Potential Transformer- an instrumental transformer

A line diagram from power generation to end user(consumer) been shown below which shows the flow of high voltage.

At starting, At generating side, Generated voltage is 11kV Which is been stepped up to 220KV. At substation, voltage been stepped down to voltage levels of 66KV & 11KV.  At last, At consumer end, voltage been finally stepped down to 415 volts.

At power generating station & substation, Measuring of high voltage directly is; difficult & dangerous. That’s why to level the voltage down at measurable level, An instrumental transformers are developed. These instrument transformers further helps energy/power meters to measure the voltage indirectly.

This post is all about the instrumental transformer such as potential transformer. Further, In this post, Specifications of potential transformer is going to be discussed.

POTENTIAL TRANSFORMER

Potential Transformer (PT) is a type of instrumental transformer which is used to lower down the high level voltage at measurable low level. Which is further utilized by energy/power meter or some sort of another circuit. PT is type of step down transformer which lowers the voltage down.

To know about the working of potential transformer, We need to understand the working mechanism of Transformer , first.

Transformer – Transformer is the power electrical machine. Which transfers electrical power from one circuit to another.

Generally, Transformer has two windings primary & secondary, Both the windings are winded on common silicon steel stamping. These windings are not directly connected rather they are magnetically coupled to each other. By changing the winding configuration, Voltage level can easily get adjusted !

Transformer without any movement, changes the voltage level on the secondary side while maintaining the constant frequency on both the sides. Both primary & secondary windings have number of turns. The variation of voltage & current on secondary side depend upon the ratio of number of turns.

• Secondary side’s voltage increases & current decreases when secondary side’s number of turns are greater than primary side’s number of turns.
• Secondary side’s voltage decreases & current increases when  secondary side’s number of turns are lower than primary side’s number of turns.

Next, Schematic diagram of transformer is discussed !

 

Transformer actually follows the equation, Vp/Vs=Np/Ns=Is/Ip

Where,

Vp= Primary Voltage

Vs= Secondary Voltage

Np= Primary number of turns

Ns= Secondary number of turns

Ip= Primary side current

Is= Secondary side current

The phenomenon Np>Ns makes Vp>Vs ; is used in Potential transformer.

TECHNICAL TERMS OF POTENTIAL TRNASFORMER

• Transformer’s Insulation level voltage
  Insulation voltage level defines the bearing capacity of insulation. It is always higher than highest system voltage.
  
• Highest system voltage
  Highest system voltage defines the bearing capacity of system. It is always higher than working system’s voltage.
  
• Frequency
  Frequency defines the supply frequency.
  
• Rating
  Rating defines the apparent power of the transformer.
  
• Voltage Ratio
  It is the ratio of voltage on primary side to the voltage on secondary side.

 

DISCUSSING THE NAME PLATES OF POTENTIAL TRANSFORMERS

In an image below, A name plate of potential transformer is been shown which is having a voltage ratio of 220KV/110. This Potential transformer is running in SUB-STATION.

Technical details of the Transformer

• Transformer’s Insulation level voltage : 400KV
• Highest system voltage : 245KV
• Frequency : 50Hz
• Per phase rating : 200VA
• Voltage Ratio : 220000/110
• Oil Quantity : 250±5%Litres
• Total Weight : 1100±5%Kg

This potential transformer contains two windings on secondary. Both the windings are same, having equal number of turns & equal rating.

In an another image below, Another name plate of potential transformer been shown which is having a voltage ratio of 11KV/110. This transformer is running on (VCB) Vacuum circuit breaker so that 11KV  voltage get easily converted in to measurable level of 110Volts.

• Rated voltage : 11KV
• Highest system Voltage : 12KV
• Rated frequency : 50Hz
• Voltage Ratio : 11KV/110
• Rating : 100
• Accuracy class : 0.5
• Connections on both sides : STAR/STAR

SUMMARY

The post discusses the challenges of delivering electricity from distant power generation facilities such as thermal, hydropower, or nuclear plants to consumers. High voltage transmission is essential to minimize losses over long distances, typically starting from 11kV and stepping up to 220kV before being reduced at substations for consumer use.

Instrument transformers, particularly potential transformers (PT), are crucial for safely measuring high voltages by stepping them down to a measurable level. The post also explains the operational principles of transformers, including voltage ratios and technical specifications of various potential transformers, emphasizing their role in the electrical distribution system.