SUBSTATION WITH POWER TRANSFORMER

SUBSTATION WITH POWER TRANSFORMER

A substation in a power system is the one that usually houses the Power transformer. This is also the place where the transformation of voltage occurs. Power transformers found in substations may be either autotransformers or multi-winding conventional transformers. A three-phase installation may consist of a three-phase unit or three single-phase units. The decision as to what type of transformer to purchase depends on such factors as initial installed cost, maintenance costs, operating cost (efficiency), reliability, etc. Three-phase units have lower construction and maintenance costs and can be built to the same efficiency ratings as single-phase units. The initial cost of a three-phase transformer is usually approximately one-third less than four single-phase units. Additionally, the exposure of three-phase units to long outages can be minimized system-wide when a mobile substation or transformer is available for backup in case of failure. RUS Bulletin 61-16, “Guide for Economic Evaluation of Distribution Transformers,” provides information for evaluating substation transformer costs.

Various electrical equipment not only transformers are placed in a substation like switches, breakers, controls, etc. According to “Design Guide for Rural Substation”, a substation is a part of a system and not an entity to itself. In a typical electric utility, there exists at least one substation. It is carefully designed so that the effects of an outage (caused by the failure of a single component such as a transformer, transmission line, or distribution line) will result in minimal interruption of service and affect the fewest customers possible. Substations may be categorized as distribution substation, transmission substation, switching substation or a combination thereof. (RUS Bulletin 1724E-300 Issued June 2001)

Distribution Substations

A distribution substation is a combination of switching, controlling, and voltage step-down equipment arranged to reduce sub-transmission voltage to primary distribution voltage for residential, farm, commercial, and industrial loads. Rural distribution substation capacities vary. Substations generally include one l.5 MVA to three 5 MVA transformers. These substations may be supplied radially, tapped from a sub-transmission line, or may have two sources of supply. Most cooperatives’ substations have 12,470Y/7200-volt or 24,490Y/14,400-volt distribution circuits. A special class of distribution substation would include a dedicated customer substation. This substation would be similar to a distribution substation except that all of its capacity would be reserved for the service of one customer. The secondary voltages of a dedicated substation would also be modified to match special requirements of the customer. Coordination with the customer is of primary importance in determining the technical requirements. Confirmation of the technical terms being used is likely to be required since electrical engineers in differing industries may use the same terms to describe similar, yet technically different, criteria.

Transmission Substations

A transmission substation is a combination of switching, controlling, and voltage step-down equipment arranged to reduce transmission voltage to sub-transmission voltage for distribution of electrical energy to distribution substations. Transmission substations frequently have two or more large transformers. Transmission substations function as bulk power distribution centers, and their importance in the system often justifies bus and switching arrangements that are much more elaborate than distribution substations.

Furthermore, a transmission substation can be either a Step-up Transmission substation or a Step-down Transmission substation.

Step-Up Transmission Substation

The purpose of this kind of substation is to transform the voltage level that it receives from a generating facility from its generating voltage which is normally smaller stepping it up to a voltage that is worthy for transmission. Large power transformers are used to increase the generating voltage for transmission to distant locations.

The typical voltages are:

High voltage (HV) ac:                             69kV, 115kV, 138kV, 161kV, 230kv

Extra-high voltage (EHV) ac:                   345kV, 500 kV, 765 kV

Ultra-high voltage (UHV) ac:                     1100 kV, 1500 kV

Direct-current high voltage (dc HV):        +250kV, +400kV, +500 kV

Step-Down Transmission Substation

Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for sub-transmission lines or distribution lines. The step-down substation can change the transmission voltage to a sub-transmission voltage, usually 69 kV. The sub-transmission voltage lines can then serve as a source to distribution substations. Sometimes, power is tapped from the sub-transmission line for use in an industrial facility along the way. Otherwise, the power goes to a distribution substation.

Switching Substations

A switching substation is a combination of switching and controlling equipment arranged to provide circuit protection and system switching flexibility. Switching stations are becoming common on cooperatives’ transmission systems. Flexible switching arrangements in a transmission network can aid in maintaining reliable service under certain abnormal or maintenance conditions.

  

Reference: http://www.powertransformer.us/substationtransformers.htm , RUS Bulletin 1724E-300 Issued June 2001