Overvoltage Protection

Transcription

1 Overvoltage Protection S T U D E N T M A N U A L March 31, 2005

2 2 STUDENT TRAINING MANUAL 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

3 OVERVOLTAGE PROTECTION 3 STUDENT RESOURCES Learning Steps None 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.

4 4 STUDENT TRAINING MANUAL Lesson 1: Overvoltage Problems Learning Objective:Identify potential overvoltage situations. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction Lightning flashes represent a difference of electrical potential between clouds or between the clouds and the earth. Lightning possesses the same principle as electricity, and always flows through the path of least resistance. Power lines and electrical facilities provide a path of low resistance to the earth s ground for the lightning to take. These lightning strikes, which contact power lines and electrical facilities, 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 other causes of overvoltage. These overvoltages are difficult to protect against - customers with sensitive equipment may install their own surge or overvoltage protection. 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 the time required to restore power.

5 OVERVOLTAGE PROTECTION 5 Lesson 2: Function of a Shield Wire Learning Objective:Describe the function of a shield wire. Learning Method:Self-learning + On-the-job Evaluation 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 each structure. The shield wire is connected to ground which provides the lightning with a low resistance path to ground. The shield wire protects the insulators, which in turn protects the conductors, the structures and other apparatus from the high voltage caused by the lightning strike.

6 6 STUDENT TRAINING MANUAL When lightning strikes a line conductor, the only thing preventing the overvoltage from flowing to the earth s ground is the line insulation. On steel structures, the insulation from ground is the suspension or deadend insulators. On lines constructed with wood poles, the insulation from ground would be the insulators and the poles themselves. If there was no shield wire, the lightning overvoltage would either destroy the insulators or reduce their insulating capability. Shield wires are highly effective, and are generally installed on transmission 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.

7 OVERVOLTAGE PROTECTION 7 Lesson 3: Applications of a Shield Wire Learning Objective:Describe the two applications of a shield wire. Learning Method:Self-learning + On-the-job Evaluation 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 metal connection at the top of each structure. The structure is connected to a ground rod (located at the base of the structure) by a ground wire. The steel structure is also part of the grounding path. Wooden Poles The shield wire on wooden poles is connected to the top of each pole using a metal clamp. The ground wire extending from the metal clamp

8 8 STUDENT TRAINING MANUAL 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 for communication purposes. This shield wire is connected to the structure by an insulated mounting bracket. This bracket is grounded, and the design is such that the lightning high voltage will flash over the insulator and 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 of each structure with an insulator mounting bracket. The structure is connected to a ground rod (located at the base of the structure) by a ground wire. The steel structure is part of the grounding path. Wooden Poles

9 OVERVOLTAGE PROTECTION 9 On wood poles, the shield wire is connected to the top of each pole with an insulator mounting bracket. The ground wire extending from the insulator mounting bracket is connected to the ground rod at the base of the pole. STOP DANGER Extreme caution must be taken when working with this particular application, as a potential difference may be present in these shield wires and serious injury could result.

10 10 STUDENT TRAINING MANUAL Lesson 4: Function of a Lightning Arrestor Learning Objective:Describe the function of a lightning arrestor. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction The intent of a lightning arrestor is to relieve power lines of high voltage stress, and in doing so, protect electrical apparatus and equipment. With a properly installed arrestor, the overvoltage situation quickly subsides and the system returns to normal. An arrestor is installed in parallel by connecting the top to the incoming power line, and the bottom to the ground wire and system neutral, if available. Normal Conditions

11 OVERVOLTAGE PROTECTION 11 An arrestor has a spark gap that is designed to provide an open circuit between the line and ground for normal operating conditions. The normal voltage of a system should not flash over this gap. Overvoltage Conditions Overvoltage conditions may cause the spark gap in the arrestor to flash over and dissipate the high voltage into the earth using a ground wire and adjoining neutral connections. This spark gap makes an open circuit which allows the system to return to normal after the overvoltage condition has subsided.

12 12 STUDENT TRAINING MANUAL Lesson 5: Types and Applications of Lightning Arrestors Learning Objective:Describe the two types of lightning arrestors and their applications. Learning Method:Self-learning + On-the-job Evaluation 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 lightning arrestors is the same; however, they are designed differently to be used on a variety of underground and overhead applications.

13 OVERVOLTAGE PROTECTION 13 Underground Type

14 14 STUDENT TRAINING MANUAL Overhead Type Underground arrestors are used at the end of underground primary cables 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 for overvoltage protection, but also serves as an insulating cap for underground transformer bushings. There are two main classes of overhead arrestors used on power systems: station class distribution class

15 OVERVOLTAGE PROTECTION 15 Station Class

16 16 STUDENT TRAINING MANUAL Station class arrestors are generally used to protect equipment in substations and switching stations.

17 OVERVOLTAGE PROTECTION 17 Distribution Class

18 18 STUDENT TRAINING MANUAL Distribution class arrestors are generally used to protect distribution equipment 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 of arrestor required depends on the applied line voltage, as well as the flash over voltage rating required before the arrestor operates. This information is provided in the packaging of the arrestor.

19 OVERVOLTAGE PROTECTION 19 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 to achieve 100%.

20 20 STUDENT TRAINING MANUAL 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.

21 OVERVOLTAGE PROTECTION 21 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 T / F T / F T / F T / F T / F T / F 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. 13. A steel structure is part of the path to ground for the high voltage caused by lightning. 14. A lightning arrestor s function is to provide a method of relieving power lines from high voltage stress. 15. Under normal conditions, the arrestor provides an open circuit between the line and the ground. 16. An overvoltage condition causes the arrestor s spark gap to flash over and dissipate the high voltage into the earth. 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. 18. The two main types of arrestors are overhead and underground. 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.

22 22 STUDENT TRAINING MANUAL T / F 22. The two main classes of overhead arrestors are station and distribution.

23 OVERVOLTAGE PROTECTION 23 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 customers. 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

24 24 STUDENT TRAINING MANUAL 22. T