tailieunhanh - Electrical Power Systems Quality, Second Edition phần 3

Hiệu suất điện áp cơ bản các giải pháp bảo hộ ở cấp người dùng Cuối Đánh giá kinh tế khác nhau Ride-Thông qua các giải pháp thay thế động cơ-Bắt đầu Thanh toán bù trừ các vấn đề sụt Tiện ích hệ thống Fault-Tài liệu tham khảo Chương 4: Nguồn quá áp thoáng qua của quá áp thoáng qua nguyên tắc | Voltage Sags and Interruptions 104 Chapter Three Phase A Voltage kV 30 20 10 0 -10 -20 -30 0 20 40 60 80 100 Time ms Phase A Current 40 20 0 -20 -40 -60 ------1-----1-----1-----1------ 0 20 40 60 80 100 Time ms Figure Typical current-limiting fuse operation showing brief sag followed by peak arc voltage when the fuse clears. the public is generally much less understanding about an interruption on a clear day. Ignoring third-harmonic currents The level of third-harmonic currents has been increasing due to the increase in the numbers of computers and other types of electronic loads on the system. The residual current sum of the three-phase currents on many feeders contains as much third harmonic as it does fundamental frequency. A common case is to find each of the phase currents to be moderately distorted with a THD of 7 to 8 percent consisting primarily of the third harmonic. The third-harmonic currents sum directly in the neutral so that the third harmonic is 20 to 25 percent of the phase current which is often as large or larger than the fundamental frequency current in the neutral see Chaps. 5 and 6 . Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any Use is subject to the Terms of Use as given at the website. Voltage Sags and Interruptions Voltage Sags and Interruptions 105 Because the third-harmonic current is predominantly zero-sequence it affects the ground-fault relaying. There have been incidents where there have been false trips and lockout due to excessive harmonic currents in the ground-relaying circuit. At least one of the events we have investigated has been correlated with capacitor switching where it is suspected that the third-harmonic current was amplified somewhat due to resonance. There may be many more events that we have not heard about and it is expected that the problem will only get worse in the future. The simplest .