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March 31, 2005 STUDENT MANUAL Overvoltage Protection
March 31, 2005
S T U D E N T M A N U A L
Overvoltage Protection
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 Load Checks
Objectives: From memory, you will be able to describe the electrical system requirements for overvoltage protection.
Rationale: An electrical system is susceptible to many types of overvoltage problems. Proper installation of overvoltage protection devices will keep damage to the system and customer’s equipment to a minimum.
Learning Objectives• Identify potential overvoltage situations.• Describe the function of a shield wire.• Describe the two applications of a shield wire.• Describe the function of a lightning arrestor.• Describe the two types of lightning arrestors and their applications.
Learning Methods• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job• Self-learning + On-the-job
EVALUATION METHODS
• Written test• Written test• Written test• Written test• Written test
O V E R V O L T A G E P R O T E C T I O N 3
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
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.
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
4 S T U D E N T T R A I N I N G M A N U A L
Lesson 1: Overvoltage ProblemsLearning Objective:Identify potential overvoltage situations.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test
Introduction
Lightning flashes represent a difference of electrical potential betweenclouds or between the clouds and the earth. Lightning possesses thesame principle as electricity, and always flows through the path of leastresistance. Power lines and electrical facilities provide a path of lowresistance to the earth’s ground for the lightning to take.
These lightning strikes, which contact power lines and electricalfacilities, are the main cause of overvoltage in a power system.
Electrical facilities can be protected from the lightning by the use of:
• shield wires• lightning arrestors
System faults, poor voltage regulation, and switching surges are othercauses of overvoltage.
These overvoltages are difficult to protect against - customers withsensitive equipment may install their own surge or overvoltageprotection.
Overvoltage can cause serious damage to:
• customer equipment• equipment and apparatus in any electrical system such as
transformers, power poles, and the insulating capability of insulators
Another major concern caused by overvoltage is power outages and thetime required to restore power.
O V E R V O L T A G E P R O T E C T I O N 5
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Lesson 2: Function of a Shield WireLearning Objective:Describe the function of a shield wire.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test
Introduction
A shield wire is a conductor mounted above (and running parallel with)the energized conductor, and is connected to the highest point of eachstructure.
The shield wire is connected to ground which provides the lightningwith a low resistance path to ground. The shield wire protects theinsulators, which in turn protects the conductors, the structures and otherapparatus from the high voltage caused by the lightning strike.
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
6 S T U D E N T T R A I N I N G M A N U A L
When lightning strikes a line conductor, the only thing preventing theovervoltage from flowing to the earth’s ground is the line insulation. Onsteel structures, the insulation from ground is the suspension or dead-end insulators. On lines constructed with wood poles, the insulationfrom ground would be the insulators and the poles themselves. If therewas no shield wire, the lightning overvoltage would either destroy theinsulators or reduce their insulating capability.
Shield wires are highly effective, and are generally installed ontransmission and other crucial lines because:
• These lines affect the delivery of electricity to a large number of customers.
• These lines take a long period of time to repair if damaged.• There is a high cost to install a shield wire on all lines, so only the
major ones are most commonly protected.
O V E R V O L T A G E P R O T E C T I O N 7
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Lesson 3: Applications of a Shield WireLearning Objective:Describe the two applications of a shield wire.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test
Directly Connected Shield Wire
Steel Towers or Steel Poles
The shield wire on steel towers or steel poles is connected by a metalconnection at the top of each structure. The structure is connected to aground rod (located at the base of the structure) by a ground wire. Thesteel structure is also part of the grounding path.
Wooden Poles
The shield wire on wooden poles is connected to the top of each poleusing a metal clamp. The ground wire extending from the metal clamp
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
8 S T U D E N T T R A I N I N G M A N U A L
is connected to the ground rod at the base of the pole.
Insulated Shield Wire
On some transmission lines, the shield wire is also used forcommunication purposes. This shield wire is connected to the structureby an insulated mounting bracket. This bracket is grounded, and thedesign is such that the lightning high voltage will flash over the insulatorand follow the low resistance path to ground provided.
Steel Towers or Steel Poles
On steel towers or steel poles, the shield wire is connected at the top ofeach structure with an insulator mounting bracket. The structure isconnected to a ground rod (located at the base of the structure) by aground wire. The steel structure is part of the grounding path.
Wooden Poles
O V E R V O L T A G E P R O T E C T I O N 9
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
On wood poles, the shield wire is connected to the top of each pole withan insulator mounting bracket. The ground wire extending from theinsulator mounting bracket is connected to the ground rod at the base ofthe pole.
DANGERSTOP
Extreme caution must be taken when working with thisparticular application, as a potential difference may bepresent in these shield wires and serious injury couldresult.
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
10 S T U D E N T T R A I N I N G M A N U A L
Lesson 4: Function of a Lightning ArrestorLearning Objective:Describe the function of a lightning arrestor.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test
Introduction
The intent of a lightning arrestor is to relieve power lines of high voltagestress, and in doing so, protect electrical apparatus and equipment. Witha properly installed arrestor, the overvoltage situation quickly subsidesand the system returns to normal.
An arrestor is installed in parallel by connecting the top to the incomingpower line, and the bottom to the ground wire and system neutral, ifavailable.
Normal Conditions
O V E R V O L T A G E P R O T E C T I O N 11
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
An arrestor has a spark gap that is designed to provide an open circuitbetween the line and ground for normal operating conditions. Thenormal voltage of a system should not flash over this gap.
