Analysis of Power Distribution Systems (The impact of distributed generation)

Distribution System Structure

The distribution substation is the interconnection element between the distribution system and the upstream power delivery system. At the substation the step-down (HV/MV) transformer reduces the subtransmission voltage level to an appropriate value for primary distribution lines.

Different protection, switching, and measurement equipment is installed at the substation to ensure a safe operation. The primary distribution lines spread across the consumption area served by the substation, these primary distributions lines are also known as feeders.

One or more lateral lines (or laterals) branch from distribution feeders and extend until they reach the step-down (MV/LV) distribution transformers, which are responsible for performing the final voltage reduction in order to obtain a voltage level adequate for customer use (e.g. 400 and 230 V).

Overhead lines are primarily used in rural circuits, whereas in urban circuits distribution lines are mostly underground. In suburban areas there can be a mixture of overhead and underground circuits. Big industrial zones are usually served by dedicated circuits as they represent large loads that can affect the service of other loads.

Figure 1 presents the typical configuration of a power distribution system, including the substation and the layout of one distribution feeder.

Reliability Assessment of Distribution Systems

Reliability analysis is a fundamental piece of distribution system planning and design. Reliability models and studies can be useful, among other aspects, to identify design limitations, quantify equipment improvements or determine the impact of system expansion.

The importance of reliability assessment is increasing with the advent of the smart grid. The future distribution system offers a potential of improved reliability by implementing rapid fault location and isolation, and service restoration.

It is widely accepted that distributed generation (DG) can have a positive impact on the distribution system since it can support voltage, reduce losses, provide backup power, provide ancillary services, or defer distribution system upgrade.

Two main modes of DG connection can be distinguished:

1. Operating as a backup source within a microgrid;
2. Operating in parallel with the distribution system.

In the first case, the generation units are locally operated and can be allowed to inject power to the system. If they are correctly controlled, they can have a positive impact on distribution system reliability.

A generation unit operating in parallel to the system can be forced to be disconnected in case of system fault, and the benefit to the system reliability is not so obvious and can be negative.