single-phase instrument transformers - saskpower 3 book 1/10...transformer window twice in the same...

March 10, 2005

S T U D E N T M A N U A L

Single-Phase Instrument

Transformers

Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved

2 S T U D E N T T R A I N I N G M A N U A L

Prerequisites: • Single-Phase Transformer Operating Principles module

Objectives: Given a Revenue Metering Connection Diagrams manual, you will be able to select and connect a single-phase meter service using voltage and current instrument transformers.

Rationale: Some services require the use of special metering using voltage and/or current instrument transformers. Improper connections can easily go unnoticed and cause dramatic errors in customer billing, resulting in loss of revenue or customer relations concerns.

Learning Objectives• Explain the construction and function of potential transformers.• Explain the construction and function of current transformers.• Explain the construction and function of single-phase instrument

meters.• Select and connect all components in a single-phase instrument

connected service.• Determine the billing multiplier for an instrument connected single-

phase service.• Calculate the consumption and cost of an instrument connected

single-phase service.

Learning Methods• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job• On-the-Job Demonstration and Practice• Self-learning + On-the-job• Self-learning + On-the-job

EVALUATION METHODS

• Written test

S I N G L E - P H A S E I N S T R U M E N T T R A N S F O R M E R S 3


• Written test• Written test• On-the-Job Evaluation• Written test• Written test

STUDENT RESOURCES

• Electric Service Guide• Revenue Metering Connection Diagrams

Learning Steps1. Read the Learning Guide.2. Follow the steps outlined in the Learning Guide.3. Clarify any questions or concerns you may have.4. Complete the Practice and Feedback.5. Complete the Evaluation.

Introduction

Instrument transformers are used when the load being metered is greaterthen the capacity of the self contained meter. They simply change themagnitudes of voltage and current to a level that can be safely metered.Two types of instrument transformers are:

• Potential transformers• Current transformers



Lesson 1: Potential TransformerLearning Objective:Explain the construction and function of potential transformers.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

A potential transformer is very similar to a standard distributiontransformer, both in its construction and operation.

It contains a primary and secondary coil and the ratio is such that itproduces either 115V or 120V on the secondary winding.

As with distribution transformers, the primary coil is connected eitherline to line or line to ground. The main difference between a distributiontransformer and a metering potential transformer is that the latter has avery low capacity (rated in volt-amps) and is extremely accurate in itsratio of transformation.



Figure 1. Line to Line Connection



Figure 2. Line to Ground Connection

Ratios

As with distribution transformers, the ratio of a potential transformer isobtained by dividing the voltage rating of the secondary winding intothat of the primary winding.

Example:

A transformer with a voltage rating of 480V on the primary winding and120V on the secondary winding would have the following ratio:



Potential Transformer Selection (PT)

Determine the conditions that require the use of a potential transformerand if needed, determine the proper size of potential transformer

Conditions Requiring a Potential Transformer

A potential transformer is required when a high voltage line must bereduced to utilize a low voltage meter.

Size Selection of Potential Transformers

To determine the proper PT size, the amount of voltage phase-to-phaseand/or phase-to-ground must be checked (use of a voltmeter or priorknowledge of the circuit). The amount of voltage determines whichmetering diagram to select and which potential transformer is required.Check the PT nameplate for sizing and capacity ratings.

Reference

For further information, refer to connection diagrams CD-136and CD-237 in the Revenue Metering manual.



Figure 3. PT Nameplate

Figure 4. PT Nameplate



Lesson 2: Current TransformersLearning Objective:Explain the construction and function of current transformers.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

Current transformers are designed so the primary winding is in serieswith the line and is rated as to the amount of current that can be carriedin the primary winding. The ratio of CTs are 100/5, 50/5, 20/5, etc.

A rating of 100/5 indicates the transformer is fully loaded when 100amps is flowing in the primary winding.

With a full load on the primary, there will be 5 amps flowing on thesecondary. Some types of current transformers are:

• 2-wire current transformer• 3-wire current transformer• open window type current transformer

CAUTION!Current transformers must always be hooked inseries.

DANGERSTOP

If a 2-wire, self-contained meter was accidentallyinstalled on a circuit with a current transformer, a deadshort would result across the terminals, putting theoperator at risk and damaging equipment.



2-Wire Current Transformer

Figure 5. 2-Wire Current Transformer

A 2-wire current transformer consists of one primary winding and onesecondary winding.

