To address the system operability challenges due to the continuous reduction of system inertia by increasing penetration of renewable power generation, this paper proposes a new synthetic inertia control (SIC) strategy for doubly-fed induction generator (DFIG) based wind turbine generators (WTG). The proposed SIC scheme enables individual DFIG generators to contribute an effective inertial response which helps to stabilise the rate of change of frequency (RoCoF) and minimise the maximum frequency deviations in particular under severe disturbance conditions. To achieve the proposed SIC, Lithium-ion supercapacitors are required in the DC link of the DFIG power electronic conversion system to increase extra energy storage capability. The DFIG's DC voltage operates at various levels following the derived SIC algorithms for energy exchange with the connected AC grid, and the conventional control system for DFIG is modified to implement the algorithms. The functions and system benefits of SIC scheme is verified using a DFIG model connected to a power system and compared with the simulations of conventional DFIG control scheme. Practical issues such as size, cost, weight, connection configuration and charging/discharging rate of lithium-ion capacitors for the SIC are discussed and relevant recommendations are presented.

X-ray computed tomography and serial block-face SEM have provided detailed three-dimensional reconstructions of electrical trees for the first time. The application of finite element analysis (FEA) to the analysis of electrical fields in an epoxy block containing a tree is considered. Illustrations are provided by way of a number of case studies. It is shown that the limitations of FEA do not arise from the discrete nature of the meshing: rather uncertainties are more concerned with material properties in high fields on the micrometer scale, the limitations imposed by the pixel size of the imaging technique, and the discrete nature of the image reconstruction technique. For a dynamic model of tree growth space charge dynamics on the same physical scale need also to be modelled. A meshing strategy is used, calibrated against the charge simulation method, to ensure accurate but manageable computations in critical parts of a tree such as branch tips. Examples of field values are given using geometric constructs and low-field material characteristics as illustrative values. The field variation around a conducting tree structure, including the maximum field direction as a branch starts to bifurcate, is determined as an example. These yield values in excess of those expected if space charge movement was considered, but consistent with analytical calculations.

The surface potential gradient is a critical design parameter for planning overhead lines since it determines the level of corona loss, radio interference, and audible noise. The majority of existing surface gradient calculations are based on analytical methods which restrict their application in simulating complex surface geometry. This paper proposes a novel method which utilizes both analytical and numerical procedures to predict the surface gradient. With this method, stranding shape, protrusions, the proximity of tower, the type of tower, bundle spacing, and bundle arrangement can be taken into consideration when calculating surface potential gradients. A sensitivity study on the factors affecting surface gradient distribution is performed.

Emerging technologies associated with conductors and transmission towers require a re-evaluation of models of audible noise generated by overhead line conductors. Measurements on which existing commercial models were developed were typically performed on a single, classical design of conductor around 50 years ago. Within this paper, an ac testing facility employing a semianechoic chamber and a novel measurement procedure are proposed to give accurate determination of acoustic emission from overhead line conductors. The reproducibility of these measurements is significantly above traditional testing methods, but is shown to be heavily dependent on the chosen wetting technique. It is found that to achieve an accurate result, background acoustic noise control is essential especially for lower frequencies of power frequency harmonics. A direct comparison is reported for conductor samples with various physical characteristics. The analysis of results gives power utilities valuable information for selecting quieter conductors for overhead lines. The magnitude of high-frequency audible noise generated above 1 kHz is shown to be closely related to corona discharge activity, while the acoustic emission below 1 kHz is determined by the size distribution of droplets.

The relationship between the size of water droplets on the surface of ac high-voltage conductors, their vibration, and corona onset is investigated. Measurements have been carried out on individual droplets in high-voltage experiments, including high-speed photography, and discharge inception measurements. These have been correlated with acoustic emission from larger arrays of droplets on overhead transmission line conductors. A numerical model based on the finite element method has also been developed to simulate the vibration of the droplet enabling evaluation of the electric field and the characteristics of the droplet vibration. It is shown that the size of the water droplet is critical in its mechanical response to the field and that corona does not necessarily occur when the droplet is deformed into its most conical state as is the case under dc fields. For larger droplets the phase shift between the field and droplet vibration results in the maximum field occurring when the droplet is in a “flattened” profile rather than when it is “pointed.” The phase relationship between the droplet vibration and the electric field thus controls the onset of corona. It is argued that conductors which facilitate the development of small, uniform, stable droplets on their surface are the cause of abnormally high levels of low-frequency audible noise sometimes observed by transmission utilities. Moreover, the narrow resonant response of the droplet may lead to a difference in acoustic emissions experienced between power utilities with 50 and 60 Hz system frequencies.

Research work on audible noise emitted from overhead line conductors is reviewed, and a unique experimental set up employing a semi-anechoic chamber and corona cage is described. Acoustically, this facility isolates undesirable background noise and provides a free-field test space inside the anechoic chamber. Electrically, the corona cage simulates a 3 m section of 400 kV overhead line conductors by achieving the equivalent surface gradient. UV imaging, acoustic measurements and a partial discharge detection system are employed as instrumentation. The acoustic and electrical performance is demonstrated through a series of experiments. One conductor sample is used as an example to illustrate results under various spray conditions.