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Electrical Machines LSEGG216A 9080V Auto & Instrument Transformers Introduction • • Identify auto-transformers, voltage transformers and current transformers from their winding diagrams. Calculate the voltage and current in the windings of an autotransformer. List the advantages and disadvantages of an autotransformer. State the AS/NZS3000 requirements with respect to transformers. Describe the construction of voltage transformers. State the ratings of voltage transformers. Describe the construction of current transformers. State the ratings of current transformers. List the precautionary measures taken to connect and disconnect instrument transformers. Complete connection diagrams for instrument transformers. • List the applications for auto-transformers and instrument transformers • • • • • • • • Auto Transformers What are They? Transformer that acts like an isolation transformer by changing • • • Voltage Levels Current Levels Impedance values But does not isolate between the Primary and the Secondary Auto Transformer does not have this coil Isolation Transformer As a result: • Requires less copper • Lighter • I2R losses are less • More efficient Auto Transformer But No isolation 230th turn 10A 300V 300T 230V 7.6A 300 T 230 T 10 A 230 T 300 T 300 V 10A 7.6A Current in this part of the coil = 10 – 7.6 = 2.33A Auto Transformers Where are they used? • In power distribution lines to counteract line Z • Motor starting circuits If there is an insulation breakdown between coils the supply voltage may be imposed onto the low voltage load It is recommended that the voltage reduction should only be by a maximum of 25% Instrument Transformers PT CT Used to reduce voltage or current to provide metering or protection Regardless of the input level the output always stays the same PTs = 110V CTs = 5A Voltage Transformers Potential Transformer PT Primary Voltage System Phase Voltage Secondary Voltage 110V Burden Ratings generally less than 500VA Current Transformers There are two Types CT • Conventional winding • Toroidal Used for two reasons • Metering • Protection All CTs Must be very accurate with regard to • Turns ratio • Characteristics change little with load changes • Phase angle between primary & secondary very small Difference Between Protection & Metering CTs A 100:5 CT is used to measure current A 2kA fault appears A Metering CT will saturate producing no more than 5A Metering equipment is protected More Accurate A Protection CT will NOT saturate & try to produce 100A Protection equipment will react quicker Dangers of CTs Secondary coil must never be open circuited with supply available • Short circuit secondary coil then remove meter. • CT has to remain short circuited until another meter is obtained • This will not damage the CT