Solving Harmonic Resonance Problems on the Medium Voltage System

Harmonic problems are increasing on MV systems, especially with the application of power factor correction capacitors or extensive underground networks that resulting resonances close to the fifth harmonic. This paper describes two case studies of solving harmonic resonance problems on distribution systems with the design of harmonic filters that provide both power factor correction and harmonic control on the distribution system. The solution on the primary distribution system can be much more economic than trying to solve the problem within customer facilities. General guidelines for the application of harmonic filters on the primary distribution system are developed. INTRODUCTION Many distribution systems are experiencing increasing levels of harmonic generation and resulting harmonic voltage distortion. Harmonics are generated by all types of customers. Residential and commercial customers have more and more electronic load and industrial customers have a wide range of nonlinear loads that can be important. These can include ASDs, dc drives, process rectifiers, induction furnaces, welders and arc furnaces. Harmonic limits for small loads (e.g. IEC 61000-3-2 [1]) and guidelines for harmonic injection from customer facilities (e.g. IEC 61000-3-6 [2], IEEE 519 [3]) provide some control of harmonic generation. However, the level of harmonic generation can still be significant and it becomes especially important when the distribution network can exhibit resonance conditions that magnify the harmonic currents and create high voltage distortion levels. When these resonance conditions exist, it can be difficult to control the harmonics to acceptable levels using conventional methods of harmonic control in customer facilities. In fact, the resonance conditions on the primary distribution system can result in overloading of conventional harmonic filters and power factor correction within customer facilities. This paper describes two case studies of solving harmonic resonance problems on distribution systems with the design of harmonic filters that provide both power factor correction and harmonic control on the distribution system. The solution on the primary distribution system can be much more economic than trying to solve the problem within customer facilities. General guidelines for the application of harmonic filters on the primary distribution system are developed. CASE STUDY 1 – RESONANCE CAUSED BY SUBSTATION CAPACITOR BANKS The first case study involves a medium voltage (MV) distribution system that supplies a combination of industrial, commercial, and residential customers. A simplified one line diagram of the system is shown in Figure 1.