Instrument Power

Location:

United States

Posted:

February 14, 2013

Contact this candidate

Resume:

Are you using the right CT's and PT's

for your application?

John Levine, P.E.

Levine Lectronics and Lectric

E-mail: ****@*-*.***

GE Multilin / Instrument Transformers, Inc.

GE Multilin

Instrument Transformers, Inc.

Alternate Title of Presentation

Now I know what I do not know about

CT s and PT s.

As engineers we do not need to know all the

answers just where to get them.

GE Multilin

Instrument Transformers, Inc.

Current & Voltage Transformer

Basics (IEEE Standards)

GE Multilin

Instrument Transformers, Inc.

Agenda

Current Transformers

Voltage Transformers

Required Information for Specifying CTs & VTs

Take Home Rules for CTs & VTs

Applications where used

GE Multilin

Instrument Transformers, Inc.

Why use Instrument Transformers?

Circuit Isolation

Reduce voltage and currents

to reasonable working levels.

Phasor combinations for

summing and measuring

power

GE Multilin

Instrument Transformers, Inc.

CURRENT TRANSFORMERS

TYPES OF C.T. CONSTRUCTION

The most common type of C.T. construction is the DOUGHNUT

type. It is constructed of an iron toroid, which forms the core of the

transformer, and is wound with secondary turns.

Secondary Winding Primary Conductor

Iron Core

GE Multilin

Instrument Transformers, Inc.

Transformer ratio (TR)

Primary Current

Transformer Ratio =

Secondary Current

Primary Current

(5 amps)

(100 amps)

100 = 100:5 or 20:1

GE Multilin

Instrument Transformers, Inc.

Polarity

Direction of

Secondary Current

Direction of

Primary Current

IEEE Secondary

IEC Polarity

IEEE H1

Primary Marks

Polarity

IEC P1

Marks

Remember:

Primary current into polarity =

Secondary current out of polarity 8

GE Multilin

Instrument Transformers, Inc.

Polarity Direction of

Secondary Current

IEEE Secondary

IEC Polarity

IEEE H1

Primary Marks

Polarity

IEC P1

Marks

Direction of

Remember: Primary Current

Primary current into non-polarity =

Secondary current out of non-polarity 9

GE Multilin

Instrument Transformers, Inc.

Generator typical wiring

GE Multilin

Instrument Transformers, Inc.

What is your application?

If your application is metering, how high do I

need to go in current? 2 times ?

If your application is protective relaying, how

high do I need to go in current? 20 times ? 40

times ?

GE Multilin

Instrument Transformers, Inc.

CT Rating Factor (RF) -- IEEE

Rated current x (RF) =

Maximum continuous current carrying

capability:

Without exceeding temperature limits

Without loss of published accuracy class

Typical rating factors for Metering CTs are:

1.0, 1.33, 1.5, 2.0, 3.0, 4.0

GE Multilin

Instrument Transformers, Inc.

CT Rating Factor (RF) -- IEEE

IEEE C57.13 Accuracy

0.3% @ BX.X RF 4.0

0.60

0.6%

Accuracy

Accuracy Class - %

0.30

Region 0.3% Accuracy

Region

10% 100% 200% 300% 400%

0.30

0.60

1.0 2.0 3.0 4.0

Rating Factor 13

GE Multilin

Instrument Transformers, Inc.

Short-Time Thermal Current Rating

One (1) second thermal rating

Expressed as value of RMS primary current

Main influencing factor:

CT primary & secondary wire size

Can be converted to thermal rating for any

time period (t) up to five (5) seconds:

(New RF at new Ambient/Stated RF at 30 degrees C)2=

85-New Ambient/New Ambient

Example: CT with rating factor of 4 at 30 degrees = rating factor of 2.95 at

55 degrees

X2/42=85-55/55 X=2.95

GE Multilin

Instrument Transformers, Inc.

CT Metering Accuracy

Actual secondary Rated secondary

current current

Difference in % is known as

the Accuracy

of the CT

GE Multilin

Instrument Transformers, Inc.

