Protective voltage transformer

							   Voltage Transformers for
   Power System Relaying



Tim Brown
Idaho Power Company




            Introduction
 Voltage Transformers (VTs) Basics and Use
 in Power System Relaying
 VT Basics, Types, Models, Transient
 Performance and Examples
 VT Performance Important in High Speed
 Digital Relays
               Agenda
What are VTs used for?
VT Types
VT Accuracy
Standards
Ferro Resonance
Transient Performance




              Overview
VTs are Needed to Give an Accurate
Representation of the System Voltage
A Clear Understanding of VTs is Required
to Specify them for a Particular Application
            Vocabulary
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




             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
 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.




  Basic Transformer Model
                      Op
            Ip


                           Np



                                Oe
            Is

       Zb        Vs        Ns




                      Os
 Wire Wound Transformers
         (con’t)
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




         Capacitive VTs
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 Tappings on T or
  Separate Auto-Transformer on Sec. Winding.
               CVT Basic Circuit
Line Voltage
                                               Ferro-resonace
                C1                              Suppression
                              L                    Circuit
      Coupling
     Capacitors      Compensating Reactor
                C2                Step-Down
                                  Transfomer




           CVT Circuit Diagram

           Ce          L           Ri          Rs




      Vi                                Ze              ZB
    Capacitive VTs (con’t)
– Ferro Resonance Suppression Circuit will be
  Discussed Later
– 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.




            CVTs (con’t)
– At Low System Voltage, the Core Flux and
  Permeability are Reduced. Exciting Impedance
  Decreases and Intermediate Current and
  Voltage go up Mag. and Phase Error.
Standards
– Steady State Classifications based on Burden
  type
– No Standard for Transient Response
            CVTs (con’t)
Ferro-resonance
– 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.




            CVTs (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
                    Ferro-Resonance
                   Suppression Circuits

                                 Relay Voltage                            Relay Voltage

Step-Down Transformer
                                     L       Step-Down Transformer
      Secondary                                    Secondary
                                                                     Lf   Rf
                        C

                                         R                                       R



                            Active                                   Passive




                              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.
               CVTs (con’t)
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.




               CVTs (con’t)
– 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.
              CVTs (con’t)
    Coupling Capacitors, High C 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.




 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.
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.
             BPD (con’t)
Transient Response
 – Low Capacitance Values Resulting from the
   Method Obtained.
 – Lower C Devices have Worse Transient
   Response.




              Summary
VT Types - WWT, CVT, and BPD
VT Accuracy
VT Ferro-Resonance
VT Transient Performance
Apply Knowledge - Relaying or Metering
Any Questions?
Where to get more information
 Power System Protection: Principles and
 Components, IEE, Peter Perigrinus Ltd.
 Capacitive Voltage Transformers: Transient
 Overreach Concerns and Solutions for
 Distance Relaying, Daqing Hou and Jeff
 Roberts, 22nd Annual Western Protective
 Relay Conference.
 Bushing Potential Device: Type PBA2,
 Westinghouse I.B 33-357-1B




   More Information (con’t)
 Bushing Potential Device: Type KA-108,
 General Electric GEI-93192F