2040 zyxwvutsrqpo IEEE Transactions on Power Delivery, Vol. 10, No. zyx 4, October zy 1995 Firing Angle Modulation for Eliminating "ransformer Dc Currents in Coupled Ac-Dc Systems A.M. Gole (Member) R. Verdolin (Non-Member) E. Kuffel (Fellow) Department of Electrical and Computer Engineering University of Manitoba, Winnipeg, MB, Canada, R3T 2N2 zyxw Abstract - A method of eliminating dc components of the cur- rents in the transformer windings of a dc converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency com- ponent on the dc side may not remove this dc component. The impact of such control action at one converter on the other con- verters in the dc transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of non-characteristic harmonics on both the ac and dc sides. The study is performed using an electro- magnetic transients simulation program and theoretical calcula- zyxwvu tions. KevwordS; HVdc Transmission, Firing Angle Modulation, Ac- Dc Hybrid Systems, Harmonic Elimination, Electromagnetic Transients Simulation. I. INTRODUCTION It is a well known property of the dc conversion process that a current component of frequency f on the dc side appears on the ac side as a component of frequency fkfo ; zyxwvut fo being the fundamental ac frequency, taken here to be 60 Hz [ 1,2]. If the dc transmission line lies adjacent to an ac transmission line, there is a possibility for a 60 Hz current to be induced into the dc line. This current would then appear in the secondary winding of the converter transformer as a dc component (and also a 120 Hz component) superposed on the fundamental 60 Hz component. This dc current compo- nent eventually flows as transformer magnetizing current and offsets the knee of the flux-current characteristic and if excessive, causes unsymmetrical saturation of the converter transformer. This can lead to an increase in audible noise and result in possible loss of life expectancy of the trans- former[2]. Other reported problems are inaccurate control and protection measurements because of saturation of current transformers on the ac side. Because of the very low magne- tizing current requirement of HVDC transformers, even a few tens of amperes of dc current is considered by some as excessive. A typical value of 0.1% of the rated ac current for this dc offset is often cited as the limit [2]. There are several instances where a dc and ac line follow adjacent comdors and such induction effects could be detrimental. With the added difficulty of obtaining transmis- sion line rights of way there is considerable interest in having hybrid ac-dc transmission systems in which ac and dc con- ductors are placed on the same tower structure [3,4,5]. The coupling of the 60 Hz component would be even more severe in such cases. Several alternatives have been proposed for mitigat- ing the dc currents in the transformer windings. One pro- posed alternative is to use dc side bloclung filters for the 60 Hz induced currents. This scheme sometimes results in increased dc side overvoltages. Another alternative is to transpose the ac lines. This option does not completely elimi- nate the induced harmonics especially when ac and dc con- ductors are on the same tower as in hybrid ac-dc lines. Another possible alternative is to use modulation of the con- verter's firing angle. This method appears to have the advan- tage of not requiring extensive modifications to the power equipment as in the other choices. We explore this alternative in this paper. Some theoretical results are presented on an idealized system to understand the essential phenomena. Subsequent investigations are carried out on a realistic sys- tem using an electromagnetic transients simulation program (PSCADIEMTDC) [6]. The results indicate that it is possible to eliminate the dc currents in the transformer using such a scheme. However, there are some undesirable side-effects such as the generation of non-characteristic harmonics. 11. THE STUDY SYSTEM This study was motivated by the ac coupling observed in a study involving hybrid ac-dc transmission in which ac and dc lines are located on the same tower [7]. This ing 230 kV ac three phase transmission line spanning a dis- tance of 390 km to which is added a bipolar dc circuit rated at 95 WM 275-8 PWRD A paper recommended and approved by the IEEE Transmission and Distribution Committee of the IEEE Power Engineering Society for presentat- to February 2, 1995, New York, NY. Manuscript sub- mitted ~~l~ zyxwvu 26, 1994; made available for printing January 4, 1995. ion at the 995 IEEEIPES Winter Meeting, January 29 zyxwvu 9 system is shown in Fig. 1. The system consists of a pre-exist- k250 kV, 600 MW. The dc conductors are piaced on the same tower as the ac conductors after a suitable modification to the tower structure. 0885-8977195/$04.00 0 1995 IEEE