IEEE CT Metering Accuracy

Accuracy Application

Class 0.15 High Accuracy Metering

0.15S Special High Accuracy Metering

0.3 Revenue Metering

0.6 Indicating Instruments

1.2 Indicating Instruments

* All accuracy classes defined by IEEE C57.13 or C57.13.6

* Accuracy classes include both ratio & phase angle error 16

GE Multilin

Instrument Transformers, Inc.

IEEE CT Metering Accuracy

Burden

Load connected to CT secondary

Includes devices & connecting leads

Expressed in ohms

Standard values = B0.1, B0.2, B0.5, B0.9, B1.8

E0.04, E0.2

All burdens defined by IEEE C57.13 or C57.13.6 for 60 Hz only

GE Multilin

Instrument Transformers, Inc.

IEEE CT Metering Accuracy

Standard IEEE CT Burdens (5 Amp)

(Per IEEE Std. C57.13-1993 & C57.13.6)

Application Burden Impedance VA @ Power

Designation (Ohms) 5 amps Factor

Metering B0.1 0.1 2.5 0.9

B0.2 0.2 5 0.9

B0.5 0.5 12.5 0.9

B0.9 0.9 22.5 0.9

B1.8 1.8 45 0.9

E0.2 0.2 5 1.0

E0.04 0.04 1 1.0

GE Multilin

Instrument Transformers, Inc.

CT CLASSIFICATION for Metering

A current transformer for metering purposes may

typically have an accuracy of 0.3%. The C.T. must

maintain this accuracy for normal load currents,

provided the rated burden on the C.T. is not exceeded.

It is quite acceptable, and in fact desirable, for the C.T.

to saturate when fault current flows. The accuracy for a

typical metering C.T. is specified as:

0.3 M 0.9

O.3% METERING O.9 OHMS BURDEN

This metering C.T. has an accuracy of 0.3% when the

connected burden does not exceed 0.9 OHMS. Transformers, Inc.

GE Multilin

Instrument

IEEE CT Metering Accuracy

Accuracy expressed as: Typical

Examples

Accuracy Class Burden (Ohms) 0.3B0.2

(0.3, 0.6, 1.2) (B0.1, B0.2, B0.5, 0.6B0.9

B0.9, B1.8)

1.2B1.8

0.15E0.2

(0.15*, 0.15S^) E0.2, E0.04)

* Accuracy class is stated at 100% rated current

* At 10% rated current, twice the error is allowed (5% for 0.15 class)

^ Accuracy class is stated at 100% to 5% rated current

GE Multilin

Instrument Transformers, Inc.

IEEE CT Metering Accuracy

IEEE C57.13 Accuracy

0.3 @ BX.X; RF 4.0

0.60

0.6%

Accuracy

Accuracy Class - %

0.30 Region

0.3% Accuracy

Region

10% 100% 200% 300% 400%

0.30

No accuracy guaranteed at

current levels less than 10%

0.60

1.0 2.0 3.0 4.0

Rating Factor 21

GE Multilin

Instrument Transformers, Inc.

IEEE CT Metering Accuracy

CT Parallelogram IEEE C57.13

0.3 class

10%

parallelogram

100%

0.6 class

parallelogram

Note:

Burden must

be specified

Corrected to 0.3% Class

GE Multilin

Instrument Transformers, Inc.

IEEE CT Relay Accuracy

Protection CT

Standard Relay Accuracy Classes

C or T100

C or T200

What do these mean?

C or T400

C or T800

GE Multilin

Instrument Transformers, Inc.

IEEE CT Relay Accuracy

Relay class (C or T ) designates minimum

secondary terminal volts

At 20 times rated current

Without exceeding 10% ratio error

Into a maximum specified burden

Now that everyone is totally confused

let s look at some simple examples 27

GE Multilin

Instrument Transformers, Inc.

CT CLASSIFICATION for RELAYING

Protection Class CT s - Must supply 20 times rated current

10 C 400

Format

Accuracy Voltage at 20 times CT

Letter

T = Determined by test

C = Calculated

K = Calculated

L = Low internal secondary impedance

H = High internal secondary impedance 28

GE Multilin

Instrument Transformers, Inc.

IEEE CT Relay Accuracy

C or T100 example

Secondary current

100 amps (20 x 5)

Terminal Volts = 100

Burden of

Devices Primary current Total Ext

24,000 amps Burden

(20 x 1200) 1.0

CT Burden of

1200:5 Leads C or T100

Terminal Volts = (20 times rated) (Total external burden)

100 Volts = (100 amps) (1.0 ) 29

GE Multilin

Instrument Transformers, Inc.

