Electric Power Systems Research 77 (2007) 1329–1336 Transient of modified HVDC converters Hussein. D. Al-Majali a,∗ , Sulaiman. M. Al-Dhalaan b a Department of Electrical Eng., Faculty of Eng. Mu’tah University, Al-Karak 61710, Jordan b The General Organization for Tech. Educ & Vocational Traning, Riyadh, Saudi Arabia Received 3 April 2005; received in revised form 28 September 2006; accepted 10 October 2006 Available online 13 November 2006 Abstract The bridge circuit used in high voltage direct current (HVDC) converters may be modified by including by-pass thyristor valves connected to tapping points on the secondary windings of the transformer to give a reduction in both harmonic generation and reactive volt–ampere absorption for the rectifier and inverter modes of operation. In this paper, control strategies for the transient response of the HVDC converter with either thyristor or GTO thyristor by-pass valves are investigated using several types of firing angle control logic. Fast and continuous control of the DC voltage is possible using the proposed scheme without the conventional on load tap-changer on the converter transformer. Experimental results on a laboratory model compare well with the predicted values. © 2006 Elsevier B.V. All rights reserved. Keywords: HVDC converter; Transient response; Control strategies 1. Introduction The converter bridge controllers used for high voltage direct current transmission (HVDC) has normally a basic control sys- tem that controls the direct current in the rectifier and the extinction angle in the inverter. Appropriate control character- istics, invariably constant current and constant extinction angle features are achieved by fast-acting control of the valve firing angles [1]. Larger slow changes of AC voltage are handled by changing the transformer tap setting [2]. The control of HVDC has been discussed in a number of works [3–5]. By controlling the HVDC system, some of the most important requirements should be achieved, these are: (1) symmetrical firing of the valve to reduce the abnormal harmonics generated by the converter, (2) keeping the power factor as high as possible, therefore minimizing the reactive volt–ampere consumption in the converter, (3) controlling the DC line current to transmit power with sufficient speed and accuracy of response, (4) protecting inverter operation from commutation failures, (5) providing continuous operating range ∗ Corresponding author. E-mail address: halmajali@mutah.edu.jo (Hussein.D. Al-Majali). from full rectification to full inversion, (6) allowing smooth transition from current control to extinction angle control. Previous papers [6,7] have considered a modified bridge cir- cuit for use in HVDC converter. By including a ‘by-pass valve’ using either additional thyristors or GTO thyristors. The reactive volt–ampere (VAR) absorption is reduced by about 35% when valves with a turn-off facility are used and by 20% when conven- tional thyristor valves are used, compared with a conventional converter. Similar results were expected in the inverter mode of operation. At a typical operating angle, α = 15 ◦ , there is some reduction of the harmonics content of the supply current 12% for fifth harmonic and 7% for seventh harmonic. An additional fea- ture of the scheme is that the transformer tap-changer could have a reduced range with fewer taps or even be eliminated. In partic- ular, it is advantageous to have a by-pass device with a turn-off facility. The previous presented in refs. [8,9] have considered only the steady-state performance of the modified bridge circuit and theoretical values were validated by experimental results using a laboratory model. In this paper, a laboratory model is developed to handle sudden changes (transient response) of system voltage. Sev- eral types of firing angle control logic are investigated and experimental results confirm the theoretical prediction. Tran- sient response characteristics for both conventional thyristor and GTO thyristor by-pass valves are presented. 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.10.002