These transformers are normally used on 2-wire, single-phase circuits inconjunction with 2-wire meters. They are also used on all types of three-phase circuits.



3-Wire Current Transformer

Figure 6. 3-Wire Current Transformer

A 3-wire current transformer consists of two primary windings and onesecondary winding.

These transformers are normally used on 3-wire, single-phase circuits inconjunction with 2-wire meters, but were also used as a past standardthree-phase 4-wire delta configuration.



Open Window Type Current Transformer

Figure 7. Window Type CT

This particular current transformer consists of a circular iron core withmany secondary turns wrapped around it. It has no primary winding.This is provided by the phase conductor of the circuit being measured.The figure above illustrates the construction of this particulartransformer.

The ratio of this current transformer may be altered by varying thenumber of wires passing through the window of the transformer. Theresulting ratio is not affected by the direction of the wires in the window,but the direction of the current in the conductor will affect the metering.

Determining the Operating Ratio

The operating ratio of a window type current transformer may bedetermined as follows:

If a 200/5 amp current transformer has one conductor through itswindow, the operating ratio is 200/5 or 40/1, as shown on the nameplate.

However, if two conductors are passed through the same 200/5transformer, the new operating ratio is:



This means that with 100 amps in the conductor, there will be 5 amps inthe secondary.

Figure 8. Window CT and 2-Wire Circuit

If the nameplate ratio of this transformer is 200/5 with 100 amps flowingon the primary, there will be 2.5 amps flowing on the secondary andthrough the meter coil.

Example:

Assume a 50 amp circuit has a 240 volt load (no 120 volt loads).Assume the connections are made as in the previous figure, and that theJKP_O current transformer has a nameplate ratio of 200/5. What is thesecondary current in the meter coil?



The above diagram shows the same transformer connected for 3-wireoperation in a 3-wire, single-phase circuit.

This is a special connection since it uses two conductors - one from eachphase to make a 3-wire current transformer. Each conductor must passthrough in opposite directions to ensure proper registration for this typeof metering.



Follow the path of the current as it passes through the currenttransformer window twice in the same direction.

Current Transformer Selection (CT)

When using a current transformer, first determine the conditions presentand determine the proper size of current transformer to use.

Some conditions which require a current transformer include:

• A need to reduce line currents too large for a standard meter.• To insulate the meter from high voltages on high voltage lines

(above 480V). This is usually the reason why a current transformerwith a 1:1 ratio is used. This makes it possible to avoid connectingammeters or instrument current coils directly in high voltage lines.

• On circuits where the wires are too large to fit a standard meterterminal.

Selecting the Size of a Current Transformer

To determine the proper CT size, the amount of current flow in the twophases (ammeter check or prior knowledge) and the amount of voltage(phase-to-phase or phase-to-ground) must be known. For example, aservice has 205 amps on one phase and 225 amps on the other phase.

If the amperage is between a CT ratio of 200:5, but smaller than the nextsize of CT ratio 300:5, always select the higher size of CT ratio 300:5.Check the CT nameplate for proper sizing and capacity ratings. Thiswill ensure the coils are not subject to unnecessary stress.



Figure 9. CT Nameplate



Lesson 3: Instrument MetersLearning Objective:Explain the construction and function of single-phase instrument

meters.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

The meter used with the instrument transformers on a single-phaseservice is always a 2-wire meter, regardless of whether the circuit beingmeasured is a 2-wire or 3-wire circuit. These meters are a transformertype meter, with connections and ratings different from that of astandard, self contained, 2-wire meter.

Meter Selection

Instrument meter selection is determined by the voltage required tomeasure 120 volts or 240 volts. If a potential transformer isincorporated to reduce the voltage to 120 volts, a 120 volt transformerrated meter is required. On a 120/240V service, with a currenttransformer installed, a 240 volt 2-wire meter is required. Check themeter nameplate for size and capacity ratings.

Connecting Instrument Transformers

When connecting instrument transformers, the Revenue Meteringmanual is used to ensure the proper connections are made. Certainfeatures and procedures unique to the installation of instrumenttransformers and meters should also be considered.

Unique Features

Instrument transformers possess built in safety features which include:

• Polarity markings• Shorting links (bars)• Wire color coding

Polarity Markings

Polarity markings identify the path the voltage or current flow takes



through the primary coil into the instrument transformer’s windings andout the secondary bushing or terminal. Instrument transformers areusually subtractive polarity, whereby voltage or current flow enters theH 1 primary bushing and exits the secondary X 1 bushing. A white dotnear the primary H 1 bushing and near the secondary X 1 bushingindicates the correct path of voltage or current flow. In order to meter aservice properly, it is imperative to adhere to the instrumenttransformer’s polarity markings.

