Voltage Transformers. VT Types

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1 Voltage Transformers VT - Voltage Transformer, also know as Potential Transformer (PT) CVT - Capacitive Voltage Transformer also know as CCVT (Capacitive Coupled VT) BPD - Bushing Potential Device also know as CBD (Capacitive Bushing Device) Residual Voltage - Voltage on the secondary after the primary voltage has been removed Instrument Transformers 1 VT Types Wire Wound Transformer (WWT)- Traditional Transformer, primary and a secondary winding with an iron core Capacitive Voltage Transformer - Device with coupling capacitors, wire wound transformer. Bushing Potential Device - Similar to CVT, capacitive coupling from tap points in bushing of PCB Instrument Transformers 2 1

2 Wire Wound Transformers (WWT) Not used as a stand alone transformer rated for Line to Ground voltage. Wide Operating Range. 5% - 150% to 190% of Rated Voltage. Sized for Low Flux Density for this Wide Range. Expensive, and Large. Used with coupling capacitors in a CVT. Instrument Transformers 3 Basic Transformer Model Ip Op Np Is Oe Zb Vs Ns Os Instrument Transformers 4 2

3 Wire Wound Transformers Ideal Because of Accuracy in Reproducing Transients on the Secondary. Errors» no load Errors, Voltage Drop due to Exciting Current in Primary» load Errors, Voltage Drop in both Windings due to High Burden Currents Instrument Transformers 5 Capacitive VT Model Basic Circuit and Circuit Diagram L, Inductive Reactance» Tuned to Resonate with Equivalent C at System Frequency.» Prevents Phase Shift.» External or Internal to Wire Wound T.» Protects C2, Saturate if the Secondary of T is Shorted. Tuning of T ratio using Taps on T or Separate Auto- Transformer on Sec. Winding. Instrument Transformers 6 3

4 CVT Basic Circuit Line Voltage C Coupling Capacitors C 1 2 L Compensating Reactor Step-Down Transfomer Ferro-resonace Suppression Circuit Instrument Transformers 7 CVT Circuit Diagram Ce L Ri Rs Vi Z e Z B Instrument Transformers 8 4

5 Capacitive VTs (con t) Typical values of C1 are around 2000 pf. Accuracy/Errors» Typical Change of Phase Error at Rated Burden (150 VA.85 pf) 30 min/hz at 145kV 10 min/hz at 400kV.» Modern Transformers - Smaller Series Reactance, gives 1.5% and 60 min. Instrument Transformers 9 CVT Errors At Low System Voltage, the Core Flux and Permeability are Reduced. Exciting Impedance Decreases Intermediate Current and Voltage go up Causing Mag. and Phase Error. Standards» Steady State Classifications based on Burden type» No Standard for Transient Response Instrument Transformers 10 5

6 Ferroresonance Resulting from:» Primary Reclosing» Variations in Secondary Load (Short Circuits) DC Voltage Applied to Intermediate Voltage Terminal Saturates Core of WWT Reducing Exciting Impedance and Causing Large Inrush Currents. Instrument Transformers 11 Ferroresonance (con t) When Current Peaks and Starts to Decay, Core Magnetic Field will Collapse, Charging the Capacitor in the Opposite Polarity. 3rd Harmonic Voltage Common Output Voltage Increase of 25% to 50%. Common in Circuit with Low Losses. Increase the Resistive Load on the VT. Aux. VTs Aggravate the Condition. Ferro-resonance Suppression Circuit» Two types Active and Passive Instrument Transformers 12 6

7 Ferroresonance Suppression Circuits Relay Voltage Relay Voltage Step-Down Transformer Secondary C L Step-Down Transformer Secondary L f R f R R Active Passive Instrument Transformers 13 CVTs (con t) Active Circuit, No Affect of Fundamental Voltage, Low Impedance path for Off-Nominal Frequencies, Attenuates Off-Nominal Frequency Voltages. Passive Circuit, High Secondary Voltage, Flash Gap to Loading Resistor, Saturable Inductor will Saturate to Remove the Oscillations. Instrument Transformers 14 7

8 Transient Response Capacitors and Inductor Impedances, Large compared to the burden for CVTs. Results in Oscillations in Secondary when the Primary Voltage Changes Rapidly. Residual voltage due to Discharge of Energy Devices. Transient Overreach- CVT Transients Reduce Fundamental Component of Fault Voltage. Decreases Impedance, Zone 1 Element picks up for Fault out of Zone of Protection. Instrument Transformers 15 Solutions Reduce Zone 1 Impedance Reach - Becomes Ineffective.» Time Delay to Block Tripping During Transient Time - Close-In Fault Clearing Times. CVT Transients depends on Point On Wave of the Fault. Voltage Zero Faults Worse than Voltage Peaks. CVT Component Contributions to Transient Response» Turns Ratio, Higher for Better Isolation Between Capacitors and Burden. Instrument Transformers 16 8

9 CVTs (con t) Coupling Capacitors, High Capacitance CVTs Decrease Transient Magnitude.» Ferro-resonance Suppression Circuit Active Circuit acts like a Band-Pass Filter, Extra Time Delay in Output. Added Energy Storage Devices Contribute Adversely to the Transient. Passive Circuit has Little Effect on the Transient.» Output Burden, Parallel Burdens are Worse for Residual Voltages. Provide Additional Discharge Paths with Little Resistance for Damping. Instrument Transformers 17 Bushing Potential Device Model» Same as CVT» Mounts on or Near Circuit Breaker Bushing» Capacitances C1 and C2 Chosen from Tap Points on Side of Bushing. Instrument Transformers 18 9

10 Bushing Potential Device Accuracy/Errors» Steady State Accuracy's Similar to those in a CVT» Suggest Having Burden with a pf of 100% and Load Device to Burden Rating, to Reduce Transients. Instrument Transformers 19 BPD Transient Response» Low Capacitance Values Resulting from the Method Obtained.» Lower C Devices have Worse Transient Response. Instrument Transformers 20 10

11 Transient Response of CCVT s CCVT Transient Response is Dependent on:» Stack Capacitance» Intermediate Tap Voltage» Secondary Burden Instrument Transformers 21 11