Remote control systems and communication devices in secondary substations

SCADA, DMS and RTU

This technical article deals with the devices that make possible the interface with the primary process (e.g. cables and switchgear) in a secondary substation from a central point, like central control databases/systems that facilitate all kinds of stakeholders with data.

To enable this functionality, there has to be a chain of devices starting with the sensor in or on the primary infrastructure and ending on the screen of a stakeholder.

This article will use a top-down approach with a brief description of SCADA/DMS systems because they are currently the most common central systems. Then, an overview of the most common data communication technologies is presented.

Finally, the heart of this system is described: the Remote Terminal Unit (RTU).

RTU is the intermediary between the primary process and the central system to connect all the data and make logical operations with the data.

Contents:

1. SCADA and DMS
2. Communication to Secondary Substations
3. Remote Terminal Unit (RTU)
4. Measuring, Monitoring and Control
5. UPS
6. Main applications

1. SCADA and DMS

Traditionally, if secondary substations are equipped with any form of automation, they send data to a Supervisory Control and Data Acquisition (SCADA) system. This SCADA system makes it possible for controllers to watch over the grid 24/7.

A SCADA system uses a Real Time Database for real time operation. Of course, relevant data are shared with off line databases, allowing back office analysis.

The system monitors certain conditions, so as to determine if an alarm event has occurred (alarm handling functions) and, possibly, to perform consequently proper actions (e.g., activation of automatic procedures).

SCADA system’s databases and software programs offer a human machine interface (HMI) to provide schematics, trending, diagnostic data and so on.

Where many SCADA systems are quite straightforward, Distribution Management Systems (DMS) can provide the controllers (and other stakeholders) real-time and offline calculations such as state estimation, power flow, optimal switching etc.

A DMS system is fed by SCADA and corporate archives with dynamic data such as status of switches/breakers, measurement etc. and more, “static data” such as network size, load and generation profiles, characteristic of conductors etc.

The different SCADA and DMS solutions adopted have direct impact on:

• Naming convention
• Network representation (diagrams, schemes, …)
• Devices configuration (directly by the SCADA and/or by means of concentrators and/or converters/data gateways)
• Network management
• Network operation, both in case of manual and remote controlled operations
• Update of the connection state of the network
• Automation functionalities, both at grid and substation level.

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2. Communication to Secondary Substations

External communication is intended as the link between the substation and other systems. This can be communication between substations to a SCADA/DMS system or to a remote management tool.

Different types of data communication technologies will be described:

1. xDSL
2. FO (Fiber Optic) Glass fiber
3. PowerLine Carrier or Broadband over PowerLine
4. Dial-up
5. WiMax
6. GPRS/UMTS/LTE

2.1 xDSL

Digital subscriber line (DSL) has a relative high-speed technology based on two-wire twisted pair lines like existing telephone cables.

Distribution system operators can use this technology from third party suppliers or can use the technology on their own old existing networks. This can be an interesting solution for areas with no wireless communication or when there is an existing network available.

2.2 FO (Fiber Optic) Glass fiber

Glass fiber is based on optical signals. Therefore, it’s ideal for high speed and long distance and with lowest latency of all communication systems.

This way of communication is not so common for secondary substations because it’s quite costly to install them, if not done while installing power cables.

2.3 PLC/BPL

PowerLine Carrier (PLC) or Broadband over PowerLine (BPL) uses the energy grid for communication.

Usually, the data from the secondary substation are communicated over the MV network to a primary substation from where they are communicated through a different technology.

2.4 Dial-up

Older serial connections use dial-up modems. These can be wired or wireless. In most wireless cases, the dial-up connection is triggered by events. Dial-up connections should be included in security checks.

2.5 WiMax

In rural areas, WiMax is an alternative to make wireless communication reliable and independent from public communication networks. Speed and latency are similar to Fiber Optic communication.

Applications using WiMax are up-to-date mainly on overhead lines including tower substations.

2.6 GPRS/UMTS/LTE

For countries with a good mobile network, this is often used for the automation of secondary substations. The advantage is that it’s affordable and widely available. The disadvantage is that it’s harder to get secured and the availability depends on the traffic of other users of the network.

Earlier generations are the 3G UMTS technology and the 2G GPRS technology.