Distribution Automation Handbook
ELEMENTS OF POWER DISTRIBUTION SYSTEMS:
Primary Distribution Substations:
A primary distribution substation is the connection point of a distribution system to a transmission or a sub-transmission network. Outgoing feeders from a primary distribution substation are typically feeding secondary distribution substations and bigger, most often industrial type, consumers directly. What is considered to be the voltage level for a primary distribution substation varies country by country and depends on the whole electricity network structure and extent and historical and organizational issues. To give some kind of a picture of the voltage levels, below is an example about the system level allocation in a certain country.
| Switchgear Type and Busbar Arrangement |
A typical primary distribution substation would include air-insulated outdoor-type highvoltage side (HV) and a metal-enclosed air-insulated indoor-type medium-voltage switchgear
(MV). Due to specific reasons, like space limitations, environmental aspects and security, the
substation can be built using Gas Insulated Switchgear (GIS) technology. Utilizing GIS technology, both the high-voltage part and the medium-voltage part can be built using metalenclosed indoor-type switchgear. The GIS technology allows placing the whole substation installation inside a building, either on the ground surface or below the ground level.
Figure 3.2 shows a typical primary distribution substation located in a sparsely populated area. The high-voltage side (110 kV) is an outdoor-type air-insulated switchyard and the medium-voltage side (20 kV) is metal-enclosed air-insulated switchgear located inside the building.
In addition to the high-voltage air-insulated or gas-insulated installation possibilities, also a so-called hybrid solution is available. This solution consists of highly integrated gas-insulated outdoor-mounted bay units. One bay unit includes circuit breaker, disconnector(s), measuring transformers and the local control and interface cabinet in one transportation unit. The unit has been factory-assembled and tested, offering standard connection points to protection and remote control circuits. This solution utilizes air-insulated busbars for connecting the different type of units together to achieve the requested substation layout solution and functionality. Different standard bay units are available for different feeder types and substation layout solutions.
While designing the construction of a primary distribution substation, there are a number of different busbar arrangement alternatives for both voltage levels. The choice between the different alternatives is always a compromise where issues like the initial investment cost, operational flexibility, equipment maintenance and availability have to be considered. It is obvious that the different criteria in the consideration have different weight factors depending on the actual customers to be supplied. Industrial type of customers present high demands on the availability and operational flexibility, whereas customer types like recreational housing areas impose higher demands on the investment costs.
The main alternatives for busbar arrangements are covered shortly in the following. For each
of the alternatives, a summary of advantages and drawbacks are listed. The actual configuration of a complete substation can be a mixture of the different alternatives in respect of high voltage and medium-voltage installation.