With the rapid growth of distribution network installations, there is a need to monitor and control electrical equipment within the network. Prefabricated substations have the facility to be monitored, controlled or even have complete network management in order to offer full reliability to power providers.
Recent 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.
NERSA has introduced the NRS048 standard to manage the quality of electricity provided to customers. Utilities 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 containerised 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 optimise 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 utility connectivity. The automated measurement, monitoring, control and communications capabilities of intelligent electronic devices (IEDs) 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.
Fig. 3: CSS being delivered to site.
Dividing 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.
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.
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 CSS
In a zoned distribution network, the CSS with its built in technology becomes a key node, and act as a “smart” CSS (Fig. 1).
Fig. 2 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.
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.
Fig. 4: CSS lowered into position.
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 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.
Fig. 5: CSS in position.
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 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.
Benefits of containerised substations
Manufactured, tested and commissioned by the OEM
This will ensure that no last minute work has to be done on site, or that the unit has to be returned to the OEM for repairs of faults found at commissioning.
Portability
The substations can be disconnected and transported on a lowbed truck to a new site (Figs. 3, 4, and 5). This opens up scores of possibilities for utilities and the national generation to have one or two spare substations in stock to replace damaged or burned substations.
Quick and easy installation
Very little civil preparation or work is required to have a fully functional substation on site. The container can be lowered with a crane into the correct place with ease.
Totally scalable to suit any need and extendable designs are not uncommon
Specially designed busbar interconnections are used to connect one substation to another, alternatively cabling can be used as per normal.
Planning
With new sensor technology utilised by ABB, it is possible to build a one-size-fits-all substation for all MV requirements. The future of municipal and national utilities is to have stock of “plug and play” substations.
Longevity
The containers have a long lifetime made from durable shipping container materials. E-houses can also be custom built from stainless materials.
Typical applications
Large events
A CSS can be used to connect MV gen-sets to the grid. Frequency monitoring will start the generator set before load shedding occurs. Built in web based interfaces provide fast and inexpensive control over a system from the control room
Open pit mining
A CSS can be mounted on a skid. Cable connections entering at the bottom of the CSS will be closed off with glands to prevent dust entering the CSS, which will reduce the need for increased servicing requirements. Built in web based interfaces provide fast and inexpensive control over the system from the control room, while protecting the network.
Mega and large utilities
A CSS can be fitted with a standard 36 kV UniGear Digital board which can be used for currents up to-2500 A and voltages up to 33 kV). The CSS can contain simplified switching between transformer, feeder and motor protection, with web based interfaces which provide fast and inexpensive control over the system from the control room, while protecting the network. The CSS can be utilised to do upgrades at integral parts of the system or where thefts or faults destroy whole substations.
Upgrading sensitive loads or generation substations
Two CSSs can be used to sectionalise busbars. One side can be switched off, allowing connections to be moved to the other side before reapplying power. Once the other side has been switched over and made live, work can be done on the substation without risk.
Conclusion
The use of containerised substations saves utilities and large energy users the costly long lead times usually associated with substation upgrades, and can be custom made to suit specific needs. This means that when the CSS arrives on site, everything will be in order. Utilising sensor technology ensures that the MV electrical projects are scope creep immune.
Contact Tamara Chetty, ABB, Tel 010 202-5093, tamara.chetty@za.abb.com
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