Much expectation has been placed in smart grids for achieving higher efficiency of all major current and future equipment, for the most part by enabling better power management of energy utilities and consumers and secondly by development of more efficient grid components. The latter so-called ‘technical energy losses’ are due to energy dissipated in transmission and distribution lines, transformers, converters and inverters, and are either permanent or variable, i.e., varying with the amount of electricity distributed.
Characterisation methods for wasted energy and energy efficiency of converters (rectifiers and inverters) still need development. Evaluation of efficiency has two major aspects. First, total input power versus useful power, the difference being the loss is best measured at fundamental frequency only. Second, during product development, identifying exactly where losses occur, which necessitates accurate wide-band measurement of active power instead of relying only on simulations. This can be exemplified by the case of a high voltage DC (HVDC) substation. The AC grid will supply AC power at fundamental frequency. The HVDC converter inevitably creates harmonics in its operation. If not completely filtered, these harmonics will be injected back into the AC grid causing power loss in the grid. Measuring converter input AC power with a wide-band measuring system will then measure the power of the fundamental drawn from the grid minus the power of the harmonics re-injected into the grid, causing losses elsewhere and leading to an optimistic figure on efficiency.
Evaluation of converter efficiency requires accurate measurement of power in a wide frequency range in presence of highly distorted voltage and current waveforms. These characterisation methods must account for the specific final application and actual working conditions, including non-stationary situations. The complexity of the required measurements, combined with the lack of comprehensive standards or metrological traceability offered by the National Metrology Institutes (NMIs), result in declared efficiency values that are neither traceable nor obtained by standardised procedures for accuracy evaluation.
Storage systems have been the preferred approach for mitigating demand vs. supply with increasing shares of renewable energy in the distribution network. Electric vehicle fleets can potentially serve the electrical grid as an independent distributed energy source, by delivering the energy stored in their batteries according to the concept of vehicle to grid (V2G). In both cases, evaluating the efficiency of the storage system involves measurement of DC and AC power under highly dynamic and distorted conditions.
