Three-Phase Transformer Load Checks 3 Book 1/6... · Objectives: Given an alive substation or...

March 10, 2005

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

Three-Phase Transformer Load

Checks

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: • None.

Objectives: Given an alive substation or transformer bank, tools and equipment and a formula sheet, you will be able to determine three-phase load in kVA.

Rationale: Substations and transformer banks are an integral part of a distribution system with loads changing constantly. Monitoring the load on the transformers gives us information to supply and maintain customer voltages.

Learning Objectives• Explain the procedure used to determine coil and line current

values.• Explain the procedure used to determine coil and line voltage.• Describe the procedure to determine the load in kVA on an alive

substation or three-phase transformer bank.• Demonstrate the procedure to determine the load in kVA on an alive

substation or three-phase transformer bank.• Explain the rated three-phase capacity of a substation or transformer

bank.• Explain the fuse and riser sizing for a transformer bank.

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

EVALUATION METHODS

• Written test• Written test• Written test.

T H R E E - P H A S E T R A N S F O R M E R L O A D C H E C K S 3


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

STUDENT RESOURCES

• None.

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.



Lesson 1: Coil and Line Current ValuesLearning Objective:Explain the procedure used to determine coil and line current

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

Introduction

The first step in dealing with three-phase transformer or transformerbank load checks is to read the nameplate and identify the primary andsecondary connections.

Although the clip-on ammeter may be used on these higher voltagelevels, the voltmeter may not. Most voltmeters can be used on voltagesup to 1000 volts. Check the manufacturer’s recommendations beforeusing it.

The next step is to draw a three-phase connection diagram and recordthe measured values on the diagram to prevent confusion. Alwaysremember to convert line values to coil values, and visa versa, whennecessary.

Reference

For more information on three-phase transformer bankconnections, refer to the Construction Standards Manual, A-08-00.

DANGERSTOP

Remember, if the voltage levels are higher than 300volts, you are required to wear Class 1 rubber glovesup to 5,000 volts.



Figure 1. Delta-star configuration - follow rules of each

Current readings may be taken from the secondary risers with a clip-onammeter. If the values are taken from the primary risers, use theammertong hotstick with the ammeter in place. As each reading istaken, record the values on the vector diagram.

If the current cannot be measured on either the primary or secondaryrisers, there may be a metering cabinet attached to the secondary withcurrent transformers in series with the metering equipment. If youobtain a current reading at the current transformer, remember tomultiply the measured value by the current transformer ratio.



Lesson 2: Coil and Line VoltageLearning Objective:Explain the procedure used to determine coil and line voltage.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

When measuring the voltage of a transformer bank or three-phasetransformer, refer to the nameplate for information on the primary andsecondary connections. Once again, the voltage levels may require theuse of rubber gloves.

If the secondary bushings are accessible, you can obtain a voltagereading from them. To calculate the primary voltage from these values,use the turns ratio formula. If the secondary bushings are somewhatinaccessible, there may be potential transformers in the meteringcabinet. If the voltages are taken here, remember to multiply the valuesby the potential transformer ratio.

These values should always be applied to the vector diagram to preventconfusion, as well as to offer information on where the readings weretaken from.

Look closely at the nameplate information and/or transformer bankconfigurations to ensure accuracy in the values. Once again, rememberto convert coil values to line values when applicable

---Note---A simple method of understanding coil or line values is to lookat the riser connections on a delta configuration. If the current ismeasured on the riser before the connection, it is a coil value. Ifthe current is measured after the connection, it is a line value.Similarly, in a wye system, if the voltage is measured line toground (neutral), it is a coil value. If the voltage is measured lineto line (hot leg to hot leg), it is a line value.



Lesson 3: Determining the LoadLearning Objective:Describe the procedure to determine the load in kVA on an alive

substation or three-phase transformer bank.Learning Method:Self learning and on the job.Evaluation Method:Written test.

Introduction

When calculating the load of a substation or three-phase transformerbank in kVA, two options are available.

The first option is to use the three-phase equation:

Line values - three-phase formula:

Coil values - three-phase formula:

The second option is to determine the load on the individualtransformers and add these together.

When calculating the capacity of a three-phase bank with a 4-wire deltaconnected secondary, it is best to use coil values to calculate the load oneach individual transformer and add the capacities together.

The following example illustrates three-phase load calculations usingboth methods.



Figure 2. 240V Three-phase 4-wire Delta Connection

Remember, we are dealing with winding values and this is a deltasystem.

Therefore:

Alternatively

Transformer 1:



Transformer 2:

Transformer 3:

The addition of the three transformers = 42.84kVA.

On certain occasions, you may be faced with the problem of combiningprimary readings with secondary readings. In this scenario, convert thevalues to either primary or secondary values.

This can be accomplished by utilizing the transformer turns ratioformula:

Remember to use coil values when performing the conversion. Theanswer can then be converted to line values if necessary.

Example:

A 2,400V open delta-open delta transformer bank is connected to supplya 480V three-phase 3-wire. The input voltage of the primary coil is2,308V. The coil current of the secondary on the first transformer is82A. The second transformer is measured at 85A. What is the load, inkVA, on the transformer bank?



