9. PRIMARY EQUIPMENT & SECONDARY EQUIPMENT

11. KNOWLEDGE OF TRANSFORMER

Definition of Transformer
A transformer is a static machine used for transforming power from one circuit to another without changing frequency. This is a very basic definition of transformer.

The history of transformer was commenced in the year 1880. In the year 1950, 400KV electrical power transformer was introduced in high voltage electrical power system. In the early 1970s, unit rating as large as 1100MVA was produced and 800KV and even higher KV class transformers were manufactured in year of 1980.

Generation of electrical power in low voltage level is very much cost effective. Hence electrical power is generated in low voltage level. Theoretically, this low voltage level power can be transmitted to the receiving end. But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I2R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. These are the uses of electrical power transformer in the electrical power system.

Two winding transformers are generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use auto transformer where the ratio between high voltage and low voltage is less than 2. Again three phase single unit transformer is more cost effective than a bank of three single phase transformer unit in a three phase system. But still it is preferable to use than the later where power dealing is very large since such large size of three phase single unit power transformer may not be easily transported from manufacturer’s place to work site.

Types of Transformer

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows,

Step Up Transformer & Step Down Transformer – Generally used for stepping up and down the voltage level of power in transmission and distribution power network.

Three Phase Transformer & Single Phase Transformer – Former is generally used in three phase power system as it is cost effective than later but when size matters, it is preferable to use bank of three single phase transformer as it is easier to transport three single phase unit separately than one single three phase unit. 

Electrical Power Transformer, Distribution Transformer & Instrument Transformer - Transformer is generally used in transmission network which is normally known as power transformer, distribution transformer is used in distribution network and this is lower rating transformer and current transformer & potential transformer, we use for relay and protection purpose in electrical power system and in different instruments in industries are called instrument transformer.

Two Winding Transformer & Auto Transformer – Former is generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use later where the ratio between high voltage and low voltage is less than 2.

Outdoor Transformer & Indoor Transformer – Transformers that are designed for installing at outdoor are outdoor transformers and transformers designed for installing at indoor are indoor transformers.
 

12. WHAT IS THE DIFFERENCE BETWEEN POWER TRANSFORMER AND DISTRIBUTION TRANSFORMER

 Those transformers installed at the ending or receiving end of long high voltage transmission lines are the power transformers(usually higher voltage and higher capacity). The distribution transformers (generally pole mounted with lower voltage and lower capacity) are those installed in the location of the city to provide utilization voltage at the consumer terminals.

•  Power transformers are used in transmission network of higher voltages for step-up and step down application (550kV,400 kV, 230kV, 200 kV, 110 kV,90kV, 66 kV,35kV 33kV, 30kV) and are generally rated above 200MVA.

• Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 15kV, 6.6 kV, 3.3 kV, 440V, 400V,230V) and are generally rated less than 200 MVA.

• A power transformer usually has one primary and one secondary, and one input and output. A distribution transformer may have one primary and one divided or “Tapped” secondary, or two or more secondaries.

• Power transformers generally operate at nearly full – load. However, a distribution transformer operates at light loads during major parts of the day.

• The performance of the power transformers is generally judged from commercial efficiency whereas the performance of a distribution transformer is judged from all – day – efficiency.

• The rating of a high transformer is many times greater than that of distribution transformer.

• In Power Transformer the flux density is higher than the distribution transformer.

• Power transformer’s primary winding always connected in star and secondary winding in delta while in distribution transformer primary winding connected in delta and secondary in star.

• In The Sub station end of the transmission line, The Power Transformer Connection is Star-Delta.( For the purpose of Step down the Voltage Level)

• In the star up of the  Transmission line (H-T), The Connection of the power Transformer is Delta – Star (For the purpose of Step Up the Voltage Level) But in case of Distribution Transformer, But Generally it is used in there-phase Step down distribution transformer( Delta – Star).
  
  Click here for image of Power transformer of CTSS: Power Transformer Images.
  Click here for image of Distribution transformer of CTSS: Distribution Transformer Images.
  

Along with the rapidly increasing requirement of electrical power supply and higher criterion towards power  quality, problems of negative, harmonic, and reactive power are caught to people’s attention. The main causes of  the drop of power quality are all kinds of asymmetry and non-linear loads in power system, which includes the AC  single-phase electric railway traction supply system and petrochemical industry single-phase power supply system, etc. Those asymmetry loads bring disadvantage to the power system’s operation.

Scott-T transformer, also called T connection transformer, is one of the earliest Three phase to Two phase balance transformer. The first use of Scott-t transformer can be traced back to the beginning of electricity industry development, when the three phases transmission method just established. During that time Scott-T transformer has been applied into the field of interconnection between Three phases system and Two phases system(or two single phase systems) . It is used to supply single phase power by transferring the three phases system with minimal retroaction on the quality of whole electricity grid system.

CTSS Scott-T transformers are designed in accordance with applicable IEC 60076 standards.

Design

A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.

Scott T Transformers require a three phase power input and provide two equal single phase outputs called Main and Teaser. The MAIN and Teaser outputs are 90 degrees out of phase. The MAIN and the Teaser outputs must not be connected in parallel or in series as it creates a vector current imbalance on the primary side.

MAIN and Teaser outputs are on separate cores.

The Scott-t transformer is normally constituted by two single phase transformers. Connect the end of HV winding of one transformer to another transformer`s middle position of HV winding to form T connection of three phase HV winding.the inner turns of HV winding is dissymmetry and relationship between turns is

N_BD=N_DC,

N_AD=√3N_BC/2

The voltage and current in HV winding is different with the one in normal type single phase transformer.

Each LV winding is simple single phase winding and with turns relationship Nac=Nbc.

LV windings do not have electric connection between each other and with same voltage and current which is same with normal type single phase transformer. But the voltage and current in HV winding is different with the one in normal type single phase transformer.

Application

The Scott-T transformer is widely employed in the Railway traction system, Petrochemical industry, oil well, and smelt industry (especially power frequency induction furnace), where the input is three phase power but output should be single phase load (or two single phase loads). These single phase load normally possess the characteristics of  Heavy load, variable, low power factor, high harmonic, which result in Negative sequence current and consequently cause the asymmetry working of power system and tamper with the reliability of power grid and debase power quality and more power loss.

For the reason that the input is three phase A, B, C but the output is single phase loads, which will definitely cause the imbalance of the input three phase current and overload of one of the phase, and consequently burn the transformer or disastrously effect the transformer performance and lifetime, as well as the negative effect on the power grid quality. However, the emergence and application of Scott-T transformer, to a great extent, solve the imbalance technical puzzle and is playing crucial role until now.