Harmonics regulation and mitigation in low-voltage distribution systems of large buildings

Abstract

Electronic equipment such as computers, fluorescent lightings, and solid-state motor drives continues to proliferate in modern buildings. Such equipment generates significant harmonic currents in the power distribution systems of the buildings, and may cause malfunction or failure of sensitive equipment. In high-rise commercial buildings, the harmonics situation is even worse due to their "high-rise" nature. The long feeders/rising mains as well as the distribution transformers cause significant harmonic voltage drops along these circuits, and distort the power supply especially on the upper floors in the buildings. Disputes on harmonics are consequently unavoidable due to the ambiguous responsibility among the utility, the landlord and the tenant. This thesis discussed the control of harmonics in the low-voltage distribution systems of high-rise commercial buildings from three different aspects, that is, (1) electrical wiring to minimize the harmonic pollution, (2) harmonic regulation in buildings to manage the harmonics effectively, and (3) harmonic filtering with the proposed voltage-detection-based shunt active power filter (SAPF) to mitigate the harmonics in the distribution systems. Electric cables are widely used for the power distribution in buildings. It is necessary to characterize the harmonic impedance of these cables in order to mitigate effectively the harmonic propagation within the building distribution systems. An experimental investigation was made to identify the harmonic impedance of typical power cables used as feeders and rising mains in the buildings. An impedance database was built for commonly-used single-core and multi-core armored cables with the cross-section areas of 95mm2, 120mm2, 150mm2, 185mm2, 240mm2, 300mm2, 400mm2 and 630mm2. The impact of the cable type, the arrangement formation and the setup environment on the harmonic impedance was revealed as well. Those results and conclusions are helpful in the design of wiring systems in large buildings so as to prevent harmonic pollution effectively. They are also useful in conducting harmonic assessment for harmonic management in the building distribution systems. To manage harmonics in commercial buildings effectively, harmonic regulation based on UK Engineering Recommendation G5/4 was recommended for the building distribution systems. The harmonic limits of both voltage and the current as well as the procedures of harmonic assessment in buildings were proposed according to other widely used standards such as the IEEE519-1992 standard and IEC standards on harmonics. The proposed harmonic-assessment scheme provides a method to assess the connection of the harmonic-producing equipment into the existing system. To solve the disputes on harmonics responsibilities among the parties in the building were recommended Depending on the types of the distribution systems, the guidelines of equipment selection and installation were provided for achieving a compatible environment in the buildings. The procedure was demonstrated in a typical large commercial building. In order to comply with the harmonic limits within commercial buildings, effective mitigation measures should be made available for landlords to control excessive harmonic pollution in the building distribution systems. In this thesis, a SAPF based on voltage detection was proposed to control harmonic voltage over the whole building distribution system. The control strategy based on a multi-channel and complex-gain control was developed. The multi-channel complex-gain controller was based on the harmonic synchronous reference frames, and provided an independent control on both the magnitude and the phase of the reference currents. This control method is helpful as well in improving stability margins. Apart from harmonic mitigation over the distribution systems, the proposed SAPF also has the capability of damping harmonic resonance. A three-dimensional space vector PWM technique was adopted in the proposed SAPF. Theoretical derivation of the proposed control strategy was conducted, and analytical models were deduced to analyze both the characteristics of the proposed SAPF and the performance of harmonic voltage control along the long distribution circuits. Computer models on the platform of MATLAB/SIMULINK were developed, and digital simulations were performed to validate the analytical results. The prototype of the SAPF using a DSP chip of TMS320F2812 was developed. The proposed control algorithms were successfully implemented. Finally the laboratory testing system including the line simulator was set up. Experiments were conducted to validate the proposed control method and filtering performance of the SAPF connected in a distribution system. The proposed approaches may be extended for harmonic control in utility distribution systems especially in a deregulated market, as these systems essentially are similar in terms of supply and demand relationship.