Shorting Links (Bars)

A current transformer must always have the secondary coil short-circuited during installation or while in a circuit with the meter removed.CTs must not be energized with the secondary open circuited for thefollowing reasons:

• The core may become magnetized, thereby altering thecharacteristics of the transformer.

• High voltage can occur in an open circuited secondary and maypuncture the winding insulation.

• It can severely shock anyone coming into contact with the winding.

Example:

E = I x R (R is very high because of the open circuit)



Figure 10. Shorting Bar

Some types of current transformers are equipped with shorting links(bars) on the secondary terminals and must be closed before thesecondary leads are removed. On other types, the secondary isautomatically short circuited when the terminal cover is removed.Sockets used on single-phase metering installations where currenttransformers are required have a built-in short circuiting device on theleft hand side of the meter socket. This automatically short circuits thecurrent transformer secondary circuit when the meter is removed.

Sockets used on three-phase installations require a test block switch.

---Note---These sockets should not be used as an initial method of shortcircuiting the CT. They are a precautionary method only.



Wire Color Coding

The Revenue Meter manual has codes on the wiring diagram with lettersrepresenting the color of wire to use (ie: R/BK refers to a red wire witha black stripe). This eliminates confusion when connecting and helpsensure proper metering connections.

Installation Procedure

When installing instrument transformers to meter a service, correctconnections must be made to ensure proper metering. First determinethe service voltage and peak load for the service. Meter wiring must bein accordance with the Revenue Metering manual.

Once the metering connections have all been made and the meter hasbeen installed, energize the service. Check the disc rotation to ensurethe meter is turning in the direction indicated by the arrow.

CAUTION!Always ensure the current transformer shorting linksare short circuited until the meter has been installedand the circuit is complete.

---Note---If the disc is rotating backwards, switch either the current leadsor potential leads to correct the problem.



Lesson 4: Connecting Instrument TransformersLearning Objective:Select and connect all components in a single-phase instrument

connected service.Learning Method:On-the-Job Demonstration and PracticeEvaluation Method:On-the-Job Evaluation



Skills Practice

1. Determine:

1. Service voltage size, phase-to-phase and/or phase-to-ground (prior knowledge or voltmeter reading)

2. Service current flow in each phase (prior knowledge orammeter reading)

3. Correct revenue metering diagram

2. Select:

1. Correct instrument transformer sizes

2. Meter

3. Correct wire (color code)

3. Power disconnected:

1. Connect instrument transformer and meter according to themetering diagram

2. Open shorting links on CT

3. Install tamper proof seals

4. Energize the service

5. Perform meter check



Lesson 5: Billing Multiplier (BM)Learning Objective:Determine the billing multiplier for an instrument connected

single-phase service.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

To calculate the cost and consumption of an instrument metered service,an operator must use instrument ratios.

• Multiply constants when using CTs and PTs.• Calculate the cost of an instrument meter service.

Multiplying Constants

When current and potential transformers are used in a metering circuit,the consumption is multiplied by the billing multiplier to calculate thetotal consumption.

The BM is calculated as follows:

BM = CT ratio x PT ratio x inherent meter multiplier

Example:

Present reading = 3856

Previous reading = 3428

Potential transformer = 480/120 volt = 4:1 ratio

Current transformer = 50/5 amp = 10:1 ratio

Reference

For further information, refer to the Revenue Metering manual.The billing multiplier calculation is always illustrated at thebottom of the page.



Inherent meter multiplier = 1

BM = 10 x 4 x 1

BM = 40

Consumption = (3856 - 3428) x BM

Consumption = 428 x 40

Consumption = 17,120 kilowatt hours

Although the two readings are usually taken one month apart, anyinterval of time may be used depending upon the rate given to thecustomer.

The factor (CT ratio x PT ratio x inherent meter multiplier) in theprevious equation is the multiplying constant in the entire meteringinstallation and will be referred to as the overall multiplying constant.The term billing multiplier is more generally used.



Lesson 6: Calculating the Cost of an Instrument Meter ServiceLearning Objective:Calculate the consumption and cost of an instrument connected

single-phase service.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

Once the billing multiplier has been determined, it can be applied to theconsumption reading. The cost formula can be used to determine thecost (in dollars) for that particular service.

