Smart Compact Secondary Substations for future distribution networks

As various sources of renewable power generation are introduced, the medium-voltage (MV) distribution network must adapt to maintain high quality of supply through new network concepts and automated solutions. In this article, Martin Stefanka of ABB PPS outlines the key role of the 'smart' Compact Secondary Substation (CSS) in future self-healing power distribution networks.

There is increasing focus on power quality in the distribution network, as its key nodes provide the connection points for a wide variety of loads as well as a growing number of unpredictable renewable power sources. The introduction of distributed generation in distribution networks requires protection and control systems that can reliably locate and isolate any faults.

Traditional distribution networksTraditionally, distribution networks have been operated either as open rings (as shown in Figure 1) or as radial feed networks (as shown in Figure 2).

Figure 1. Traditional ring distribution network.

Figure 2. Traditional radial distribution network.

Both network topologies use components with basic protection, in the form of fuses, with limited or no automation built in. Generally Distribution Network Operators (DNOs) are not aware of events that occur behind the primary substation. In expanding distribution networks with a high number of supply points, this presents a challenging environment in which to secure stability of supply.

For both ring and radial networks, time-consuming manual work is needed to locate and restore supply after a fault has occurred. Typically, the DNO is not aware of faults in the secondary distribution network until customers report that they are without power, potentially leading to long outages.

Solutions to automate distribution networks have been available for some years, but this has usually been achieved by combining loose and independent elements into systems. The cost–benefit of such systems has, in most cases, not supported full-scale investment.

A combination of reduced manpower available for network maintenance and the introduction of new energy sources is creating a challenging environment for the future distribution networks, characterized by:

an increased focus on power quality, with the integration of renewable energy sources

penalties for loss of energy supply

fused protection requiring manual fuse replacement in the event of failure

a need to integrate monitoring and automation functions at an acceptable cost.

Future MV distribution networksRecent years have seen the distribution network grow in complexity, as distributed generation has been introduced. This directly influences the energy flow in the distribution network as well as the electrical parameters used for the protection schemes.

New approaches to protection are needed to properly protect the complete system, independent of network topology and the number of power generation points. Cost-effective line differential protection systems are required for closed ring distribution networks with changing energy flows.

New network elements are needed to efficiently control renewable energy sources connected to secondary distribution networks. Small energy storage solutions of up to 2 MW could play an important role in balancing peaks of supply and demand as well as contribute to supply quality – controlling voltage, power factor or harmonics. This will, however, require connectivity to DMS systems with the intelligence needed to calculate the active power (P) and/or reactive power (Q) requirements according to the actual situation and the available P and/or Q in energy storage systems. As well as implementing proper protection, control and monitoring as renewable sources are introduced, solutions also need to help balance the need to meet growing energy demands with the need to reduce carbon dioxide emissions.

To reduce the risk and length of supply failures, many national regulators have introduced penalties for non-delivered energy. DNOs therefore need solutions that maintain or increase the quality of energy supplied as their MV distribution network become more complex.

A key part of the solution is the Smart Compact Secondary Substation (CSS), which can be placed at selected key network nodes to provide remote monitoring and control. This will require simple, cost-effective signal collection, processing and communication, along with optimize overall control of the distribution network to simplify reconfiguration of the network after a failure.

In addition, the intelligent Ring Main Unit (RMU) will play an important role in future MV distribution networks as a 'plug-and-play' solution for DMS connectivity. The automated measurement, monitoring, control and communications capabilities of Intelligent Electronic Devices (IED) provide all the information needed to implement automated fault identification, fault isolation and power restoration. As a result, power outages can be shortened and system reliability improved significantly.

Today, the typical distribution network RMU includes Load Break Switches (LBS). In a future network, it may be beneficial to operate the distribution ring without any open point to reduce

power losses, meaning circuit breakers will provide a better solution. This also opens up the possibility of fault handling without customer impact.

Distribution network zoningDividing distribution networks into zones – separated by active and intelligent components – provides a way to handle fault situations in an optimal way. Optimal in this context means having as few affected consumers as possible, fast power restoration and the involvement of as few personnel as possible.

