Yousef Pipelzadeh

Wide-area power oscillation damping control through HVDC: A case study on Australian equivalent system

A case study on the Australian equivalent system - recently adopted as an IEEE benchmark for stability studies - is presented here to illustrate power oscillation damping through HVDC links. Wide-area signals are used for supplementary modulation of the HVDC power order driven through a multivariable controller. The test case is set up in DIgSILENT and benchmarked against the standard results. Linearized model around the nominal condition is obtained by using subspace identification on a set of measurements derived out of probing the system through the HVDC link. Modal analysis is carried out to identify a set of appropriate feedback signals for the controller. Pole-placement technique is used to design a multi-input, single-output (MISO) controller in two different frameworks - i) observer driven state feedback and ii) optimized lead-lag compensators. The effectiveness of HVDC modulation for power oscillation damping is validated through eigen-value analysis and non-linear simulation (in DIgSILENT) over a range of operating conditions.

Control Coordination Within a VSC HVDC Link for Power Oscillation Damping: A Robust Decentralized Approach Using Homotopy

Power oscillations can be damped effectively through modulation of both active and reactive power of a voltage source converter based high voltage direct current link. The challenge, however, is how to coordinate the control action properly at the two ends of the link without using a centralized control scheme, which requires fast communication of control signals to remote actuator (converters) sites. A full centralized controller may result in a closed-loop performance worse than that of an open loop in case of a communication loss of feedback signal(s). Alternatively, with a block-diagonal control structure, the individual control loops are decoupled from each other, which is not only easier to implement in a decentralized way, but also shown to guarantee a certain level of performance. Here, the concept of homotopy is applied to obtain a single block-diagonal controller from a set of full controllers, individually designed to ensure specified closed-loop performance for a set of operating conditions. Simulation studies in DIgSILENT PowerFactory are carried out on two test systems to demonstrate both the robustness and control coordination in a decentralized framework.

Blending HVDC-Link Energy Storage and Offshore Wind Turbine Inertia for Fast Frequency Response

This paper explores the benefits of combining the dc-link energy storage of a voltage source converter-based high-voltage dc (VSC-HVDC) link and the kinetic energy storage from wind turbines to facilitate in fast primary frequency control and system inertia to an ac network. Alongside physical and analytical justifications, a method is proposed which blends the energy stored in the HVDC link with the power control capabilities of the wind turbines to provide frequency response that is fast while not requiring excessive volume of capacitance nor demanding performance requirements on the wind turbines.

Inertial response from remote offshore wind farms connected through VSC-HVDC links: A Communication-less scheme

A communication-less scheme that allows remote offshore wind farms connected through HVDC links to participate in primary frequency control and contribute to system inertia is discussed in this paper. As a HVDC link decouples the offshore system from the onshore side, real-time communication of onshore frequency is normally required for the primary frequency control loop of the wind farms. Dependance on remote communication which could be unreliable at times is a problem. To obviate the need for communication, appropriate droop control on the offshore and onshore converters is used in this paper to translate the variation in onshore frequency to an equivalent variation on the offshore end. Thus the need for communicating onshore frequency to the offshore side is mitigated which ensures reliable operation. Such a communication-less scheme is compared against the conventional approach involving remote communication of onshore grid frequency to the wind farm site. Along side physical and analytical justification, a case study is presented to demonstrate that the communication-less scheme performs similar to the conventional one in terms of reducing grid frequency variations.

Influence of frequency-droop supplementary control on disturbance propagation through VSC HVDC links

The construction of the European Super Grid is a valuable solution to deal with the growth of electricity demand and the increasing penetration of renewable energies. Key feature is the introduction of large capacity High Voltage Direct Current (HVDC) links between countries bordering the North Sea. This paper investigates the level of coupling of two AC grids connected via an HVDC link. Two reduced dynamic equivalent transmission systems resembling Great Britain and Scandinavia are developed in DIgSILENT PowerFactory. A balanced monopolar HVDC link equipped with two-level Voltage Source Converter (VSC) is used to connect the two grids. A frequency droop control that modulates the power transfer along the DC link has been designed for the converters to provide frequency support between the grids. A variety of disturbances were applied to the AC grids to test the extent to which faults in one grid propagate to the other. It was found that the presence of frequency droop control increases significantly the degree of coupling between the two grids.

