Control of both active and reactive power in voltage source converter (VSC)-based high-voltage direct current (HVDC) links could be very effective for system stability improvement. The challenge, however, is to properly allocate the overall control duty among the available control variables in order to minimise the total control effort and hence allow use of less expensive converters (actuators). Here relative gain array and residue analysis are used to identify the most appropriate control loops avoiding possible interactions. Optimal allocation of the secondary control duty between the two ends of the VSC HVDC link is demonstrated. Active and reactive power modulation at the rectifier end, in a certain proportion, turns out to be most effective. Two scenarios, with normal and heavy loading conditions, are considered to justify the generality of the conclusions. Subspace-based multi-input–multi-output system identification is used to estimate and validate linearised state-space models through pseudo random binary sequence probing. Linear analysis is substantiated with non-linear simulations in DIgSILENT PowerFactory with detailed representation of HVDC links.

Interconnection of High Voltage Direct Current (HVDC) links into networks is key to a future HVDC grid spanning large regions. Various factors including the evolution of cable technology will lead to a variety of DC voltages and a need to provide a DC/DC step up/down at high power ratings (beyond 500 MW). This paper analyses a High Voltage (HV) DC/DC converter utilising an intermediate AC link between two modular multi-level converters (MMC) based on the Alternate Arm Converter (AAC) topology. System operation is demonstrated using a detailed Simulink simulation. Power loss estimates derived from the simulation are used to illustrate the dependence of efficiency on design choices and operating point. Furthermore, the system is shown to block a DC-side fault and prevent its affect propagation through the system. (6 pages)

The damping of multi-modal oscillations through supplementary control of multiple HVDC systems is presented. The resulting controller produced is a fixed low-order decentralised controller, capable of providing adequate damping towards the low-frequency power oscillations. Linear analysis is substantiated with non-linear simulations in DIgSILENT PowerFactory with detailed representation of HVDC links. (5 pages)

The connection of Distributed Generators (DGs) to distribution network creates technical concerns for Distribution Network Operators (DNOs) which primarily include power flow management, loss increase and voltage management problems. Active Network Management (ANM) system provides monitoring and control of the distribution network as well as providing the infrastructure and technology for full integration of DGs to the distribution network. Optimal Power Flow (OPF) is a valuable tool in providing optimal control solutions for active network management system applications. In this paper, Terminal Voltage Regulator Mode (TVRM) and fixed Power Factor Regulation Mode (PFRM) were incorporated in the main optimisation routine to extract the maximum real power output capacity. The main contribution was to test different droop characteristics including one with local voltage regulator dead-bands associated to a particular DG. The DG operation methods of PFRM and TVRM are then compared together. Numerical result obtained from tests on two U.K. representative distribution networks show that TVRM has the potential in offering more capacity exploitation and improving network flexibility for additional DG connection. (6 pages)

Growth of distributed generation has led to distribution systems with a mixture of rotating machine generators and inverter interfaced generators. The stability of such networks needs to be studied through the analysis of state-space models, and so suitable models of inverters are needed to complement the well-established models of rotating machines. As machine models include features such as automatic voltage regulators and wash-out functions, the inverter model also includes phase-locking functions and internal control loops. The model for voltage source inverters with an internal current control loop, an outer power regulation loop, a measurement of average power and a phase-locked loop has been developed. The model is presented in detail and is formed with a state-vector, similar to that used for rotating machines. The model includes nonlinear terms but can be linearised about an operating point. The state-space model is verified against a component-level time-step simulation in Simulink/PLECS.

In this paper, a systematic way of developing a small-signal state-space model of the inverter-based microgrids is presented. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed 'modes') indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. Results from the model are verified against a prototype microgrid experimental setup.

The use of the micro-grid paradigm with extensive inverter interfaced generation raises the problem of severely restricted fault levels when operating in a power island. This paper presents a review of the conventional distribution network protection practices and then discusses their limitations when applied to inverter dominated micro-grids. The use of voltage measurement based fault detection is considered and is followed by consideration of how to apply this technique in conjunction with an adaptive form of protection. A potential solution for small micro-grids is presented in the form of voltage controlled overcurrent devices to enable the use of lower current threshold settings. Key issues for the design of network protection within micro-grids are summarised.

There have been many variants of the active power filter proposed and these variations cover both the circuit topology and the control system employed. Some of the control variants reflect different control objectives but there are still many variants within similar objectives. The available control techniques are described and contrasted in a structured way to identify their performance strengths. Objectives are classified by the supply current components to be corrected and by the response required to distorted grid voltage. The various signal transformations are described in terms of their impact on the distortion identification problem. Time-domain, frequency-domain, instantaneous power and impedance synthesis methods are examined. Additional control functions such as DC-bus voltage and current reference following are also discussed. It is found that a key difference between control methods is the way in which current distortion is treated in the presence of distorted grid voltage.

The fitting of an active power filter (APF) to mitigate the effects of a diode or thyristor bridge-rectifier is predicated on the assumption that the rating of the filter is reasonable (i.e. small) compared with the rating of the existing bridge rectifier or of a replacement active rectifier. The ratings of both shunt and series APFs are analysed in a variety of operating conditions. Ratings are assessed through peak voltage and mean current as appropriate for junction semiconductor devices. Rms ratings are also given because of their familiarity. The series APF, appropriate for the compensation of harmonic voltage sources, is of a generally higher rating than the shunt APF, appropriate for harmonic current sources. The use of a shunt APF where a series APF is appropriate results in poor device utilisation. The semiconductor ratings of an APF can be reduced by using hybrid schemes. Removing large-amplitude, low-order current harmonics with harmonic traps is effective for the shunt APF. Reducing peak voltages by attenuating high-frequency components is effective with the series APF. Arranging rectifiers into multipulse arrangements is seen to be effective for both shunt and series APFs. The ratings are approximately halved for the 12-pulse case.

The static VAr compensator (SVC) recently commissioned by the UK National Grid Company plc incorporates a three-phase chain-cell STATCOM. The installation demonstrates the advantages of the chain-cell converter in providing dynamically variable exchange of reactive power with a transmission system to facilitate voltage control. However, the chain-cell converter in this form is not able to exchange significant real power with the transmission system. Such ability would be of great benefit in operations ranging from damping transients through to load levelling.. This paper demonstrates that the chain-cell converter, when modified for the task of both real and reactive power exchange, has the potential to compensate for many transient and dynamic effects in a transmission system. The required cell ratings are discussed in terms of the usable stored energy of the converter and compared to other converters used for interfacing storage elements. A state-space model is also presented for incorporation into system studies.