The transmission and distribution sector is facing the constant challenge of delivering reliable as well as affordable electrical power. Hence it is always looking for ways to improve on both. Standards-based certification of equipment has traditionally been viewed as the most trusted way to ensure the quality of components that make up the grid and contribute to reliability. But what form should certification take? And how can components best be assessed against the requirements of the relevant standards?
Laboratory-based testing has always been and remains the ‘gold standard’ for equipment certification. Exposing a piece of equipment to realistic conditions that mimic both normal operation and fault situations, yet in a controlled environment, provides the most complete picture of how it will behave in the field. At the same time, continuing advances in computer technology and modelling algorithms raise the question: will computer simulation eventually replace lab testing? CIGRÉ explored this very question in a past report by Working Group A3.20. The paper focused on the use of simulation tools to assess high voltage circuit breakers. In particular, it compared results from simulated and physical dielectric tests on an SF6 breaker with simplified geometry. Simulations were carried out independently by experts, using several commercial software packages. Simulation results were then compared against each other and also against results from physical tests carried out by KEMA Laboratories, acting as independent party. The study found that simulation tools proved to be excellent at predicting electric fields and resulting stresses inside a breaker chamber under static conditions. But they were significantly worse at predicting dynamic stresses. In other words, simulation tools can accurately determine stress when a component is functioning normally – right up to the point of a failure. However, they cannot reliably predict what will happen once that component has started to fail and therefore will struggle when it comes to determining withstand capabilities. In fact, withstand predictions in the CIGRÉ report varied by around 40% of the (simulated) average value and also deviated from measured performance by up to another 40%.
Clearly, simulation is a valuable development tool. But it cannot match the accuracy and reliability of physical type tests when assessing performance of high voltage components. In fact, CIGRÉ WG A3.20 concluded: “At present, it is not envisaged that simulation can replace type tests”. This highlights how difficult it is to predict how equipment will behave during operation, particularly under fault conditions. It also has consequences for alternative means to assess high voltage equipment that rely on calculation and explains why physical testing is still the most trusted means of determining quality of grid components. Such a conclusion is reinforced by another CIGRÉ study, this time looking at the short circuit behaviour of transformers. The study found that, among all large power transformers that failed due to short circuit, one-third had successfully passed a design review. But none had undergone a physical short circuit test.
The obvious conclusion is that physical testing can uncover issues that might otherwise remain hidden. Equipment certification based on laboratory testing is the best way to give all stakeholders more confidence in the components that make up a power grid. While testing is often seen as the high cost option, given the steep potential cost of major failures in the energy supply – including repairs, fines, liability claims and damage to reputation – it remains the most cost-effective solution to assess the quality and performance of grid components.
DNV GL KEMA Laboratories