March 31, 2005

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

Three-Phase Underground

Operating

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

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Prerequisites: • Single-Phase Underground Operating module

Objectives: Given an underground distribution system, the Construction Standards Manual, tools and equipment, and an electrical facilities map, you will be able to perform the necessary switching procedures to operate a three-phase underground system.

Rationale: There are a variety of procedures required for the operating, troubleshooting and maintenance of a three-phase underground distribution system. All of these procedures require the ability to perform a combination of basic operating techniques. Since many components of the underground system are not visible, it is important to develop and follow standard switching procedures.

Learning Objectives• Explain the procedure to fuse and operate a three-phase

underground transformer.• Demonstrate the procedure to fuse and operate a three-phase

underground transformer.

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

EVALUATION METHODS

• Written test• On-the-Job Evaluation

STUDENT RESOURCES

• None

Learning Steps

T H R E E - P H A S E U N D E R G R O U N D O P E R A T I N G 3

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

1. 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.

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

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Lesson 1: Three-Phase Underground OperatingLearning Objective:Explain the procedure to fuse and operate a three-phase

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

Introduction

The majority of three-phase underground operations and switching isdone when a fault exists on the system. Other switching requirementsmay be completed to:

• meet load demands• isolate cables and apparatus being worked on• isolate cables and apparatus in close proximity to work being

performed

It is important for the personnel performing the task to know andunderstand the procedures to follow. Following the correct operatingprocedure will ensure worker and public safety.

The operations and switching required to maintain an undergroundthree-phase system include:

• fusing transformers• isolating and energizing cables and apparatus• closing, opening or moving a normal open

Most of these tasks will be completed in either a dead front, dead breaktransformer or dead front, load break transformer, since this is a locationin which fusing and switching operations can be easily performed. Also,with three-phase underground, proper phasing and phase rotation mustbe understood and adhered to.

Three-Phase Underground Transformer Operating Procedures

The operation of group operated oil switches may vary. Follow thedirections of the manufacturer located on the transformer switch.

T H R E E - P H A S E U N D E R G R O U N D O P E R A T I N G 5

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

Most three-phase underground transformers are equipped with an off-load tap changer. To change the tap setting, isolate the transformer.

Three-Phase Underground Transformer Fusing

The basic procedure for fusing and operating a three-phase transformeris very similar to that of a single-phase underground transformer.

The main difference in three-phase applications is that operations arecompleted with three cables or three fuses.

Both the dead front, dead break transformer and the dead front, loadbreak transformer have two high voltage group operated oil switches,which are designed to isolate the high voltage bushings of thetransformer from its internal three-phase buss work. Another switch is

---Note---The position of the tap changer must be noted when changing outa transformer.

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

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used to isolate the transformer windings from the internal buss work,thereby isolating the bayonet fuses.

Three-Phase Fusing Procedures

The following procedures are to be adhered to when replacing thebayonet fuses in three-phase underground transformers.

Prior to removing a bayonet fuse from a three-phase transformer, ensurethe transformer oil switch is off and the pressure relief valve has beenoperated.

Change the bayonet fuse in accordance with the Construction Standards

---Note---Open and block the transformer door.

T H R E E - P H A S E U N D E R G R O U N D O P E R A T I N G 7

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

manual.

Underground Faults

When operating in an underground three-phase system, it is important torecognize the causes of a fault and the steps required to locate a fault.

Upon determining the location of the fault, develop a switching plan toisolate the faulted cable within the underground system. This enablesservice restoration to as many customers as possible.

The following can be utilized to determine the origin of the problem:

• takeoff fuses

CAUTION!Do not use the bayonet fuse to pick up the transformerload; always use the transformer oil switch.

CAUTION!When a fault occurs on a three-phase system, single-phasing problems may arise.

CAUTION!When underground cables on a three-phase systemare severed completely due to digging operations, allcables must be phased out. This is to ensure thecables are not cross-phased and proper phaserotation.

---Note---When faults occur on a three-phase system, customers may havepartial power. The fault indicators will identify which phase(s)the fault(s) has occurred on.

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

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• fault indicators• voltage checks

Once the faulty section of cable has been isolated, the loop feed systemcan be used to create an alternate cable feed.

CAUTION!All safety precautions and switching procedures mustbe adhered to.

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

Lesson 2: Demonstrate Three-Phase Underground OperatingLearning Objective:Demonstrate the procedure to fuse and operate a three-phase

underground transformer.Learning Method:On-the-Job Demonstration and PracticeEvaluation Method:On-the-Job Evaluation

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

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Skills Practice

1. Tools:

1. Clean and approve 8 foot grip-all hotstick

2. Wrench (crescent or box end)

2. Removal of fuse:

1. Open and block the doors.

2. Open the transformer oil switch.

3. Operate the transformer relief valve.

4. Unlock the fuse handle eye.

5. Rotate the handle 90 degrees.

6. Pull the bayonet fuse cartridge out part way and allow theoil to drain.

7. Remove the bayonet fuse cartridge completely.

8. Unscrew the fuse cartridge container and end plug with awrench and remove the fuse.

3. Installation of fuse:

1. Install a new fuse and tighten the end plug (expand thescalloped end ferrule). Reconnect the fuse cartridgecontainer with a wrench.

2. Align the bayonet fuse assembly with the housing andinsert properly.

3. Turn the locking handle and engage the latch.

4. Use appropriate switching methods to energize the

T H R E E - P H A S E U N D E R G R O U N D O P E R A T I N G 11

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

transformer from a remote location.

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

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Summary

To summarize this module, you have learned:

• The procedure to fuse and operate a three-phase underground transformer.

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%.

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

Review Questions

T / F 1. Three-phase underground transformers have three internal switches which can be operated externally.

T / F 2. The dead front, load break transformer is the only three-phase underground transformer with bayonet fuses.

T / F 3. After re-fusing a bayonet fuse at a three-phase underground transformer, the operator should re-energize using the transformer’s cable switch at the re-fusing location.

T / F 4. Most three-phase underground transformers have an off-load tap changer.

T / F 5. When a fault occurs on a three-phase underground cable, single phasing will never occur.

6. An operator is required to operate transformer oil switches with:(a) Class 2 rubber gloves.(b) 8 foot grip-all hotstick.(c) Elbow puller.(d) All of these

7. Other than a fault situation, other switching requirements may include:(a) Meeting load demands.(b) Isolating cables and apparatus being worked on.(c) Isolating cables and apparatus in close proximity

to work being performed.(d) All of these

T / F 8. With three-phase underground, proper phasing and phase rotation must be understood and adhered to.

T / F 9. The doors must be opened and blocked when operating in a three-phase underground transformer.

T / F 10. Bayonet fuses may be used to pick up load.

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

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Review Question Solutions

1. T

2. F

3. F

4. T

5. F

6. 8 foot grip-all hotstick.

7. All of these

8. T

9. T

10. F