IEEE CT Relay Accuracy

C or T200 example

Secondary current

100 amps (20 x 5)

Terminal Volts = 200

Burden of

Devices Primary current Total Ext

24,000 amps Burden

(20 x 1200) 2.0

CT Burden of

1200:5 Leads C or T200

Terminal volts = (20 times rated) (Total external burden)

200 Volts = (100 amps) (2.0 ) 30

GE Multilin

Instrument Transformers, Inc.

IEEE CT Relay Accuracy

Standard IEEE CT Burdens (5 Amp)

(Per IEEE Std. C57.13-1993)

Application Burden Impedance VA @ Power

Designation (Ohms) 5 amps Factor

Relaying B1 1 25 0.5

B2 2 50 0.5

B4 4 100 0.5

B8 8 200 0.5

GE Multilin

Instrument Transformers, Inc.

IEEE CT Relay Accuracy

Excitation curve includes

voltage required to overcome

internal resistance (DCR) of CT.

Approximately 32 volts.

10% ratio error = (20 x 5) (10%)

= (100) (0.10)

= 10 amps

How many terminal volts

would you estimate

this CT can produce?

GE Multilin

Instrument Transformers, Inc.

1000:5 CT

IB >4.5

amps

1000 Amps 5amps

With 10% error, IB is anywhere from 4.5 to 5.5 amps

Most Protection CT s also have a Metering CT accuracy so

you could use this value for calculating the error at lower

currents

GE Multilin

Instrument Transformers, Inc.

CT Burden Calculation

How do we calculate this?

Secondary current

Burden of

Primary

Devices Current

Total

Burden Z T

CT Burden of

Leads 34

GE Multilin

Instrument Transformers, Inc.

CT Burden Calculation

Z T = RCT + RL + ZB

ZT = Total burden in ohms (vector summation of

resistance and inductance components)

RCT = CT secondary resistance in ohms @75 deg C (DCR)

RL = Resistance of leads in ohms (Total loop distance)

ZB = Device impedance in ohms

Assumption: 3 phase CTs are Y connected

GE Multilin

Instrument Transformers, Inc.

GE Multilin Electronic Relay Burden

VA = VI. V= IR, So 0.2 = I*I*R.

.2/25 = .008 ohms GE Multilin

Instrument Transformers, Inc.

100:5 C.T. Secondary Winding Resistance (DCR) = .062 ohm

Resistance of Cable from C.T. to Relay and back = .1 ohms

Resistance of Relay Coil = .02 ohms

Total Resistance = .182 ohms

.02

.062

If we have a fault of 2,000 amps and the C.T. ratio is 100:5 then the C.T.

secondary current is 100 amps. Therefore we will produce a voltage of 100

amps x .182 ohms = 18.2 Volts. To prevent CT saturation, select a CT with a

knee point above 18.2 Volts.

GE Multilin

Instrument Transformers, Inc.

780-102 is a 1000 to 5 CT, Class C100

1000:5 C.T. Secondary Winding Resistance (DCR) = .32 ohm

Resistance of Cable from C.T. to Relay and back = .1 ohms

Resistance of Relay Coil = .008 ohms

Total Resistance = .428 ohms

.32 .008

If we have a fault of 20,000 amps and the C.T. ratio is 1000:5 then the C.T.

secondary current is 100 amps. Therefore we will produce a voltage of 100

amps x .428 ohms = 42.8 Volts. To prevent CT saturation, select a CT with a

knee point above 42.8 Volts.

What happens if the fault current is 40,000 amps?

GE Multilin

Instrument Transformers, Inc.

Factors Influencing CT Accuracy

Frequency

Low frequency and High accuracy are not friends!

Current Ratio

Low ratio and high accuracy are not friends!

Burden

High burden and High Accuracy are not friends!

GE Multilin

Instrument Transformers, Inc.

CT Saturation

What is CT Saturation?

GE Multilin

Instrument Transformers, Inc.

Power Systems Relay Committee (PSRC)

Saturation Calculator

GE Multilin

Instrument Transformers, Inc.

