Increasing the power transfer capability of an ac transmission line using a parallel small power dc link Transactions of the IEE Sri Lanka, vol 3, No 2, Apr 2001, p52-55 – J R Lucas, H J C Peiris 1 Increasing the power transfer capability of an ac transmission line using a parallel small power dc link J Rohan Lucas and H Jahan C Peiris Department of Electrical Engineering University of Moratuwa, Sri Lanka ABSTRACT The paper presents a method of increasing the power transfer level of an ac transmission line by using a parallel-small power dc link to improve the ac system small-signal stability. In order to demonstrate the validity of the proposed method, computer simulated dynamic response of the parallel ac-dc power system is compared with that of the ac system alone. In this study, the power flow in the small-power dc link is modulated by adding an auxiliary signal to the current reference of the rectifier firing angle controller to improve the damping of power flow oscillations in the ac transmission line. This modulation control signal is derived in response to the frequency oscillations in the rectifier end ac system (rate of change of generator angle oscillations). The simulations performed revealed that the use of a small-parallel dc link transferring about 2% of the normal ac power substantially increases the power transfer capability of the ac line. SYMBOLS αR = Alpha order at rectifier β = Damping Constant βR = Beta at rectifier δ 0 = Steady state Transmission angle ∆δ = Small change in Transmission angle ω 0 = Steady state angular frequency H = Inertia Constant Idc meas = Measured Direct Current Idc Idc ref = Reference value of Idc Idc δ = Idc modulation signal Ks = Synchronizing Coefficient ∆Pac = Small change in AC Power Transmitted ∆Pdc = Small change in DC Control Power ∆Pm = Small change in Mechanical Power V1, V2 = Bus bar voltages on AC line Vdc0 = Nominal DC voltage X = Transmission system reactance 1.0 INTRODUCTION The automatic voltage regulators present in the generating units in power systems could have an adverse effect on system small-signal stability. Hence, poorly damped oscillations of low frequency can occur which often persist for long periods of time and can sometimes limit the power transfer capability[1]. Power system stabilizers (PSS) have been widely utilized to improve damping of these oscillations, through modulation of the generator excitation [2,3]. To cause damping, PSS must provide an electrical torque on the rotor proportional to the speed variations. Additional damping is required under conditions of weak transmission and heavy load; for example, when attempting to transmit power over long transmission lines from remote generating plants or over relatively weak ties between systems. Contingencies such as line outages, often precipitate such conditions. Hence, systems which normally have adequate damping can often benefit from stabilizers during such abnormal conditions. The capability of an HVDC link to rapidly modulate the power flow, in response to control signals, has been utilized for some time to improve the dynamic stability of AC/DC systems[4,5]. Studies [6-9] have also shown that modulation signals incorporated to converter controls, that are derived in response to signals such as frequency deviation and rate of change of AC power transmitted provide additional damping. For example; (i) in the HVDC Intertie between Pacific North-West and South-West regions of the United States, a modulation signal derived in response to the rate of change of power in the parallel ac intertie is fed to the converter controls to improve the damping of low frequency oscillations between the two regions [4,9], (ii) in the Eel River back-to-back HVDC system which interconnects the Hydro Quebec system with the New Brunswick system in Canada, a modulation signal derived based on the frequencies of both ac systems is fed to the converter controls to improve the dynamic stability of the two systems [4] and (iii) in the CPA- UPA (CU) HVDC system between North Dakota and Minneapolis in United States, a modulation signal derived in response to rectifier end ac system frequency changes is fed to converter controls to provide damping for disturbances in the North Dakota ac system [4]. Nevertheless, due to the high cost of converters, HVDC transmission has been typically used for interconnecting asynchronous ac systems and for economic transmission of bulk power usually over long distances. However due to the availability of low cost- low power converters at low voltage, the hvdc schemes of low capacity at low voltage may now be used to improve the dynamic stability of existing AC systems. Moreover, such a DC line can be implemented using the same towers utilized for parallel AC transmission since the size/weight of the conductors are less due to the small power capacity of the DC link. Hence, such a damping improvement scheme would be economically feasible.