Continuing with the previous example, the consumption of the meter is428kWh and the demand reading is 1600. Calculate the cost of theservice if the billing multiplier is 40 and the cost per kWh is 7 cents forthe first 10,000kWh and 3 cents for the remaining consumption. Thedemand charge is $0.00 for the first 50kVA and $13.01 per kVA on theremaining demand.

Consumption = Read x BM

Consumption = 428 x 40

Consumption = 17120kWh

Demand = Read x BM

Demand = 1600 x 40

Demand = 64000VA

Demand = 64kVA

Cost = Consumption x Rate

Cost = (10,000 x .07) + (17,120 - 10,000) x .03

Cost = 700 + 213.6

Cost = $913.60



Demand Charge = Consumption x Rate

Demand Charge = (64 - 50) x $13.01

Demand Charge = 14 x $13.01

Demand Charge = $182.14

Total Cost = Consumption + Demand

Total Cost = $913.60 + $182.14

Total Cost = $1095.74



Summary

To summarize this module, you have learned:

• The construction and function of potential transformers.• The construction and function of current transformers.• The construction and function of single-phase instrument meters.• To select and connect all components in a single-phase instrument

connected service.• To determine the overall billing multiplier for an instrument

connected single-phase service.• To calculate the consumption and cost of an instrument connected

single-phase service.

Practice Feedback

Review the lesson, ask any questions and complete the self-test.

Evaluation

When you are ready, complete the final test. You are expected toachieve 100%.



Review Questions

1. A potential transformer is used to reduce:(a) Current.(b) Power.(c) Resistance.(d) Voltage.

T / F 2. The primary side of a potential transformer can only be connected phase-to-ground.

T / F 3. A potential transformer can be used on a 120/240 volt single-phase service.

4. A potential transformer has a primary winding rating of 2400 volts and a secondary winding rating of 120 volts. The PT ratio is:(a) 20:1.(b) 1:1.(c) 10:1.(d) 2400:1.

5. Types of current transformers are:(a) 2-wire and 3-wire.(b) 3-wire, 2-wire and open window.(c) 3-wire and open window.(d) Open window and 2-wire.

6. The operating ratio of a 600 amp service using an open window type current transformer with two cables running through it is:(a) 100:5.(b) 200:5.(c) 300:5.(d) 600:5.



7. A 2-wire CT with 225 amps flowing in the primary winding has a ratio of 300:5. The amperage at the meter is:(a) 225 amps.(b) 5 amps.(c) 3.75 amps.(d) 4.25 amps.

T / F 8. When selecting a meter, an operator should check the meter nameplate.

9. When making instrument transformer connections, an operator should be aware of:(a) Color wire coding, polarity markings and meter

box size.(b) Polarity marks, shorting links and wire color

coding.(c) Shorting links, size of meter box and polarity

marks.(d) None of these

10. A single-phase service is metered using two 400/5 current transformers and a 600/120V potential transformer. The meter inherent multiplier is 1.5. The billing multiplier is:(a) 400.(b) 600.(c) 100.(d) 1000.

11. A single-phase service contains a 480/120V PT, a 200/5 2-wire CT and a meter with an inherent multiplier of 2. The previous read was 4,328kWh and the present read is 4,386kWh. The billing multiplier is:(a) 480.(b) 200.(c) 320.(d) 2.



12. A single-phase service contains a 480/120V PT, a 200/5 2-wire CT and a meter with an inherent multiplier of 2. The previous read was 4,328kWh and the present read is 4,386kWh. The actual amount of kilowatts consumed is:(a) 18,560kWh.(b) 58kWh.(c) 96,000kWh.(d) 960kWh.

13. A single-phase service contains a 480/120V PT, a 200/5 2-wire CT and a meter with an inherent multiplier of 2. The previous read was 4,328kWh and the present read is 4,386kWh. If the cost per kWh is 3 cents, the kilowatt consumption cost is:(a) $556.80.(b) $320.00.(c) $960.00.(d) $58.00.



Review Question Solutions

1. Voltage.

2. F

3. T

4. 20:1.

5. 3-wire, 2-wire and open window.

6. 300:5.

7. 3.75 amps.

8. T

9. Polarity marks, shorting links and wire color coding.

10. 600.

11. 320.

12. 18,560kWh.

13. $556.80.

single-phase instrument transformers - saskpower 3 book 1/10...transformer window twice in the same...

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