Factors Affecting Degree of Saturation

and Time to Saturation

DC Offset

Fault Magnitude (symmetrical current)

100 to 5 CT@20 times= 2000 amps, at 20,000 amps we have 200 times CT

CT Turns Ratio

Secondary Burden

CT Accuracy

Remanence Flux

Can occur if current interrupted when core is saturated

If DC flows in windings during testing

Need a voltage above 60% of knee point to reduce the remanence to

less than 10% of saturation flux density. 42

GE Multilin

Instrument Transformers, Inc.

Tips for Avoiding CT Saturation

Use higher ratio CTs

Use separate set of high ratio CTs for high fault

current tripping

Reduce secondary burden

Select low burden relays & meters

Distribute single phase burdens among phases

Increase size of secondary leads

Reduce length of secondary leads

Use step down auxiliary CTs

GE Multilin

Instrument Transformers, Inc.

Potential Transformers

GE Multilin

Instrument Transformers, Inc.

Definitions

Voltage Transformer (VT)

An instrument transformer used to reflect a primary

voltage into a secondary voltage through a magnetic medium.

Always connected in parallel with primary conductor across a

circuit load.

Secondary (measuring) voltage is usually 115 or 120

volts nominally. The secondary voltage level is selected for ease

of measurement and safety.

Control Power Transformer (CPT)

Designed to provide power for contractors, relays and

devices with high inrush currents, Regulation is not as critical.

GE Multilin

Instrument Transformers, Inc.

GE Multilin

Instrument Transformers, Inc.

POTENTIAL TRANSFORMERS

14,400/120 = 120/1

4200/120 = 35/1

2400/120 = 20/1

Relay

GE Multilin

Instrument Transformers, Inc.

IEEE VT Accuracy Class

Metering Accuracy

Classes (% error)

0.3

Defined by IEEE C57.13

0.6

Applicable from 90% to 110%

rated voltage

1.2

0.15 Defined by IEEE C57.13.6

GE Multilin

Instrument Transformers, Inc.

IEEE VT Accuracy Class

Metering Accuracy

Classurden

B Burdens VA PF

W 12.5 0.10

X 25 0.70

M 35 0.20

Y 75 0.85

Z 200 0.85

ZZ 400 0.85

These standard burden designations have no significance 49

at frequencies other than 60 Hz. GE Multilin

Instrument Transformers, Inc.

IEEE VT Accuracy Class

Expressed as:

Accuracy Class + Burden Code

0.3 W,X,Y

0.6 Z

1.2 ZZ

These standard designations have no significance

at frequencies other than 60 Hz.

GE Multilin

Instrument Transformers, Inc.

GE Multilin

Instrument Transformers, Inc.

VT Installation Guidelines

Caution:

Rated voltage: Do not operate above 110%

Line to ground rated:

Do not connect line to line

Do not use on ungrounded systems

w/o consulting factory

Rated Frequency: Do not operate below rated

frequency w/o consulting 52

factory GE Multilin

Instrument Transformers, Inc.

IEEE VT Groups

VT No. of Connection Neutral Notes

Group Bushing Grounding

Method

open

1 2 Any Withstand 25% over rated voltage on

an emergency basis

Y-Y possible

open

2 2 Any Withstand 10% over rated voltage

continuously. Primary rated for line to

Y-Y possible line voltage.

3 1 Y-Y-Y Any Outdoor, two secondary windings.

Withstand 10% over rated voltage

continuously.

4A 1 Y-Y Effectively Withstand 10% over rated voltage

continuously & 25% on an emergency

basis. For operation at 100% rated

voltage.

4B 1 Y-Y Non- Withstand 10% overvoltage

effectively continuously. For operation at 58%

Y-Broken rated voltage.

Corner

5 1 Y-Y Effectively Outdoor. Withstand 40% over rated

voltage for 1 minute and 10% over

rated voltage continuously

GE Multilin

Instrument Transformers, Inc.

VT Typical Connections

Open Delta Connection Y Y Connection

(2) Double Bushing VTs (3) Single Bushing VTs

GE Multilin

Instrument Transformers, Inc.

Required Information

for Specifying CTs & VTs

GE Multilin

Instrument Transformers, Inc.

