Technical Parameters of a Potential Transformer (PT)

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Some the important parameters are listed here for the reference only. The actual technical parameters of a potential transformer may be somewhat different depending on the actual requirements.

  • Potential Transformers are of
    1. Electromagnetic Type,
    2. Outdoor Type,
    3. Single Phase,
    4. Oil filled self cooling type,
    5. with Porcelain Housing.
  • The Standards to be followed for a Potential or Voltage Transformers
    1. IS 3156 (Part 1 to 4)
    2. IS 4146 – Application guide for Voltage Transformers
  • Temperature Rise Limits
    1. Maximum Winding Temperature Rise Limit is 50oC above ambient
    2. Maximum Oil Temperature Rise Limit is 40oC above ambient
    3. Maximum Ambient Temperature Limit is 50oC .
  • The P.T. shall have
    1. Terminal box for secondary connection,
    2. Lifting lugs,
    3. Grounding terminals,
    4. Oil sight glass,
    5. Pressure relief device,
    6. Filling and draining plugs.
  • The core of the potential transformer should be made of
    1. CRGO silicon Steel or
    2. Nanocrystalline materials.
  • The PT winding shall be made of copper.
  • The secondary terminals shall be brought out through fuse of suitable rating.
  • PT junction box to C&R panel must be connected with 4 sqmm color coded (Red, Yellow, Blue, Black) copper cable.
  • Two grounding terminals on diagonally opposite sides shall be provided.
  • The lower end of the high-voltage winding is brought out through a bushing to insulate it from the case or frame. That terminal of the high-voltage winding shall be earthed separately.
  • The PT secondary terminal box shall be made of a 3 mm thick sheet of steel. However, some utilities accept a 2 mm thick sheet, but for outdoor applications, 3 mm is preferable according to many experienced engineers.
  • The degree of protection tghe secondary terminal box shall not be less than IP-55.
  • The door shall have a braided copper wire connecting it to the main body for earthing.
  • The tank and top chamber shall be hot-dip galvanized or painted. An aluminum top chamber is also acceptable.
  • Steel surfaces exposed to insulating oil shall be coated with a heat-resistant, oil-repellent insulating varnish.
  • Voltage Factor
    1.5 times for 30 sec, 1.2 continuous (for 400 KV, 220 KV, and 132 KV)
    1.9 times for 8 Hrs, 1.2 continuous (for 33 KV)
  • PTs shall be of accuracy class 0.2 for metering and 3P for protection.