Overvoltage Conditions
Overvoltage conditions may cause the spark gap in the arrestor to flashover and dissipate the high voltage into the earth using a ground wireand adjoining neutral connections.
This spark gap makes an open circuit which allows the system to returnto normal after the overvoltage condition has subsided.
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
12 S T U D E N T T R A I N I N G M A N U A L
Lesson 5: Types and Applications of Lightning ArrestorsLearning Objective:Describe the two types of lightning arrestors and their
applications.Learning Method:Self-learning + On-the-jobEvaluation Method:Written test
Introduction
There are two types of arrestors used to protect electrical apparatus:
• underground arrestors• overhead arrestors
CAUTION!The basic operation of these two types of lightningarrestors is the same; however, they are designeddifferently to be used on a variety of underground andoverhead applications.
O V E R V O L T A G E P R O T E C T I O N 13
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Underground Type
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14 S T U D E N T T R A I N I N G M A N U A L
Underground arrestors are used at the end of underground primarycables where an elbow connection is required. They may be found at:
• a normal open in an underground loop system• an underground switching cubicle• an underground radial feed
The underground type of arrestor not only provides a means forovervoltage protection, but also serves as an insulating cap forunderground transformer bushings.
Overhead Type
There are two main classes of overhead arrestors used on powersystems:
• station class• distribution class
O V E R V O L T A G E P R O T E C T I O N 15
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Station Class
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16 S T U D E N T T R A I N I N G M A N U A L
Station class arrestors are generally used to protect equipment insubstations and switching stations.
O V E R V O L T A G E P R O T E C T I O N 17
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Distribution Class
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Distribution class arrestors are generally used to protect distributionequipment 25kV and lower, such as:
• distribution transformers• underground take-off structures• metering units
---Note---Arrestors are available in a variety of voltage sizes. The size ofarrestor required depends on the applied line voltage, as well asthe flash over voltage rating required before the arrestoroperates. This information is provided in the packaging of thearrestor.
O V E R V O L T A G E P R O T E C T I O N 19
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Summary
To summarize this module, you have learned:
• How to identify potential overvoltage situations.• Two types of overvoltage protection.• The function of a shield wire.• Two applications of a shield wire.• The function of a lightning arrestor.• The applications of two types of lightning arrestors.
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%.
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
20 S T U D E N T T R A I N I N G M A N U A L
Review Questions
1. Overvoltage may result from:(a) System faults.(b) Poor regulation.(c) Lightning.(d) All of these
2. Electrical apparatus can be protected from overvoltage by:(a) Rubber gloves and rubber hoses.(b) Fuses.(c) A shield wire and arrestors.(d) OCRs and RVEs.
T / F 3. Power lines and electrical facilities provide a path of low resistance to the earth’s ground for lightning.
T / F 4. Another major concern caused by overvoltage is power outages and the time required to restore power.
T / F 5. Switching surges are the main cause of overvoltage in a power system.
6. Shield wires are generally installed on transmission and other crucial lines because:(a) These lines deliver the electricity to a large
number of customers.(b) These lines are constructed on fifty foot or longer
poles.(c) These lines take little time to repair.(d) All of these
T / F 7. The shield wire provides the lightning with a low resistance path to ground.
T / F 8. Shield wires are generally installed on transmission and other crucial lines because these lines are constructed on 50 foot or longer poles.
O V E R V O L T A G E P R O T E C T I O N 21
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
T / F 9. The purpose of shield wire is to protect the insulation capability on the insulators in an electrical system.
T / F 10. A shield wire used for communications is installed using a direct connection to the earth’s ground.
T / F 11. Lines constructed with wood poles and a shield wire use a ground wire as part of the path to ground for the high voltage caused by lightning.
T / F 12. A shield wire’s main function is to protect against high voltage caused by lightning, but it can also be used for communication purposes.
T / F 13. A steel structure is part of the path to ground for the high voltage caused by lightning.
T / F 14. A lightning arrestor’s function is to provide a method of relieving power lines from high voltage stress.
T / F 15. Under normal conditions, the arrestor provides an open circuit between the line and the ground.
T / F 16. An overvoltage condition causes the arrestor’s spark gap to flash over and dissipate the high voltage into the earth.
T / F 17. Once an overvoltage occurs on an arrestor, the system will always have to be refused or reset before the system can return to normal.
T / F 18. The two main types of arrestors are overhead and underground.
T / F 19. Arrestors do not have any different ratings, and can be connected to any line voltage.
20. Underground arrestors are required:(a) On a three-phase transformer in a loop feed.(b) On a radial fed padmount transformer.(c) On a take-off structure.(d) All of these
T / F 21. The two main classes of overhead arrestors are transmission and distribution.
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
22 S T U D E N T T R A I N I N G M A N U A L
T / F 22. The two main classes of overhead arrestors are station and distribution.
O V E R V O L T A G E P R O T E C T I O N 23
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
Review Question Solutions
1. All of these
2. A shield wire and arrestors.
3. T
4. T
5. F
6. These lines deliver the electricity to a large number of cus-tomers.
7. T
8. F
9. T
10. F
11. T
12. T
13. T
14. T
15. T
16. T
17. F
18. T
19. F
20. On a radial fed padmount transformer.
21. F
Copyright 2004 by the Training and Development Centre, SaskPower. All Rights Reserved
24 S T U D E N T T R A I N I N G M A N U A L
22. T
