltage connections between sensor and the protection relay. Digital communication continuously supervises all signals. Errors are immediately detected and back-up schemes can be activated. The use of sensors increases the safety for personnel owing to the maximum level of secondary signals during normal operation amounting to only a few volts.

Digital MV switchgear uses the space in the substation or in an industrial facility in an efficient way.

It reduces its overall footprint by omitting the metering panel from the switchgear and integrating the compact size of voltage sensors that can be installed into the feeder busbar compartment. The switchgear room can thus be smaller or more switchgear panels can fit inside.

Future MV Switchgear

Continuous improvement process will evolve MV switchgear to new levels of efficiency and safety by using proven technologies and international standards.

The evolution from Digital MV switchgear to a self-sustained Smart MV switchgear is just around the corner.


Benton Vandiver III received a BSEE from the University of Houston in 1979.

He is currently the Technical Sales Engineer for ABB in the Southeast Region located in Houston, TX. A registered Professional Engineer in TX, he is also an IEEE / PSRC senior member. He was with Houston Lighting & Power for 14 years in Substation Engineering, with Multilin Corp. for 4 years as Project Manager and with OMICRON electronics for 22 years in various sales, marketing, and technical roles. He has authored, co-authored, and presented over 95 technical papers and published numerous industry articles.

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BeijingSifang June 2016