The zones are defined according to consumption criticality and the vulnerability to disturbance. A zone may include several traditional MV distribution rings or only parts of these rings. Zones are divided by circuit breakers, LBSs or disconnectors with remote communication and varying degrees of intelligence for protection, measurement and control.

Figure 3 shows the various zones downstream of the substation as shaded areas. Located between these zones are zone dividers with protection and breaking/reclosing or simple disconnection capabilities. All zone dividers have facilities for remote communication to transmit the status indications, control commands, measurements, and so on, required by the application. Depending on the capabilities of the zone divider equipment, the zone on the downside is either a protection zone or a control zone.

Figure 3. Main principles of zoning.

Zone borders are located according to the capability of the zone dividers, the differences in fault vulnerability between the areas and the criticality of the power supply within the areas. For example, areas can be differentiated according to fault probability, or the need to secure the supply to areas with substantial and/or critical consumption. The same criteria are used when determining whether a zone should be a protection zone or a control zone.

Communication is a central part of the zoning concept, as it is essential to know the status of the zone divider equipment and to control it. With the development of highly capable, widely available, reliable and secure public wireless networks, it is now feasible to implement communication capabilities in most nodes in a distribution network.

Smart CSSIn a zoned distribution network, the CSS with its built in technology becomes a key node, and act as a 'smart' CSS.

Figure 4 shows a typical RMU configuration with three cable switches (C) and one vacuum circuit breaker (V). Each feeder in the RMU is equipped with sensors that measure both current and voltage for all three phases: the RMU controller can monitor up to 12 current and 12 voltage inputs.

Figure 4. Remote monitoring and control in an RMU.

The position indication of each switch and breaker in the RMU is fed to the RMU controller for local as well as remote monitoring and/or control (SCADA). The RMU controller sends open/close commands to each switch or breaker in the RMU, either locally or remotely from a SCADA system.

The traditional, largely manual, procedures for reporting and restoring power outage may take several hours to complete, depending on how fast customers report the power outage and the time required for the maintenance crew to locate the fault and to restore power.

The measurement, monitoring, control and communication capabilities of RMU controllers enable automated fault identification, fault isolation and power restoration. As a result, power outages can be shortened and system reliability improved significantly. The RMU controller illustrated in Figure 4 uses a Fault Passage Indicator algorithm to detect the forward or reverse fault for a variety of earthing systems – including isolated, solidly earthed, resistance earthed, or resonant grounding.

In the case of external power failure, an RMU controller must provide a back-up power supply. This is achieved by using batteries that are monitored and charged by an integrated battery charger.

As the RMUs are spread across the distribution network, the challenge is to provide the collected data to the SCADA system and control the RMU efficiently. One way to achieve this is through alternative communication channels such as public cellular (GSM/GPRS, 3G) networks. Other communication options, including fiber-optic or other wireless technologies (such as point-to-point radio), should be supported by the RMU controller.

The RMU controller must be easy to install and operate for both new and retrofit installations. The size of the RMU controller is also a key factor: ideally, the different functionalities shown in Figure 4 should be available in one box and integrated with the RMU itself – greatly reducing installation time and cost, as well as requiring fewer components with interconnections, improving system reliability, and reducing maintenance and lifetime cost.

Currently, remote control and monitoring of a CSS is performed by the RMU only. The smart CSS will take things one step further and include both the transformer and low-voltage switchboard, as shown in Figure 5, further securing and improving energy supply quality.

Figure 5. Smart CSS.

ConclusionFuture distribution networks will be affected by the introduction of renewable power generation along with a variety of critical load types, securing quality of supply will require new levels of automation. Smart CSSs, with fully integrated RMU solutions, can be placed at key points in the network to monitor critical components and key network parameters. Solutions will be needed to upgrade traditional CSSs to Smart CSSs, without the need for a completely new MV installation.

In addition, flexible communication with a DMS system will enable collected data to be processed and defined actions to be executed automatically in the event of network problems.

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