Current Transformer (CT) RFQ Specification

Environment: Indoor Outdoor

System Voltage (kV) Power Frequency (kV) BIL (kV) Standard (Check one)

0.6 4 10 IEEE __

0.72 3 IEC __

3.6 10 40 IEC __

5.0 19 60 IEEE __

7.2 20 60 IEC __

8.7 26 75 IEEE __

12 28 75 IEC __

15 34 110 IEEE __

24 50 125 IEC __

25 40 125 IEEE __

34.5 70 150 IEEE __

34.5 70 200 IEEE __

CT Application: Metering Protection

Dimensions: Inches mm

Max. Outside: L x W x D

CT Window: Round: Diameter; Rectangular: L x W ; Primary Bar:

Current Ratio: : 5 : 1

GE Multilin

Continued next slide Instrument Transformers, Inc.

Current Transformer (CT) RFQ Specification (Continued)

Accuracy:

Indication Only: %, VA (Skip metering & protection selections)

Metering Class:

IEEE: 0.3 0.6 1.2 2.4 Other

IEC: 0.2 0.5 1.0 Other

Metering Burden:

IEEE (Ohms): B0.1 B0.2 B0.5 B0.9 B1.8 Other

IEC (VA): 2.5 5.0 10 15 30 Other

Protection Class: C (IEEE) VA, P (IEC)

Operating Frequency: 60HZ 50HZ

Rating Factor: 1.0 1.33 1.5 2.0 Other

Secondary Connections: Terminals 24 inch leads

Outer Insulation: Standard Cotton tape & varnish Polyester tape

Insulation Class: 105 0C (Standard) Other

Other Special Requirements (dimensional constraints, mounting requirements, etc):

GE Multilin

Instrument Transformers, Inc.

Voltage Transformer (VT) RFQ Specification

Environment: Indoor Outdoor

System Voltage (kV) Power Frequency (kV) BIL (kV) Standard (Check one)

0.6 4 10 IEEE __

0.72 3 IEC __

3.6 10 40 IEC __

5.0 19 60 IEEE __

7.2 20 60 IEC __

8.7 26 75 IEEE __

12 28 75 IEC __

15 34 110 IEEE __

24 50 125 IEC __

25 40 125 IEEE __

34.5 70 150 IEEE __

34.5 70 200 IEEE __

Operating Frequency: 60HZ 50HZ

Accuracy:

IEEE: W X M Y Z ZZ Other

(Enter 0.3, 0.6, 1.2, or leave blank)

IEC: 10VA 25VA 50VA 100VA 200VA 500VA Other

(Enter 0.2, 0.5, 1.0, or leave blank)

Continued next slide GE Multilin

Instrument Transformers, Inc.

Voltage Transformer (VT) RFQ Specification (Continued)

Thermal Rating: VA (Optional)

Primary Voltage: 1 bushing VAC - Phase to neutral

2 bushing VAC - Phase to phase

Secondary Voltage: 120V 115V 110V 100V

120/ 3 115/ 3 110/ 3 100/ 3

Other

Rated Voltage Factor (RVF) (1 bushing only): 1.9 for 30s

Fuses:

Primary Secondary None (600 720 V)

Primary Live parts only Switchgear Style

Note integral fusing not available above 15kV

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 1

Never open circuit a current

transformer secondary while

the primary is energized

CTs are intended to be proportional

current devices. Very high voltages can

result from open circuiting the secondary

circuit of an energized CT. Even very small

primary currents can cause damage

Consult the factory if you have questions.

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 2

Never short circuit the

secondary of an energized VT

VTs are intended to be used as proportional voltage

devices. Damaging current will result from short

circuiting the secondary circuit of an energized VT.

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 3

Metering applications

do not require a C class CT

C class ratings are specified for protection

purposes only. With some exceptions metering

class CTs are typically smaller and less expensive.

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 4

CT secondary leads must be

added to the CT burden

Electronic relays usually represent very

little burden to the CT secondary circuit.

In many cases the major burden is

caused by the CT secondary leads.

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 5

Never use a 60 Hz rated VT

on a 50 Hz System

60 Hz VTs may saturate at lower frequencies and

exceed temperature limitations. VT failure is

likely severe equipment damage is possible.

