COORDINATED POWER MODULATION OF PARALLEL MULTITERMINAL HVDC SYSTEMS EMBEDDED IN AC NETWORKS
Abstract
This research is concerned with power modulation techniques for multiterminal HVDC transmission networks, for the purpose of damping low frequency oscillations. A linear analysis of integrated AC/DC systems is made, feedback gains are obtained by linear quadratic design techniques, and comparisons are made between decentralized and centralized modulation schemes. An algorithm is proposed to implement control laws in a central supervisory control. This algorithm removes the coupling introduced by the required current balancing algorithm. A comparison of centralized modulation with a modified central supervisor and a previously developed central supervisor {6} shows a more damped system response with the modified central supervisor. Modulating currents are subjected to the converters' maximum and minimum current limits before balancing and this eliminates the sudden jumps in modulating currents which were observed in earlier studies. Simulation studies are also made for different converter locations and different DC network configurations. It is analytically shown that a mesh DC network would cause less mode shifting during modulation. The effects of AC system line reactance and converter capacitor size are also evaluated. The modulating currents used for decentralized modulation are limited because of the loss of current margin. A proposed dynamic current margin technique allows higher modulating currents and improves the system response. A centralized modulation scheme with less communication requirments is also evaluated. In this scheme, mode shifting is recognized by a comparison of the converter reference current and the actual converter current, and a corrective action is taken to reduce the mode shifting. A coordinated modulating of the DC power and the AC reactive power is also studied and the effect of reactive power modulation is evaluated by a linear anaysis of the AC/DC system and through the simulation studies. This scheme results in further improvement in damping the interarea oscillations. The problem of loss of communication for centralized modulation is also discussed, and it is shown that proposed corrective actions allow the modulation of the unaffected terminals to continue.
Degree
Ph.D.
Subject Area
Electrical engineering|Energy
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