Lab manual psd v sem experiment no 6

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Lab manual psd v sem experiment no 6

  1. 1. STANI MEMORIAL COLLEGE OF ENGINEERING & TECHNOLOGY POWER SYSTEM DESIGN LAB EXPERIMENT NO. 6 Aim: - Instrument Transformers: Design considerations of CTs & PTs for measurement and protection. Apparatus Requirements: - Current Transformer, Potential Transformer, AC supply, Watt Meters, Patch Cords. Theory:Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit. Current Transformer:A current transformer (CT) is used for measurement of electric currents. Current transformers, together with voltage transformers (VT) (potential transformers (PT)), are known as instrument transformers. When current in a circuit is too high to directly apply to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer also isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. Current transformers (CT) are a series connected type of instrument transformer. They are designed to present negligible load to the supply being measured and have an accurate current ratio and phase relationship to enable accurate secondary connected metering.
  2. 2. Potential Transformer:Potential transformers (PT) (also called voltage transformers (VT)) are a parallel connected type of instrument transformer. They are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering. The Current Error or Ratio Error in Current Transformer or CT:From passer diagram of C.T. it is clear that primary current Ip is not exactly equal to the secondary current multiplied by turns ratio, i.e. KTIs. This difference is due to the primary current is contributed by the core excitation current. The error in current transformer introduced due to this difference is called current error of CT or sometimes Ratio Error in Current Transformer. Voltage Error or Ratio Error in Potential Transformer (PT):The difference between the ideal value Vp/KT and actual value Vs is the voltage error or ratio error in a potential transformer, it can be expressed as: Circuit Diagram:- Fig. 6.1 Equivalent Circuit Diagram of C.T.
  3. 3. Fig 6.2 Vector Diagram of C.T. Fig. 6.3 Vector Diagram of P.T. Precautions:1. Do not touch any live conductor.
  4. 4. 2. Please keep distance from electrical equipments because voltage level very high i.e. 132 kV, 220 kV, and 400 kV. Procedure:- Error Measure in Current Transformer:I. II. III. Using a core of high permeability and low hysteresis loss magnetic materials. Keeping the rated burden to the nearer value of the actual burden. Ensuring minimum length of flux path and increasing cross – sectional area of the core, minimizing joint of the core. IV. Lowering the secondary internal impedance. Is – Secondary Current Es – Secondary induced emf Ip – primary Current Ep – primary induced emf KT – turns ratio = numbers of secondary turns/number of primary turns Io – Excitation Current Im – magnetizing component of Io Iw – core loss component of Io Φm – main flux. Let us take flux as reference. EMF Es and Ep lags behind the flux by 90o. The magnitude of the passers Es and Ep are proportional to secondary and primary turns. The excitation current Io which is made up of two components Im and Iw. The secondary current Io lags behind the secondary induced emf Es by an angle Φ s. The secondary current is now transferred to the primary side by reversing Is and multiplied by the turns ratio KT. The total current flows through the primary Ip is then vector sum of KT Is and Io.
  5. 5. Error Measure in Potential Transformer:As in the case of Current Transformer and other purpose Electrical Power Transformer, total primary current Ip is the vector sum of excitation current and the electric current equal to reversal of secondary current multiplied by the ratio 1/KT Hence, Ip = Io + Is/KT If Vp is the system voltage applied to the primary of the PT then voltage drops due to resistance and reactance of primary winding due to primary current Ip will comes into picture. After subtracting this voltage drop from Vp, Ep will appear across the primary terminals. This Ep is equal to primary induced emf. This primary emf will transform to the secondary winding by mutual induction and transformed emf is Es. Again this Es will be dropped by secondary winding resistance and reactance, and resultant will actually appear across the burden terminals and it is denoted as Vs So if system voltage is Vp, ideally Vp/KT should be the secondary voltage of PT, but in reality actual secondary voltage of PT is Vs. Is – Secondary Current Es – Secondary induced emf Vs – Secondary terminal voltage Rs – Secondary winding resistance Xs – Secondary winding reactance Ip – Primary current Ep – Primary induced emf Vp – Primary terminal voltage Rp – Primary winding resistance Xp – Primary winding reactance KT – turns ratio = numbers of primary turns/number of secondary turns Io – Excitation Current Im – magnetizing component of Io Iw – core loss component of Io Φm – main flux β – Phase angle error
  6. 6. Observation Table:- Calculation:- Result: - We have successfully studied about Instrument Transformers, Design considerations of CTs & PTs for measurement.
  7. 7. Viva- Voice Questions:- Ques. 1 What is Instrument Transformer? Ques. 2 Write the Expression of Error in C.T. Ques. 3 For which Instrument transformer Secondary must not be open? Ques. 4 Difference between C.T. and P.T.
  8. 8. Viva- Voice Answers:Ans. 1 Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuitry from the high voltages or currents. Ans. 2 Ans. 3 for Current Transformer. Ans. 4 1) Ans. 4 CT is used for Current Measurement while PT is used for Voltage Measurement. 2) CT always connected with in the series or main circuit while PT is always connected with parallel with the main circuit.

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