18-06-2014, 12:56 PM
LITERATURE SURVEY
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INTRODUCTION
Nowadays, number of high-voltage direct current (HVDC) systems is increasing rapidly throughout the world, due to their advantages in long distance and large capacity power transmission, asynchronous interconnections, and their ability to prevent inadvertent loop flows in an interconnected AC system. These definite technical and environmental advantages make HVDC transmission systems more attractive than high-voltage alternating current (HVAC) systems in many power system projects
1.3 CHAPTER REVIEW
In the following chapters, the principle behind the new transient harmonic current protection scheme has been proposed. The scheme is further explained with the help of the flowchart in the next chapter. With Matlab, the test system is modelled as a distributed parameter system. The comprehensive test studies show that the proposed scheme is simple, reliable, and practical. It can provide correct responses under various fault conditions including high ground resistance faults. Finally, the two main factors that affect performance of the protection, fault resistance, and fault location are also discussed. The relationships between the two factors and the sensitivity of transient harmonic current protection are also presented in this paper.
NOVEL TRANSIENT HARMONIC CURRENT PROTECTION PRINCIPLE
When converters at both terminals of the HVDC transmission system operate, power conversion harmonics with integral multiple frequencies of the fundamental frequency are generated in the system. For example, the output voltage of a 12-pulse converter mainly has 12th, 24th, and 36th harmonics. Harmonic voltages are generated during the conversion process in HVDC transmission systems. Superimposed on the direct voltage, they produce harmonic currents of the same orders flowing in the HVDC transmission systems. The main problem associated with harmonic currents is that they increases transmission loss, disturbs control and monitoring equipment, and cause electromagnetic pollution. To overcome these problems, shunt-connected DC filters and smoothing reactors generally are installed in HVDC systems. Usually, they are installed at both terminals of the DC transmission line with the purpose of filtering the characteristic harmonics effectively.
INTERNAL FAULT
With the internal fault illustrated in Figure 2.2, the transient harmonic current i_(kf )is generated from the fault point i_kcR while i_kcI and are generated by the two converters of the HVDC transmission system. Since there is a current loop involving the fault and DC filters, the harmonic currents will flow in the DC line and they can be captured by the transient harmonic current protection unit at the two terminals of the transmission line. The preceding analysis shows that the transient harmonic currents are different under various fault conditions. For the internal fault, the transient harmonic currents can be measured by the transient harmonic current protection unit; however, those harmonic currents will be blocked from the transient harmonic current protection unit under external faults. Therefore, internal faults and external faults can be discriminated by the transient harmonic currents.
TEST SYSTEM
The test system is modelled based on the CIGRE benchmark with MATLAB. The length of the 500kV HVDC transmission line is 2000km, and the frequency-dependent (Phase) transmission-line model is used. The sampling frequency is 4000 Hz at both terminals synchronously. The DC filter configuration and system parameters are presented in the Appendix A
CONCLUSIONS
A novel transient harmonic current protection scheme for the HVDC transmission line has been proposed. The relation between fault features and responses of the proposed protection has been investigated. Based on the test system, different operating conditions and various faults are tested. Comprehensive test studies show that the performance of the transient harmonic current protection is satisfied. It can identify the internal fault and external fault correctly and quickly and responds well to high ground resistance faults. Two main factors that affect the performance of the protection are also considered: fault resistance and fault location. The relationships between the two factors and the sensitivity of transient harmonic current protection have also been discussed.