Coordinated damping control through multiple HVDC systems: A decentralized approach

The role of multiple HVDC links — both CSC and VSC — in improving the AC system dynamic performance is presented here. An Australian equivalent system with three critical inter-area modes is considered through case studies performed in DIgSILENT PowerFactory. Subspace-based multi-input-multi-output (MIMO) system identification is used to estimate and validate linearized state-space models through injecting pseudo random binary sequence (PRBS) probing signals at the HVDC controller inputs. Choice of appropriate input-output combinations is done through modal residue analysis taking note of not only the magnitudes but also the phase angles. The decentralized control design is posed as an optimization problem and solved using an evolutionary technique. The results are validated through linear analysis and non linear simulation in DIgSILENT for two extreme (light and heavy loading) operating conditions.

Role of western HVDC link in stability of future Great Britain (GB) transmission system

Supplementary control of active power through the HVDC converter station to improve the damping of power oscillations in the GB network is demonstrated. Measurement-based system identification approach is applied to accurately estimate linear state-space models of the GB system. The linearized models are used to design controllers to improve the dynamic stability of the system using both local and remote feedback signals. The impact of possible latency in communicating the remote feedback signals is also investigated.

Corrective Control With Transient Assistive Measures: Value Assessment for Great Britain Transmission System

In this paper, the efficacy and value of using corrective control supported by transient assistive measures (TAM) is quantified in terms of the cost savings due to less constrained operation of the system. The example TAM is a rapid modulation of the power order of the high-voltage direct current (HVDC) links in the system so as to improve transient stability during corrective control. A sequential approach is used for the offline value assessment: a security constrained economic dispatch (SCED) module (master problem) determines the optimal generation dispatch, HVDC settings, and the corrective control actions to be used post-fault (generation and demand curtailed) so as to minimize the operational costs while ensuring static security. The transient stability module (slave problem) assesses the dynamic stability for the operating condition set by the SCED and, if needed, applies appropriate TAM to maintain the system transiently stable. If this is not possible, the master module uses a tighter set of security constraints to update the dispatch and other settings until the system can be stabilized. A case-study on the Great Britain system is used to demonstrate that corrective control actions supported by TAM facilitate significantly higher pre-fault power transfers whilst maintaining N-2 security.

Coordinated Control of Offshore Wind Farm and Onshore HVDC Converter for Effective Power Oscillation Damping

Damping contribution from wind farms (WFs) is likely to become a mandatory requirement as a part of the grid codes. For remote offshore WFs, connected through a voltage source converter (VSC)-based direct current link, the most convenient option for the onshore transmission system operator (TSO) is to modulate the reactive power at the onshore VSC within their own jurisdiction. In this paper, we show that supplementary control through the onshore VSC alone, although attractive for TSOs, could result in undesirable voltage variations in the onshore grid. On the other hand, modulation of active power output of the wind turbine generators (WTG) alone turns out to be inadequate due to the limited overload capability of the WTGs. Coordinated control over both onshore VSC and aggregated WF output overcomes the above limitations and is shown to be effective for power oscillation damping. A homotopy approach is used to design the coordinated controller, which can be implemented locally (at offshore WF and onshore converter site) using a decentralized architecture. This is a bilinear matrix inequality problem, which is solved by transforming these constraints into linear matrix inequality constraints. Case studies on two test systems show that the proposed controller yields similar system dynamic response as supplementary control through the WF alone.

Reduced dynamic model of a modular multilevel converter in PowerFactory

Modular Multi-level Converters (MMC) have emerged as the preferred technology for High Voltage DC transmission installations. The inclusion of these converters, characterized by complex control schemes, in large AC grids may cause AC/DC interactions that need to be fully investigated. The evaluation to what extent the MMC dynamics interact with the dynamics of a transmission network is of primary importance. It becomes critical for the grid operators, which usually rely on more traditional VSC topologies (two-level), to use such models when studying AC/DC interactions. This paper presents the development of a MMC reduced dynamic model (RDM) in PowerFactory that will facilitate the analysis of large AC systems incorporating MMC based VSC HVDC links. The MMC control scheme is designed following an alternative strategy which considers the energy balancing and the storage capability of the converter. The system is arranged as a point-to-point link, its operations are validated against a detailed equivalent circuit based model in PSCAD/EMTDC. A close match between the original system and the benchmark confirms the validity of the MMC RDM proposed.