Research Into The Influence Of Large Scale Pulsed Power System On The Power Quality

With the development of power supply and power grid, the structure and property of loads have also undergone profound changes. A great many new loads, the wide application of power electronic equipments in particular, have made power quality and the resultant stability of a power system a prominent question affecting the normal functioning of an electric power system. And this question is not only the basis of this dissertation's choice of research subject but also its research object.As an important component of a pulsed power system, Capacitor charging power supply (CCPS), through a series of power electronic conversion, ultimately stores energy of industrial frequency, in the form of high-frequency pulsating direct current, onto energy-storing capacitors through coupling to provide initial energy for the formation of pulsed power. As a load on a power grid, CCPS has the following characteristics: non-linearity and impact. The former makes the CCPS generate lots of harmonics and the latter causes the frequency and the voltage of the grid to fluctuate. And the effect exerted on voltage by the impact of reactive power is especially noticeable.In the second chapter of this dissertation, a circuit model and its corresponding mathematical model, which can reflect the working process and the power electronic properties of a CCPS, are established based on an in-depth analysis of a CCPS's working characteristics such as constant current charging, charging efficiency, charging voltage accuracy and so on. Therefore, the charging power supply is simplified into a three-phase rectifying circuit load that changes with time. The accuracy and efficiency of this model is tested in the ensuing quantitative analyses and experiments.In the third chapter, this equivalent model is adopted to calculate the current at the DC side of a single unit of CCPS and thereby infer the equation for the current at the AC side so that the Fourier coefficient for each harmonic is obtained. Quantitative analysis of a single unit of CCPS's harmonics within the 50th order is carried out to get the various parameters that signify the characteristics of harmonics. Results show that the total harmonic distortion rate at the AC side is closely linked to the harmonic-filtering circuit at the DC side and the size of the load:within a certain range, as the filter inductor enlarges, the harmonic distortion rate lessens but has a quality of saturation; as the filter inductor continues enlarging, its effect on the harmonic distortion rate will lessen to nil. It is more appropriate to adopt a 3 mH inductor in this apparatus. As the power increases, the harmonic distortion rate will decrease though the RMS of harmonic will enlarge constantly. When the charging power reaches its peak of 50 kW, the harmonic current distortion rate is about 30% while harmonic RMS current reaches about 20 A.Once harmonic current is created, it will spread on a power grid, which not only causes voltage distortion at the bus bar to which the harmonic source load is connected but may also cause voltage distortion at the upper or higher-voltage bus bars and thus affect all loads and equipments that have power electronic relations with the bus bar. To analyze this effect, the Matlab software is adopted in chapter four to calculate harmonic power flow, and the concept of harmonic transmission parameter is introduced to study the interrelations between harmonic transmission parameter and SCC short-circuit capability, line parameters, load and so on with a view to finding out the general laws governing harmonic transmission.Furthermore, the impact of active power and reactive power of a CCPS load must be taken into full account. According to analysis, when 400 CCPSs are working concurrently, the active power can rocket to 20MW or so within a minute while the reactive power can rise to 6Mvar in the same duration. When active power and reactive power have reached their respective peak value, charging is finished and they both plummet to zero, which has fairly great influence upon system frequency and in particular voltage. After taking advantage of relevant results above and considering the power factor in the presence of harmonics, the definition of power factor in the presence of harmonics is put forward, the CCPS power factor is calculated and a reactive power curve is obtained. Combined with the parameters of a power grid, the stability and range of fluctuation is analyzed and simulated. The outcome meets with that of the actual test.Aimed at the above analysis of effect and combined with present harmonic-harnessing and reactive power compensation technologies, a set of assembled apparatus (PPF+APF) is devised. It can not only filter harmonics but also compensate reactive power with fairly good effects.Meanwhile, an optimized circuit-designing scheme for a CCPS and a power distribution scheme are also put forward so as to positively lessen the effect of CCPSs on the power quality of power grid.