Methodology of Supply Group Design
Requirement
The basic requirements behind the decisions for size and structure of Supply Groups are intended to limit risk and also to manage and restrict ownership of the meters/vending.
Risk
If a Supply Group key is compromised, it will be necessary to do key changes for all customers in that area in order to convert them to a new key revision number. This has not happened yet but is an increasing possibility as the installed meter base increases and STS is accepted internationally. Any such key changes obviously requires re-coding of all the security modules with new keys and visits to every customer in the area to perform the key changes on the meter. The risk and amount of work required, is much lower if the area has several smaller supply groups instead of a few large supply groups.
Ownership
A Supply Group defines the owner of the meters in that area. Only people with that particular Supply Group key can vend to customers in your Supply Group. (It is possible to load multiple Supply Groups into a security module to enable the vending of electricity to several Supply Groups).
Whenever a geographical area has to be split into two, or if two areas have to be combined at a later date, key changes have to be performed for all the customers in those areas since the supply group code (SGC) is the only way to restrict vending to the specific area. (This requirement may arise if a portion of a prepaid area is transferred from one utility to another.) One utility is usually not allowed to vend electricity to the customers from another utility which means that the utility will not have the keys for the Supply Groups of the other utility.
Further more, if various towns have the same supply group, it will not be possible to restrict vending machines in the area to only vend for one town and not the other. This restriction may not be a serious constrained but may become a requirement if one of these towns are to receive electricity from another utility, e.g. if Eskom takes over the supply of electricity in one area. All the customers in such an area must then be changed to a separate supply group which would require visiting every meter to perform the necessary key changes.
Design Proposal
The best design for supply groups would be separate supply groups for geographically separate areas. Such supply group separations will mostly depend on the possibility of combining or division of ownership for these areas in future. The number of customers in a supply group must also be considered to limit the risk if the key is compromised. These decisions can only be made by a party with adequate knowledge of the local conditions, geographical distribution, vending infrastructure and economical climate that may result in change of ownership of these areas.
Additional Management Considerations
A further point to consider is that more supply groups for a utility will result in meters with different SGCs in the stores that must be controlled and managed. It can be difficult to ensure that meters with the correct Supply Group is installed in the correct area.
One solution might be to order all meters for new electrification projects pre-coded for the correct supply group, but to order some/all meters for replacements on the default SGC. These meters can then be re-coded when they are installed (or just before installation). There are several tools to assist a utility with such meter coding processes, e.g the Meter Initialization System (MIS), Vendstar handheld engineering terminal, or key change tokens can simply be generated from most of the standard vending systems. The MIS was developed specifically with this in mind and would be the recommended solution if many meters have to be re-coded. These tools must be tightly controlled since there is some risk associated with the ability to change meters from one key to another, changing power limits or adjusting tariff indices. However, they all provide engineering logs for trace-ability.