Design process for the new primary substation (structure selection and calculations)

Primary substation

The scope of this document is to study and develop methodology by which techno/economical dimensioning of primary substation in power system can be done. The document considers existing power supply system and changes that should be done in the process of designing of new substation.

The equipment that will be used in the substation is described. All necessary calculations are made, on the basis of which the equipment is selected according to state standards.

Objectives

Designing of a substation is a process, when you have to consider not only technical aspects of the issue, but economical aspects as well. This work considers the special features and parameters of power system that could finally affect a power substation.

The work address the following issues:

• Definition of optimal structure of power substation according to the investment cost, operational cost and reliability cost, in long run;
• Estimating techno-economic range of power substation structure depending on the total peak load of the area;
• Evaluating the effect of outage costs in techno-economic studies.

The fourth chapter considers a technical view on primary substation. The main components of primary substation are mentioned and described.

In chapter five the case studies about substation design are presented. Methodology, which was already described, is adapted to the chosen region. Chapter six includes final conclusion.

Main transformers

The function of main transformers on primary substation is to step-down incoming voltage to a suitable level. Power transformers make the biggest contribution to the total cost and represent to of the total substation cost.

The amount of main transformers is selected according to each case-area features, but, in general, it is installed from one to six transformers on primary substations to convert incoming power.

The sum of all transformers capacities is equal to a substation’s capacity.

Types and number of main transformers

The most common main transformers, which are used in usual regional and MV networks, are three-phase main transformers. The main reason is that the use of one three-phase transformer on primary substation instead of using three single-phase transformer units is more beneficial. One three-phase transformer needs less empty space, has lower losses and lower maintenance cost than three single-phase transformers of equivalent capacity.

The type of transformer on primary substation is selected according to its application and case-area features.

There are a lot of special types of transformers, which are used to accommodate special requirements. For instance, for distribution applications it is often required to convert power to two voltage ranges. In this case three-winding transformers are used to provide three voltage levels for some substations and few loads nearby.

Figure 2 shows a primary substation with three-winding transformer.

One more very important special transformers type is low footprint transformer. It is used to be fit into substations, which were originally designed for smaller units.

Another category is high impedance transformers, which are designed to limit fault currents. These transformers have a higher cost, but reduce fault currents and breaker requirements to appropriate levels.

The majority of primary substation main transformers are delta-connected on the high side and either wye-connected on the low side. If power transformer has a big capacity, wye-connection is utilized in order to decrease linear currents on the low side. The same rules are also used for three-winding transformers.

The majority of three-winding transformers also have delta-connection scheme on the high voltage side. If it is needed to decrease linear current on medium-voltage or low-voltage side, wye-connection might be used.

Moreover, main transformers of big capacity (over 60 MVA) are costly and difficult to transport. Because of that, there are some economic reasons for using two or three transformers of smaller capacity instead of using a single transformer of greater capacity.

It should be noted that the forecasted load growth is not always accurate enough, and there might be a need in increasing transformers capacity.

But, it is not easy to carry out for all types of substations. For example, the capacity of main transformers on primary substation, which is originally planned for multiple transformers, could be increased in stages by paralleling of additional transformer without serious reconstruction of primary substation.

The reconstruction of a single transformer substation will take more time and investments.