Instrument Transformer Presentation

Presentation on "Instrument Transformer" (Summer vocational training presentation) from BHEL Jhansi

1. Bundelkhand Institute of Engineering and Technology Jhansi Electrical Engineering Department

2. Instrument Transformer Guided By : Sr. Er. S. K. Roy Instrument Transformer Manufacturing Department BHEL Jhansi Presented By : Rishi Kumar 1604320042 B.Tech (EE) A Summer Vocational Training Presentation On 2 Head of Department : Dr. D. K. Srivastava Electrical Engineering Department BIET Jhansi

3. Overview Introduction to Instrument Transformer Its Classification Design & Function of CT Construction, Accuracy Limit & Equivalent CKT Safety Precaution & Usage of CT Design & Function of PT Construction & Error of PT Cause of Error in PT Conclusion Reference

4. Introduction Instrument transformers are necessary for Isolating the protection , control & measurement equipment from the high voltages of a power system Supplying the equipment with the appropriate values of current and voltage

5. Types of Instrument Transformers Two Types of Instrument Trans formers : Current Transformer (CT) Potential Transformer (PT) Current transformer & Potential transformer

6. Current Transformer (CT)

7. Like any other transformer, a current transformer has a primary winding, a magnetic core and a secondary winding The primary winding of a current transformer is connected in series with the power circuit The impedance is negligible compared with that of the power circuit The alternating current flowing in the primary produces a magnetic field in the core & induces a current in the secondary winding The CT's primary circuit consists of a single 'turn' of conductor, with a secondary of many tens or hundreds of turns The primary winding may be a permanent part of the current transformer, with a heavy copper bar to carry current through the magnetic core Design

8. Reduce power system current to lower value for measurement Insulate secondary circuits from the primary Permit the use of standard current ratings for secondary equipment Function

9. Current Transformer Construction BAR PRIMARY WOUND PRIMARY Prima r y Secondary

10. REL A Y 1A 1000A 1000 turns sec Insulation covered wire, giving inter-turn insulation & secondary to core insulation Generator, or system voltage source ‘ Feeder’ or ‘Bus-bar’ forming 1 turn of primary circuit Insulation to stop flash-over from HV primary to core & secondary circuit Bar-Primary Type CT Laminated ‘strip’ wound steel toroidal core

11. Accuracy The accuracy of a CT is directly related to a number of factors : Burden : The secondary load of a current transformer is usually called the "burden" to distinguish it from the load of the circuit whose current is being measured Rating factor : Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute maximum measurable Load External electromagnetic fields Temperature Physical configuration. The selected tap, for multi-ratio CTs

12. C.T. Equivalent Circuit Z b N P 1 Z CT S 1 V t Z e I s I p I p /N I e E s I p = Primary rating of C.T. I e = Secondary excitation current N = C.T. ratio I s = Secondary current Z b = Burden of relays in ohms E s = Secondary excitation voltage (r+jx) V t = Secondary terminal voltage Z CT = C.T. secondary winding impedance in ohms (r+jx) across the C.T. terminals Z e = Secondary excitation

13. Safety Precaution The secondary of a current transformer is not disconnected from its load while current is flowing in the primary The transformer secondary will attempt to continue driving current across the effectively infinite impedance. This will produce a high voltage across the open secondary This high peaks of voltage may not be measured by conventional voltmeter. But these high peaks of induced voltage may breakdown the CT insulation, and may case accident to personnel

14. Usage Current transformers are used: A CT for operation on a 110 kV grid Monitoring the operation of the power grid Measuring current

15. Potential Transformer (PT)

16. Design A Potential Transformer theory is just like theory of general purpose step down transformer Primary of this transformer is connected across the phases or and ground depending upon the requirement PT has lowers turns winding at its secondary In an ideal Potential Transformer when rated burden connected across the secondary the ratio of primary and secondary voltages of transformer is equal to the turns ratio But in actual transformer there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages

17. Types of Potential Transformer Two main basic types are available : Electromechanical P Ts Similar to a power transformer May not be economical above 132kV Capacitor P Ts (C P T) Used at high voltages Main difference is that C P T has a capacitor divider on the front end

18. Construction of PT The construction of a voltage transformer takes into account the following factors Output – Seldom more than 200-300VA. Cooling is rarely a problem Insulation – Designed for the system impulse voltage level. Insulation volume is often larger than the winding volume Mechanical Design – Not usually necessary to withstand short-circuit currents. Must be small to fit the space available within switchgear

19. Provides isolation from high voltages Must operate in the linear region to prevent accuracy problems - Do not over specify VT Must be capable of driving the burden, specified by relay manufacturer Protection class VT will suffice Function

20. Errors in PT I s - Secondary Current E s - Secondary induced emf V s - Secondary terminal voltage I p - Primary current E p - Primary induced emf V p - Primary terminal voltage K T - numbers of primary turns/number of secondary turns I o - Excitation Current β - phase angle error The difference between the ideal value V p /K T and actual value V s is the voltage error or ratio error in a potential transformer % voltage error = V p − K T .V s /V p X 100 % The angle ′β′ between the primary system voltage V p and the reversed secondary voltage vectors K T .V s is the phase error

21. The voltage applied to the primary of the potential transformer first drops due to internal impedance of primary Cause of Error in PT Transformed voltage across secondary winding will again drops due to internal impedance of secondary before appearing across burden terminals

22. Conclusion Current Transformer Measuring current Monitoring the operation of the power grid CT secondary should not be kept open Potential Transformer Measuring Voltage Provides isolation from high voltages

23. References "Measurement Canada Standard Dwg. No.3400 D3 Delta Connected CTs"  (PDF). Measurement Canada. Retrieved 12 December 2012. "Limits of the 0.3 and 0.6 accuracy classes for measuring current transformers" . Measurement Canada. Retrieved 18 April 2013.   "PS-E-15 — Provisional Specifications for Approval of Electronic Voltage Transformers" . Measurement Canada. Retrieved 18 April 2013. Monsef, H.; Ghomian, T. (2006). "Modified quadrature method for accurate voltage measurement in optical voltage transducer". IEE Proceedings - Generation, Transmission and Distribution.  153  (5): 524.  doi : 10.1049/ip-gtd:20050189 .  ISSN   1350-2360 . Network Protection & Automation anshu, AREVA 2002

24. Thank You