01-06-2013, 03:52 PM
EHNANCEMENT OF VOLTAGE QUALITY IN ISOLATED POWER SYSTEMS
Enhancement of Power.pdf (Size: 868.25 KB / Downloads: 56)
INTRODUCTION
The typical definition for a harmonic is a sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency. Some references refer to “clean” or “pure” power as those without any harmonics. Harmonics have been around for a long time and will continue to do so[3to5]with effects like:
• Overheated transformers, especially delta windings where triplen harmonics generated on the load side of a delta-wye transformer will circulate in the primary side.
• Nuisance operation of protective devices, including false tripping of relays and failure of a UPS to transfer properly, especially if controls incorporate zero-crossing sensing circuits.
• Bearing failure from shaft currents through uninsulated bearings of electric motors.
• Blown-fuses on PF correction caps, due to high voltage and currents from resonance with line impedance.
• Mis-operation or failure of electronic equipment
• If there are voltage sub harmonics in the range of 1-30Hz, the effect on lighting is called flicker. This is especially true at 8.8Hz, where the human eye is most sensitive, and just 0.5% variation inthe voltage is noticeable with some types of lighting.
The switching-type power supplies found in most personal computers and peripheral equipment, such as printers, while they offer many benefits in size, weight and cost, the large increase of this type of equipment over the years is largely responsible for the increased attention to harmonics.
HARMONIC MITIGATION AND POWER FLOW IN ISOLATED
POWER SYSTEM
With regard to the problem in hand, it is assumed that the nonlinear converter and the sensitive loads are balanced. In what follows, symbols with the subscript “S”denote quantities which are associated with the upstream source, “L” for those associated with the sensitive load, “D” with the downstream main converter drive and “C” with the series compensator. Subscript “H” denotes the h^th harmonic component and “I” that of the fundamental. Voltage and current Phasor are denoted with a symbol on the top of the respective quantities. Their magnitudes (rms) are shown as capital letters while their peak values are denoted with on top. Vectors are denoted by bold letters. As shown in Fig. 1, the central part of the SC is the voltage source inverter (VSI) and the energy storage system (ESS). As PWM switching scheme is often used in the VSI, harmonics are generated and filtering is required. Lf and Cf are the filter inductance and capacitance. The VSI synthesizes the required voltage quantity which would be injected in series with VL. The ESS would act as a buffer and provides the energy needed for load ride through during a voltage-sag. Conversely, during a voltage-swell, excess energy from the network would be stored in the ESS so that VL can be controlled.
CONTROL OF HARMONIC DISTORTIONS
Distorted phase voltage V_S on the upstream source-side of the sensitive load can be expressed as shown in (1) at the bottom of the page for phases a, b, and c where ω_0 is the fundamental frequency, n is the harmonic order; V_on is the zero phase sequence voltage component; V_1n and φ_1nare the peak and phase of the positive phase sequence voltage component; V_2n and φ_2n are the peak and phase of the negative phase sequence voltage component. When expressed in this manner, V_S would be completely general and would include unbalances in the network. Clearly, distorted voltage is undesirable at the sensitive load terminals.
2 Power Flow Control through SC
Having described the harmonic mitigation principle and noting that harmonic power flow would exist in the SC circuit, detailed analysis will be carried out next. For the convenience of analysis and assuming negligible unbalances in the network, a single-phase equivalent system (phase “a”) is used to describe the three-phase system shown in Fig. 1. Let the fundamental frequency component of the sensitive load current I_L1, be taken as the reference Phasor.
CONCLUSIONS
Voltage quality improvement in an isolated power system through series compensation has been investigated. It is observed that the power system contains significant proportion of fluctuating nonlinear load and high level of harmonic distortions. A method to control the injection voltage of the series compensators (SC) so that it can mitigate the effects of the harmonics has been proposed. The SC is also designed to maintain the fundamental frequency component of the terminal voltage of protected sensitive load. In the process of harmonic voltage compensation, it is shown that power exchange exists between the SC and the external network. Based on the analysis of the harmonic real power flow in the power system, it is seen that the SC would import harmonic real power from the external system. A new SC control strategy is then proposed which involves the phase adjustment of the fundamental frequency component of the sensitive load terminal voltage. Through the analysis of the power exchange, it is shown that the load ride through capability during voltage sag can be improved with the support of the harmonic real power absorbed by the SC. The capacity of the SC required is modest and, therefore, makes it a viable device for such an application. Simulations have confirmed the effectiveness of the proposed method, as it is applied on the SC to achieve improved quality of supply in the power system.