| The power industry is the country’s public utilities, supporting the national economyand social development as basic industries, with the most widely used energy resources,the development and usage level of electric energy is the symbol of national developmentlevel and comprehensive national strength. Various nonlinear devices is being used inpower system to supply efficient and energy saving methods for electric energy utilizationin a period of rapid economic development, howere, they cause the pollution in the powersystem at the same time, so the voltage distortion, voltage unbalance, voltage flicker,voltage fluctuations and voltage frequency deviation appeared along with many powerquality problems.First, the article describes the technical standards of power quality and the basicprinciples for power quality compensation technology. Active power filters (APF), staticsynchronous compensators (STATCOM) are widely used to compensate the harmonic andreactive, dynamic voltage restorers (DVR) are used to prevent voltage fluctuations in thefield of power quality. Now the grid-connected power converter played an important rolewith the enhanced power efficiency and improving the electrical environment. As we allknow, measurement and control is the core for grid-connected power converter, so thesystem must synchronize the grid phase and track grid frequency before the system will becontrolled. This paper mainly discusses the problem about phase locked-loop (PLL), thedetection of the fundamental-frequency positive-sequence component of the utility voltageunder unbalanced and distorted grid.Second, the mathematical model and the transfer function of PLLs is established, sothe time-domain and frequency-domain tracking performance has been analyzed,phase-locked loops include the phase detector (PD), loop filter (LF)and voltage-controlledoscillator (VCO).RC integrator filter, passive proportional integral filter and activeproportional integral filter is introduced for LF. Synchronous rotation using software iseasy to achieve based on the development of digital signal processing, because thedetection of conventional synchronous reference frame phase-locked loops (SRF-PLL)can’t be eliminated under unbalanced conditions, a new decoupled double synchronousreference frame phase-locked loops (DDSRF-PLL) which a decoupling network is developed in order to extract and separate the positive-and negative-sequence componentscleanly has be applied.Finally, this paper analyzes that the imbalance, harmonics and DC offset of gridgenerated the phase-locked error using the mathematical method, the frequency-biasedcharacteristic of synchronous rotation transformation is disscussed next, if the frequencyof fundamental-frequency positive-sequence component in the utility voltage is changed,the low-frequency phase error is generated and harmonic point offset, so frequencyadaptability is a key issue about PLL. In this paper, pre-filtering techniques and loop-filtering techniques are using in software phase-locked loop (SPLL) for the pollution inpower grid. Pre-filtering techniques include the open-loop synchronization algorithm baseon FIR filter and a harmonic decoupling network consisting of multiple second-ordergeneralized integrators (MSOGIs) and frequency-locked loop (FLL) which makes thesystem frequency adaptive. This paper presents loop-filtering techniques for SRF-PLLbased on notch, lead compensation and moving average filter (MAF). Two methods areverified by simulation, this paper analyzes the steady-state error with the fundamentalfrequency changing in order to study the frequency adaptivity. Phase-locked systems builtusing a digital signal processor (TMS320F2812), program uses SRF-SPLL, DDSRF-SPLLand SRF-MAF-SPLL, the results of experiment are provided in order to validate thetheoretical approaches. |
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