Distribution automation system

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The electric distribution system in the USA, Canada and many other countries of the world (Brazil, Mexico, Australia, South Africa, Korea, etc.) is significantly different to the distribution system in Europe. However, there are some European countries with a partly US-style power distribution system, for example Estonia and Latvia.

Both distribution systems include overhead line distribution and underground distribution with cables.

US-style overhead line distribution consists of distribution substations with outdoor equipment and long to very long distribution lines. In some cases, the length of these overhead lines can exceed 150 miles (240 km). Consequently, line losses and voltage drop due to line resistance and reactance are serious problems. The average number of supplied customers with these overhead lines is quite high (several thousand). As a consequence, the number of affected customers is high in case of an outage. This causes significant revenue losses for utilities and leads to decreasing customer satisfaction. To overcome the addressed problems, overhead distribution lines are segmented by Reclosers and Sectionalizers, which may be used for feeder reconfiguration in case of disturbances. Other equipment like voltage regulators (regulating transformer and controller), reactive power regulators (capacitor banks and controller), fault indicators and other equipment are used for optimal operation and fault identification and localization.

For a very long time, the above-mentioned distribution equipment has been operated locally. However, with the introduction of microprocessor based Intelligent Electronic Devices (IEDs) and the availability of affordable communication technology, Distribution Automation for fast fault detection, isolation and system reconfiguration is currently one of the major Smart Grid components.

With successful distribution automation, utilities have the opportunity to set up new business models for increased customer satisfaction, for example, the availability of highly reliable power supply for critical industry sites.

On the other hand, the power distribution structure with long distribution lines may also create significant problems, even if power is available. In summer, when all customers switch on their air conditioner, the load on distribution lines may reach dangerous dimensions, leading to thermal overload of the line and other components, and causing significant voltage stability and quality problems. In such situations, intelligent load shedding is a much-desired item. The integration of electronic meters with integrated load disconnection capability is a significant move in the right direction. However more customer-friendly solutions will be intelligent home and building focused energy management systems.

The distribution system in (middle) Europe is based on a different concept, compared to the US-style distribution system. The backbone of this structure is the highly meshed 110 kV subtransmission system, covering nearly all load areas, and the very high number of distribution substations. As a consequence, distribution lines are quite short (typically 5 km to 20 km), and the average number of customers supplied by one single distribution feeder is typically below 1 000. In addition, the connection of loads is done with precise planning and measurement, leading to highly balanced loads of distribution transformers. In contrast, the US-style distribution system is partly highly unbalanced, leading to additional power quality problems and thermal problems for transformers.

In Europe, Distribution Substations are being integrated and automated using microprocessor-based protection relays, bay controllers, remote terminal units, etc. to enable remote control and to reduce outage times. However, the (quite short) distribution feeder is not segmented, and the low voltage transformer stations are operated manually. Because of the highly advanced structure of the European Distribution System, there is no incentive for utilities to deploy the automation of distribution feeders. In case of a disturbance on a distribution feeder, the number of the affected customers is low, and the amount of revenue loss is also low.

However the increasing integration of Distributed Energy Resources (DERs), for example photovoltaic systems at low-voltage level and wind generators at medium-voltage level, causes voltage quality problems. In some of these areas, voltage magnitude is much higher than the acceptable maximum level of nominal voltage plus 10 %. With the integration of these “Generators”, the distribution system is no longer a radial system, which can be easily protected by simple non-directional Overcurrent protection relays. In the future, the application of differential protection systems will be required to meet the requirements of DERs.

In summary, the automation of the European distribution system, including low-voltage transformer houses as well as so-called micro-grids, will be strongly influenced by the acceptance and application of DER solutions.

For the automation of distribution systems, tele-control and supervision of secondary substation and transformer houses is crucial. Therefore, information exchange between those components and DMS systems shall be based on common protocols and shall be cyber-secure. The communication concepts shall be flexible for the use of different communication media and technologies due to different geographic and infrastructural conditions.

The standards listed below are available on  IEC webstore.

On the SGAM representation of the component layer (see Figure above) the current transformer, the switching element and the voltage transformer are supposed to be placed along the feeder but not in the derivation to the MV/LV transformer.

The feeder automation and smart reclosers component architecture is mostly made of three zones of components, which may be interconnected through wires or communication.

·     The Process zone includes the primary equipment of the electrical network mainly switching (i.e. circuit-breakers, switches and disconnectors) and measuring elements (i.e. current and voltage sensors/transformers). The representation on the SGAM is generic and does not necessarily correspond to any specific example.

·     The Field zone includes equipment to protect, control and monitor the process of the electrical network, mainly IEDs (which mostly handle protection, monitoring and control features like reclosing sequences), NIC (the controller of the LAN or HAN) and Router (the remote connection interface).

·      The Station zone includes the aggregation level which interfaces with other elements and systems of the distribution network. It mostly supports three main technical functions, which can be grouped or separated in different components: the RTU which serves as terminal for remote activities, the local controller which is in charge of performing automatic functions, and possibly an HMI/archiving component which offers the local operators capabilities of visualizing and archiving local data.

Communication protocols can be used either as in a) or b).

a)  Within each switching location along the feeder or within the feeders inside the substation, IEC 61850-8-1 (for any kind of data flows except sample values) and IEC 61850-9-2 (for sample values) are used to support the selected set of System Capabilities.
Considering that such a feeder may be seen as a distributed substation, many detailed guidelines provided by IEC TR 61850-90-4 can be applied.
IEC 61850 mostly replaces the former IEC 60870-5-103, used for connecting protection relays.

b)  Outside each switching location, “vertical communications” can rely on IEC 60870-5-101, or IEC 60870-5-104.
A new mapping of IEC 61850 over the web services technology (IEC 61850-8-2) has been specified, in order to enlarge (in security) the scope of application of IEC 61850 outside the substation, and more specifically address feeder automation needs.

Please refer to IEC TR 63097:2017 §4.10.4 for details on cyber-security standards and more specifically on where and how to apply the IEC 62351 series and/or other cyber-security mechanisms.

This set of standards can be positioned as depicted in the Figure above on the communication layer of SGAM.

The information layer (see Figure above) of feeder automation or smart reclosers is mostly based on the IEC 61850 information model.

We have indicated that the IEC 61850-7-4 is the core part depicting this model for each switching location along each feeder, and IEC TR 61850-90-2 for the communication to the control centre. However, other parts of the IEC 61850 series can also be used.

IEC TR 61850-90-6 is also indicated on the SGAM, which is a guide for the implementation of IEC 61850 on feeder automation.

For protocols which are not IEC 61850 native such as the IEC 60870-5-101 or IEC 60870-5-104, a mapping of IEC 61850 information model is possible using the IEC TS 61850-80-1, enabling users of these technologies to use the power of data modelling (and then more seamless integration) without changing communication technologies.