Dual Sequence Controller with Delayed Signal Cancellation in the Rotating Reference Frame

Manuscript submitted to the Twenty-second IEEE Workshop on Control and Modeling for Power Electronics (COMPEL 2021).<div>Abstract: Dual-sequence current controllers of voltage source converters (VSCs) feature two separate rotating reference frames (RRFs), commonly named dq frames, and rely on techniques that isolate the positive and negative sequences of three-phase measurements. One of these techniques is the delayed signal cancellation (DSC). It is performed in the stationary reference frame (SRF), also known as αβ frame. The DSC combines old values of one axis with new values of the other axis of the SRF. The results are, then, transformed into the RRFs for use in the current controller. This filtering process introduces an extra layer of complexity for dual-sequence current controllers, which could otherwise operate solely in the RRFs. This paper introduces a frequency adaptive DSC method that operates directly in the RRF. Moreover, an averaging of two of the proposed DSC filters with contiguous integer delays is employed for reducing discretization errors caused by grid frequency excursions. A formal proof of the equivalence between the αβ and dq DSC methods is presented. Furthermore, computer simulations of a case study support the interpretation of the results.</div>

Dual Sequence Controller with Delayed Signal Cancellation in the Rotating Reference Frame

Manuscript submitted to the Twenty-second IEEE Workshop on Control and Modeling for Power Electronics (COMPEL 2021).<div>Abstract: Dual-sequence current controllers of voltage source converters (VSCs) feature two separate rotating reference frames (RRFs), commonly named dq frames, and rely on techniques that isolate the positive and negative sequences of three-phase measurements. One of these techniques is the delayed signal cancellation (DSC). It is performed in the stationary reference frame (SRF), also known as αβ frame. The DSC combines old values of one axis with new values of the other axis of the SRF. The results are, then, transformed into the RRFs for use in the current controller. This filtering process introduces an extra layer of complexity for dual-sequence current controllers, which could otherwise operate solely in the RRFs. This paper introduces a frequency adaptive DSC method that operates directly in the RRF. Moreover, an averaging of two of the proposed DSC filters with contiguous integer delays is employed for reducing discretization errors caused by grid frequency excursions. A formal proof of the equivalence between the αβ and dq DSC methods is presented. Furthermore, computer simulations of a case study support the interpretation of the results.</div>

Understanding the Effects of Exponentially Decaying DC Currents on the Dual dq Control of Power Converters in Systems with High X/R

This paper was accepted for presentation at the IEEE CPE-POWERENG 2021 conference held in Florence, Italy, between the 14th and 16th of July 2021.<div><br></div><div>It investigates the root causes of detrimental oscillations in the dc link voltage of an energy storage system using a dual dq controller, operating at a high-voltage ac grid with high reactance-resistance ratio. Dual dq controllers are recommended in the literature for power converters operating under unbalanced, fault, or reduced voltage conditions. They employ two separated rotating reference frames, one for the positive and one for the negative sequence. The causes of the oscillations are investigated both theoretically and by time-domain computer simulations. As a result of the simulations, the performance of two dual controllers used in the industry is compared. In the presence of exponentially decaying dc currents, the filtering techniques employed by the controllers affect differently the performance of the proportional-integral regulators and disturb the feed forwarding and dq decoupling schemes. Ultimately, this results in undesirable oscillations in the dc-link voltage. This paper sheds light on how a fundamental phenomenon of three-phase ac systems can critically affect the control of power electronic converters. It provides a valuable insight into a possible root cause of oscillations in large electrical system applications with a considerable power converter penetration, such as large industrial plants striving for reducing greenhouse gas emissions.<br></div>

Understanding the Effects of Exponentially Decaying DC Currents on the Dual dq Control of Power Converters in Systems with High X/R

This paper was accepted for presentation at the IEEE CPE-POWERENG 2021 conference held in Florence, Italy, between the 14th and 16th of July 2021.<div><br></div><div>It investigates the root causes of detrimental oscillations in the dc link voltage of an energy storage system using a dual dq controller, operating at a high-voltage ac grid with high reactance-resistance ratio. Dual dq controllers are recommended in the literature for power converters operating under unbalanced, fault, or reduced voltage conditions. They employ two separated rotating reference frames, one for the positive and one for the negative sequence. The causes of the oscillations are investigated both theoretically and by time-domain computer simulations. As a result of the simulations, the performance of two dual controllers used in the industry is compared. In the presence of exponentially decaying dc currents, the filtering techniques employed by the controllers affect differently the performance of the proportional-integral regulators and disturb the feed forwarding and dq decoupling schemes. Ultimately, this results in undesirable oscillations in the dc-link voltage. This paper sheds light on how a fundamental phenomenon of three-phase ac systems can critically affect the control of power electronic converters. It provides a valuable insight into a possible root cause of oscillations in large electrical system applications with a considerable power converter penetration, such as large industrial plants striving for reducing greenhouse gas emissions.<br></div>

Measuring Electromagnetic Fields in Rotating Frames of Reference

We review the problem of transforming electromagnetic fields between inertial and rotating reference frames. We compare the method of straightforward tensor coordinate transformations adopted by Schiff in his well-known paper of 1939 with the method of Orthogonal Tetrads (OT) that was applied to this problem in 1964 by Irvine. Although both methods are mathematically rigorous, the transformed fields have different forms depending on the method adopted. We emphasize that the OT method is expected to predict the fields that would actually be measured by an observer in a rotating frame of reference. We briefly discuss existing experimental evidence that supports the OT approach, but point out that there appears to be little awareness in the physics community of this problem or its resolution. We use both methods to transform the electrostatic and magnetic fields generated by rotating charged spherical shells from an inertial into a co-rotating system. We also briefly describe how such an arrangement of shells could be used to measure rotation relative to the fixed stars.

Can an infinitely long object fit in an expanding universe?

Does space stretch its contents as the universe expands? Usually, we say the answer is no—the stretching of space is not like the stretching of a rubber sheet that might drag things with it. In this paper, we explore a potential counterexample—namely, we show that it is impossible to make an arbitrarily long object in an expanding universe, because it is impossible to hold the distant end of the object ‘stationary’ with respect to us (as defined in the Friedmann–Lemaître–Robertson–Walker metric). We show that this does not mean that expanding space has a force associated with it, rather, some fictitious forces arise due to our choice of reference frame. By choosing our usual time slice (where all comoving observers agree on the age of the universe), we choose a global frame that does not correspond to the frame of any inertial observer. As a result, simple relativistic velocity transforms generate an apparent acceleration, even where no force exists. This effect is similar to the fictitious forces that arise in describing objects in rotating reference frames, as in the case of the Coriolis effect.