Monitoring the Energy

Raymond Boyle, Principal Consultant, Wind Energy, NEL


Energy meter measurement accuracy

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National Wind Energy Centre

NEL is a world-class provider of technical consultancy, research, testing, flow measurement and programme management services to the energy, oil & gas manufacturing industries and government. The number of small and micro wind turbines is increasing steadily throughout Europe as they contribute towards various governments' renewable energy targets. These wind energy systems are also proving popular as they are a useful source of electricity that can be generated close to point of use from a sustainable resource. EU figures show that today wind power supplies more than five per cent of its overall electricity demand. The European Wind Energy Association (EWEA) believes that by 2020, wind energy will meet 15.7 per cent of EU electricity demand, growing to provide half of Europe's power by 2050. Currently, as part of the EU's drive to increase the share of renewable energy in the overall energy mix to 20 per cent, feed-in tariffs (FIT) are the most common wind power support scheme used by the various member states. This is where utility companies source some of their power via electricity generated from renewable sources produced by individuals and companies. The FIT is the price they pay the original producer for this cottage industry' electricity. A policy that the EU designed to accelerate both individuals' and corporate investment in renewable energy technologies, FIT offers long-term contracts to renewable energy producers. In countries across the EU, such government incentives have stimulated even greater interest in the deployment of renewable energy systems and small-scale wind turbines in particular. This is because a wind turbine delivering a high energy yield on a carefully selected site with a good wind regime can attract a relatively large FITs payment, resulting in a fairly short payback period.

Inaccurate measurement However, as FIT payments are based on an agreed price per unit of energy (p/kWh), it is important that the energy exported by the wind turbine system to the grid is measured accurately. This is generally achieved using a small, commercially-available energy meter. However, there are concerns within the industry that if the meter does not accurately record the energy production then the FIT payment may of course be lower or higher than it ought to be. However, the energy meters that are usually used to record wind turbine energy output are more commonly applied in circumstances where operating conditions are more steady-state in nature. This complicates matters further as the operating characteristics of both small and micro wind turbines can never be described as steady-state'. This therefore poses significant challenges in the reliable measurement of the electrical power which they deliver, usually done through a solid state inverter system. This means that in real-world use data sampling rates may therefore have a significant effect on the accuracy of the measurement in situations where such meters are being used in conjunction with a highly transient device such as a small wind turbine.

While recommendations exist for grid connection arrangements for small wind turbines, these power and energy reliability measurement issues under transient operating conditions have yet to be fully assessed. Therefore, in order to develop a better understanding of the behaviour of these energy meters, a short measurement study was undertaken by NEL, supported by the UK's National Measurement System. During these tests a small sample of energy meters were tested under laboratory and field conditions, with their performance compared against reference measurement instruments. These tests used a common hardware platform based on a specific model of a small wind turbine and the model selected for the study had a significant transient behaviour, and was therefore considered well suited to the meter assessment exercise. Laboratory-based testing involved the use of the electrical generator in a steady-state test rig and also a complete wind turbine system installed in a small wind tunnel facility. The generator system was controlled by the manufacturer's inverter and energy meters were installed at the output stage of the inverter to record energy exported to the grid. For field testing, the energy meters were connected at the grid interface of a wind turbine system at a test site. In all the tests a calibrated power analyser was also connected to the system to provide reference measurements against which the energy meter's readings could be compared. In steady-state operation, the totalised energy readings from the meters agreed quite well with the corresponding measurements from the reference instrument. However, it was observed that not all of the meters displayed the unidirectional behaviour expected, as when the generator system was in standby mode, the energy meters continued to advance in some cases. This indicated that the small amount of current drawn to power the inverter and control system (i.e. the parasitic load) contributed to the recorded energy readings. While this is an interesting observation, in reality the practical impact is insignificant since this represents a very small proportion of the total energy exported from a wind turbine system. The performance of the energy meters assessed using the wind tunnel test facility generally displayed similar behaviour to the results from steady-state testing when conditions in the wind tunnel were fairly stable. However, when the wind turbine was allowed to yaw more readily, the difference between meter and reference instrument readings tended to be greater. However, a much larger variance between meter and reference instrument readings was observed in the field testing at NEL's wind turbine test site under real wind conditions. In one case, where data from the reference instrument was analysed based on the assumption that the meter continued to advance irrespective of direction of current flow, total energy values recorded agreed to around 0.5 per cent. However, in all other cases much larger discrepancies were observed. While this study was based on only a small sample of meters assessed over a short period of time, it did highlight some important issues with their performance. To obtain a more definitive assessment of these types of meter a larger sample would therefore have to be tested over a longer period of operation and preferably in conjunction with a wind turbine system having a greater electrical generating capacity. With the EU pushing for wind to become a major player in its drive for renewable energy, the future is without doubt sustainable. However, the harsh commercial reality is that both the utility companies, and the individuals they source small wind turbine generated power from, will increasingly demand accurate measurement and equitable payment. Therefore, to ensure its financial success more studies need to be completed to increase metering accuracy and effectively support the surge in renewable power generation.