In a delta connection:

Transformer 1:

Transformer 2:

---Note---To calculate the load on an open bank, calculate the load on eachtransformer and add the two loads together. The answer is thetotal three-phase load on the transformers.



Lesson 4: Perform a Load CheckLearning Objective:Demonstrate the procedure to determine the load in kVA on an

alive substation or three-phase transformer bank.Learning Method:On-the-Job Demonstration and PracticeEvaluation Method:On-the-Job Evaluation



Skills Practice

1. Perform a load check:

1. Determine the primary and secondary configuration.

2. Determine the voltage where measured.

3. Use rubber gloves or ammertong if required.

4. Measure the line or coil voltage.

5. Measure the line or coil current.

6. Use the appropriate formula (line or coil) to calculate theload.

7. Give the answer in kVA.



Lesson 5: Three-Phase CapacityLearning Objective:Explain the rated three-phase capacity of a substation or

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

Introduction

The rated three-phase capacity of a substation or transformer bank is itscapacity at full load current. To determine this in a closed bank orsubstation, the capacity is three times the smallest transformer. In anopen bank, the capacity is 86% of two, times the smallest transformer.

Example:

An open delta-open delta bank consists of two 25kVA transformers 120/240V. Secondary Line I avg = 64A. What is the capacity of the bank?

Capacity = 50kVA x 86.6%

Capacity = 43.3kVA

What is the load on the bank?

Transformer 1:

Transformer 2:

The percentage load of a transformer bank is a ratio of the actual load onthe bank as compared to the rated capacity of the bank.



The formula to calculate the percentage load on a bank is:



Lesson 6: Fuse and Riser SizeLearning Objective:Explain the fuse and riser sizing for a transformer bank.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test

Introduction

The fuse and riser are generally based on 200% of the transformer’scapacity. It is important to choose the correct fuse and prevent thetransformer from receiving more current from the primary supply linesthan is recommended. If the current flow is too high, the fuse will isolatethe transformer from the source voltage.The fuse must be large enoughto allow sufficient current flow to the transformer to supply the load theyare designed to serve, yet small enough to prevent damage to thetransformer from overcurrent conditions caused by overload orotherwise.The riser size must be large enough to allow current to flowto the secondary side and satisfy the load. If the riser is too small on thesecondary side, insufficient current will be permitted to flow to satisfyan increasing load

Reference

For information on fuse and riser sizes, refer to theTransformation section of the Construction StandardsManual,Drawing No. A-08-00, Sheet 20 of 27 through to Sheet23 of 27.



Summary

To summarize this module, you have learned:

• To complete three-phase load checks using line or coil values.• About three-phase capacity.• Fuse and riser size selection from the Construction Standards

Manual.

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

T / F 1. The first step in taking a three-phase transformer or transformer bank load check is to read the transformer name plates and determine the secondary and primary connections.

T / F 2. Class 1 rubber gloves are required when measuring voltage levels between 300V and 5000V.

3. A balanced three-phase delta connected load draws 30A on each line conductor. The current on the secondary coils of this load is:(a) 30kVA.(b) 51.9A.(c) 30A.(d) 17.3A.

4. The voltage measured in the metering cabinet of a substation transformer rated 72,000V delta - 25,000V wye reads 128V. The power transformer ratio is 14,400V:120V. Calculate the secondary line voltage from the substation transformer.(a) 26,573V(b) 14,400V(c) 25,000(d) 15,360V

T / F 5. When current cannot be measured on either the secondary or primary risers, readings may be obtained from current transformers in an attached metering cabinet.



6. In the graphic below which formula is used to calculate a three-phase load?

(a) A(b) B(c) C(d) D

7. If a load check is taken on the secondary side of a three-phase bank, the load on the primary side would be:(a) Greater than on the secondary side.(b) Less than on the secondary side.(c) The same as on the secondary side.(d) None of these

8. To calculate the load on a three-phase 4-wire delta connected transformer bank, the current on the secondary risers of the lighting transformer must be:(a) Added to the current of the power transformers.(b) Subtracted from the current of the power

transformers.(c) Excluded from the average current on the power

transformers.(d) Averaged then added to the current of the other

transformers.



9. The capacity of an open three-phase connected 15kVA and 25kVA transformer bank is:(a) 40kVA(b) 43.3kVA(c) 30kVA(d) 25.9kVA

10. The capacity of a three-phase closed bank containing three 50kVA - 25,000V transformers is:(a) 129kVA(b) 100kVA(c) 150kVA(d) 86.6kVA

11. The correct fuse and riser size is generally based on:(a) 100% of the transformer’s capacity.(b) 150% of the transformer’s capacity.(c) 200% of the transformer’s capacity.(d) 75% of the transformer’s capacity.



Review Question Solutions

1. T

2. T

3. 17.3A.

4. 26,573V

5. T

6. D

7. The same as on the secondary side.

8. Averaged then added to the current of the other transform-ers.

9. 25.9kVA

10. 150kVA

11. 200% of the transformer’s capacity.

Date post:	19-Jul-2020
Category:	Documents
Upload:	others
View:	15 times
Download:	0 times

Three-Phase Transformer Load Checks 3 Book 1/6... · Objectives: Given an alive substation or...

Documents