The large-scale integration of power electronic-based systems poses new challenges to the stability and power quality of modern power grids. The wide timescale and frequency-coupling dynamics of electronic power converters tend to bring in harmonic instability in the form of resonances or abnormal harmonics in a wide frequency range. This paper provides a systematic analysis of harmonic stability in the future power-electronic-based power systems. The basic concept and phenomena of harmonic stability are elaborated first. It is pointed out that the harmonic stability is a breed of small-signal stability problems, featuring the waveform distortions at the frequencies above and below the fundamental frequency of the system. The linearized models of converters and system analysis methods are then discussed. It reveals that the linearized models of ac–dc converters can be generalized to the harmonic transfer function, which is mathematically derived from linear time-periodic system theory. Lastly, future challenges on the system modeling and analysis of harmonic stability in large-scale power electronic based power grids are summarized.

/pdf/harmonic-stability-in-power-electronic-based-power-systems-3hbhvwj6dc.pdf

Harmonic Stability in Power Electronic-Based Power Systems: Concept, Modeling, and Analysis

This paper reveals the existence of poorly damped resonant modes in the modular multilevel converter (MMC) that can result in harmonic instability. A small-signal model based on dynamic phasors is introduced which takes into consideration the dynamics of internal variables, such as circulating arm currents, which have a tendency for a large second harmonic component and module capacitor voltage ripple, which has other harmonic components. The small-signal model is validated by comparison with the electromagnetic transient simulation of a detailed model. It is shown that the use of an active circulating current suppressing scheme can effectively improve the damping of internal harmonic modes in comparison with a passive filter used for the same purpose. The damping of internal harmonic modes has been shown to be significantly impacted by the gains of the MMC's outer-loop controller but is less sensitive to the ac system strength or the gain of the phase-locked loop. A larger arm resistance also increases the damping of these internal modes.

Harmonic Instability in MMC-HVDC Converters Resulting From Internal Dynamics

A new HVDC-HVAC analytical model is presented in this paper. The model comprises three subsystems: AC system, PLL and DC system. The model is structured in such a manner to enable small signal analysis of HVDC-HVAC interactions and possible problems arising from these interactions, CIGRE HVDC Benchmark model is used as a test system. Model verification, performed using PSCAD/EMTDC simulations, showed good response matching for all DC and AC system variables. As an example of application of this model the influence PLL dynamics on the system stability is studied offering the important rules for tuning of PLL gains.

Analytical modelling of HVDC-HVAC systems

A frequency scanning method is introduced in the paper to obtain a more accurate frequency characteristic for identifying harmonic instability in HVDC power systems. An example of the application is used to identify the resonance frequencies in the CIGRE benchmark model. The paper shows that the benchmark model is not tuned to the resonance frequency that it was designed for. Using the scanning method, the resonance frequency of the benchmark model maybe shifted to demonstrate a simulation of core-saturation type instability. >

A frequency scanning method for the identification of harmonic instabilities in HVDC systems

Field experience is reported for a thyristor controlled series capacitor (TCSC) recently commissioned at BPA's Slatt substation. Subsynchronous resonance tests show that TCSC interactions with shaft dynamics of PGE's Boardman steam generator are well understood and are effectively avoided by normal TCSC valve firing logic. Modulation tests, performed with the Boardman plant offline, show that the TCSC can be a powerful and responsive actuator for swing damping. Security considerations did not permit lightly damped operation of the controlled plant. Close analysis indicates that the TCSC damping contribution, though small, was measurable. The best estimate is that damping for the McNary mode is 7.33% and 8.55%, for the TCSC damper loop open and closed respectively. TCSC testing and monitoring is facilitated by an advanced interactive measurement network representing BPA's approach to the information requirements of major control systems.

/pdf/modulation-and-ssr-tests-performed-on-the-bpa-500-kv-5d17sgha23.pdf

Modulation and SSR tests performed on the BPA 500 kV thyristor controlled series capacitor unit at Slatt substation

The Chateauguay HVDC back-to-back scheme with interconnections to the 765 kV transmission to the USA and to Beauharnois generators can exhibit, under certain operating conditions, second harmonic resonance problems. The author presents a thorough analysis of the problem using an eigenvalue and frequency domain approach. The analysis explains the mechanism of the excitation of the second harmonic instability by the presence of HVDC converters. The influence of changing the control parameters of the static VAr compensator at the Chateauguay terminal is also studied. Finally, an assessment is made of the effectiveness of present countermeasure schemes, namely the auxiliary DC stabilizing controls and the installation of second harmonic filters. >

A.E. Hammad

Papers

Analysis of second harmonic instability for the Chateauguay HVDC/SVC scheme