Advanced Electrical Engineering Technology

Author: Chief Editor: Yang Zhaoyi
Publisher:
Publishing Date: 2005-01-01
Features: This book is divided into seven chapters. It includes the protection, installation, wiring, and debugging of electrical equipment in substation stations, the equipment of secondary circuits in substation stations and their installation, wiring, and debugging, the normal operating state and abnormal phenomenon handling of substation equipment, the types and installation of high-voltage power cable terminal heads and intermediate joints, the installation and debugging of high-voltage motors and power driving equipment, the working principles and installation debugging of intermediate-frequency and high-frequency furnaces and steelmaking arc furnaces, the safe operation management and microcomputer monitoring of substation stations, as well as reasonable power consumption, power saving, and production efficiency improvement. This book can be used as a reference for electricians participating in national vocational skill appraisals and for graduates of electrician majors in vocational schools to prepare for examinations. It can also serve as a self-study reference for electricians.
The protection, installation, wiring, and debugging of substation electrical equipment, the equipment of secondary circuits in substation stations and their installation, wiring, and debugging, the normal operating state and abnormal phenomenon handling of substation equipment, the types and installation of high-voltage power cable terminal heads and intermediate joints, the installation and debugging of high-voltage motors and power driving equipment, the working principles and installation debugging of intermediate-frequency and high-frequency furnaces and steelmaking arc furnaces, the safe operation management and microcomputer monitoring of substation stations, as well as reasonable power consumption, power saving, and production efficiency improvement. This book can be used as a reference for electricians participating in national vocational skill appraisals and for graduates of electrician majors in vocational schools to prepare for examinations. It can also serve as a self-study reference for electricians.
Excerpt: Relay Protection Training Requirements: Understand short-circuit current calculation methods, be familiar with the principles and wiring of commonly used relay protections in substations, and learn installation and debugging methods.
Section 10kV Substation Equipment's Common Relay Protection Principle Circuit
I. Common Relay Protection Methods in 10kV Substations
1. Overcurrent Protection
When a short-circuit fault occurs in electrical equipment, a large short-circuit current is generated. Utilizing this characteristic, overcurrent protection and current speed protection are set up. The action current of overcurrent protection is set according to the maximum working current of the protected equipment (including lines). Considering that instantaneous current fluctuations may occur due to certain reasons, causing frequent tripping of circuit breakers, it is required that overcurrent protection have a time delay when it acts. To ensure selective operation of relay protection for electrical equipment at different levels, the setting of action time adopts a stepwise time delay stage difference, meaning that the action time of the upper-level protection at the power source side is longer than that of the lower-level protection. Overcurrent protection action time is divided into time-limited and inverse-time types. Time-limited refers to the fixed action time of the protection device, which is independent of the magnitude of the short-circuit current and is often achieved using time relays. Inverse-time refers to the action time of the protection device being inversely proportional to the magnitude of the short-circuit current; the larger the short-circuit current, the shorter the action time, hence the term "inverse-time overcurrent protection."
(1) Current speed protection is set according to the short-circuit current of the protected equipment. It does not rely on the time difference in setting between upper and lower-level protections to achieve selectivity and can quickly trip to cut off the fault. To prevent over-tripping, the action current must be selected to be greater than the maximum short-circuit current at the end of the protected equipment (line).
(2) Time-limited overcurrent protection
Current speed protection acts quickly but cannot protect the entire line. Overcurrent protection can protect the entire line but acts slowly, so time-limited overcurrent protection is used. The action time of time-limited current speed protection is larger than that of the instantaneous action speed protection of the next line by one time stage difference, generally taken as 0.5s, and its action current should be greater than the action current value of the instantaneous speed protection of the next line.
2. Under-voltage Protection
It is a relay protection that acts in response to a voltage drop and is commonly used in the following cases:
(1) Due to faults or other reasons, when the power supply voltage suddenly drops sharply or disappears for a moment, under-voltage protection is installed to trip for non-essential motors to ensure the self-starting of important loads.
(2) For motors that are not allowed to self-start and for those that, due to production process conditions and technical security requirements, cannot self-start after losing power, under-voltage protection should be installed to trip.
(3) For 3–10kV distribution lines, due to faults or other reasons, after instantaneous tripping, under-voltage protection is installed for general power loads to trip after the line loses power, in order to reduce the magnetizing surge current of transformers when automatic reclosing occurs, preventing excessive surge current from causing the second trip of line relay protection and failing the reclosing operation.
3. Other Protections
In addition to the above protections, there are gas protection for transformers, automatic reclosing devices for important loads or lines, and automatic backup power supply devices for some dual-power supply substations.
4. Selection of Relay Protection Methods for Electrical Equipment
The common relay protection methods for electrical equipment are shown in Table 1-1.