Protection In Distribution Grids 2.0 - Utility of Coburg Upgrades

Innovative protection concepts follow the trend towards integration, using synergies to maximize the guaranteed power supply.
Published: Mon 20 Jul 2015

As a modern enterprise the utility of Coburg has been focusing on future-oriented communication technologies at an early stage. The entire grid will be equipped with fibre. By combining protection and communication technologies this investment could now be used for the realization of a trend-setting concept for protection in medium voltage grids.

ABB realized this concept with the gateway approach of the teleprotection equipment NSD570 and the consistent use of functional capabilities of the station bus according to IEC 61850.

Distribution grid of utility of Coburg

To supply a 20kV medium voltage grid with ground resistance to a large region is a particular challenge for power protection in terms of selectivity and speed.

The 20kV Seßlach grid currently provides power to approximately 5,000 customers in an area of 90km². Due to the size of the entire grid, new sub-networks have to be run with ground resistance, as opposed to the traditional compensated way.

As the grid of the utility of Coburg includes both compensated and earth-grounded networks, teleprotection equipment has to provide a set of parameters for the low impedance and for the compensated network. This can be switched from the network control centre according to requirements.

In the course of the network expansion, fibres were lain so that the stations are interconnected and connected to the control centre in Coburg. This fibre network also provides fast communication between protection relays throughout the entire grid, which enables the realization of the modern protection concept using the international communication standard IEC 61850.

The continuous use of proven standards such as IEC 61850 brings the advantage of possible integration of devices from different vendors.

Equipment for stations in the Seßlach subnetwork

Protection concept

A protection system needs to reliably recognize all faults on each cable route and to selectively trip in the shortest possible time. Traditional techniques allow to monitor 85% of cable routes instantaneously and the remaining 15% with a delay of approximately 0.5 seconds.

For capturing the complete extent of each cable route, the utility of Coburg installed a teleprotection system between the stations in addition to the distance relays, so that the trip signals of the relays are transmitted fast and reliably to the line end. The signal transmission of station bus systems according to IEC 61850 via remote control over optical fibre is a first in medium voltage networks. The input and output coupling of the required distance protection relays are carried out via an Ethernet-based bus using GOOSE messages as opposed to many directly wired switching contacts. The routing of signals is now completely done with standardized engineering tools.

In the solution presented to the utility of Coburg the full range of functionalities of the IEC 61850 station bus is used. In addition to function and automation signals, protection signals are also transmitted using the station bus. The GOOSE interface of the teleprotection equipment NSD570 enables connection to the station bus, whereas the Ethernet/IP interface provides connection to the backbone devices of the fibre network.

Representative transmission times of the teleprotection system are <10ms, which leads to tripping times of the complete protection systems for 100% of each cable route of usually 70ms.

The trip signals (so called “commands”) are combined with certain conditions via a standardized logic. In this concrete case the POTT (permissive overreach transfer trip) procedure is used. In case of a failure, the protection relays only open the particular circuit breaker in station A and B within the line section in real time if the trip signal is also received from the opposite side via the teleprotection system.

Thanks to six distance protection functions the multifunctional protection relays REF630 offer individual adjustments of the range of the zone for phase-phase-failures and phase-earth-failures. The criterion for the fault detection is a polygonal under impedance excitation. A non-directional zero current protection is used as a backup protection with time delay.

Simplified functional diagram of permissive overreach transfer trip

Communication solution NSD570

ABB’s next generation teleprotection equipment NSD570 offers a complete set of interfaces to telecommunication systems and protection relays. Additional external devices, e.g. interface or data rate converters, are not necessary.

Traditionally the interface to the protection relay is contact-based and hard-wired per trip signal. One NSD570 can transmit up to eight simultaneous and independent commands to the remote station via such contact interfaces.

Alternatively, it is also possible to connect the NSD570 to the IEC 61850 station bus and receive maximum eight trip signals from the protection relays using GOOSE messages according to IEC 61850. The commands are filtered out of the GOOSE messages and converted into a transmission signal which is optimized for the used communication channel.

With this concept NSD570 supports the stepwise change of substations to digital technologies and IEC 61850. The large installed base of protection relays with contact-based command interfaces can be used with all available communication media, even the parallel usage of IEC 61850 compliant protection relays via the same NSD570 connection. In addition, commands from GOOSE messages can be transmitted from a substation to contact-based protection relays in a remote station.

Field mapping for the signal comparison

Engineering process and configuration tools

The engineering process is completely based on international standards and is independent from any customer or manufacturer specific aspects. This complies with the requirements of IEC 61850 for open systems and interoperability. The station description was executed in the SCL format which is defined by the standard.

In the first step the .ICD files of the NSD570 were generated and imported into the existing .SCD file of the project using ABB’s IEC 61850 configuration tool IET600. With the IET600 tool the GOOSE messages were linked according to the mapping of the fields and then a new .SCD file was generated for each station. Then the respective .SCD file was imported with the IED manager PDM600 so that the configuration of the protection relays of type REF630 could be extracted. In the same way the configuration of the teleprotection devices NSD570 was extracted from the .SCD files with the user program HMI570

The functions of the installed protection system were tested and simulated at the respective line ends with different fault simulations by synchronously controlled testing devices of the brand Omicron. The measured reaction and transmission times of the system convinced the utility of Coburg.