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 6

Exercise caution when

connecting grounded VTs

to ungrounded systems

Line to ground voltage on any VT may exceed the

primary voltage rating during a fault condition

VT must be designed for application.

GE Multilin

Instrument Transformers, Inc.

Take Home Rule # 7

It is common practice to apply

600 Volt CT to systems with

higher voltages.

This practice is done by passing fully insulated

conductors through the Window.

GE Multilin

Instrument Transformers, Inc.

QUESTIONS?

GE Multilin

Instrument Transformers, Inc.

GE Multilin

Instrument Transformers, Inc.

GE Multilin

Instrument Transformers, Inc.

LV Switchboards

LV Panelboards

Typical CT Models

5 Series

19RT 75

8SHT GE Multilin

Instrument Transformers, Inc.

LV Switchboards

LV Panelboards

Typical VT Models

460, 467, 475, 2VT460, 3VT460

GE Multilin

Instrument Transformers, Inc.

L .V. MCC s

Typical CT Models

5 Series

19RT

8SHT 77

GE Multilin

3P85 Instrument Transformers, Inc.

L .V. Switchgear

GE Multilin

Instrument Transformers, Inc.

L .V. Switchgear Current Transformers

GE 2000 amp frame

Model 560 Metering

Model 561 Protection

GE Multilin

Instrument Transformers, Inc.

L .V. Switchgear Current Transformers

GE 4000 amp frame

Model 562 Metering

Model 563 Protection

GE Multilin

Instrument Transformers, Inc.

L .V. Switchgear

No more room in the front?

We can recommend a different

model CT for the rear bus area.

GE Multilin

Instrument Transformers, Inc.

L .V. Switchgear

Typical 3 Phase VT

Installation

3VTL460

3 Phase VT

GE Multilin

Instrument Transformers, Inc.

M.V. Switchgear

Typical Line Up

Where are the CTs,

VTs, and CPTs?

GE Multilin

Instrument Transformers, Inc.

M.V. Switchgear

Current Transformers

778 / 779

680 /780 Series

Typical IEEE Metering & Protection 85

GE Multilin

Instrument Transformers, Inc.

M.V. Switchgear Current Transformers

Metering & Protection

Model 780, 778

Model 785 GE Multilin 86

Instrument Transformers, Inc.

M.V. Switchgear

Model 203 CTs

mounted in rear

bus compartment

GE Multilin

Instrument Transformers, Inc.

M.V. Switchgear

Zero Sequence CT

Model 593, 594

Is this better than a

Residual Connection?

GE Multilin

Instrument Transformers, Inc.

M.V. Switchgear Installed in enclosure

Typical VT Rollout Drawer

(2) PTG5 s in open delta

configuration

Withdrawn from

enclosure 89

GE Multilin

Instrument Transformers, Inc.

M.V. Switchgear

Typical CPT Rollout Drawer

CPT

5 & 15 kV

5 to 15 kVA

Installed in enclosure

Withdrawn from enclosure

GE Multilin

Instrument Transformers, Inc.

MV MCC s

Let s open a door

and see what s

inside

GE Multilin

Instrument Transformers, Inc.

MV MCC s

Typical CT Models

112

113

114

115

117

GE Multilin

Instrument Transformers, Inc.

Generators

Board Mounted Generator CT

GE Multilin

Instrument Transformers, Inc.

Station Class Circuit Breakers

Polyester Taped Bushing CT on

Outdoor Circuit Breaker

GE Multilin

Instrument Transformers, Inc.

Station Class Circuit Breakers

Ground Shield

Outdoor BO7 -

Replaces BCT s in shielded

aluminum housing

GE Multilin

Instrument Transformers, Inc.

Power Transformers

Slip over current transformer for installation over

exterior of outdoor bushing

Outdoor Type BO7

for Retrofit

GE Multilin

Instrument Transformers, Inc.

Power Transformers

Metering applications for indoor instrument transformers

on outdoor power transformers

Indoor PT and Bar type CT

Air terminal chamber

GE Multilin

Instrument Transformers, Inc.

GE Multilin

Instrument Transformers, Inc.

Questions?

GE Multilin

Instrument Transformers, Inc.

Contact this candidate