Today, the electrical system reactions of installations are calculated according to DACHCZ before they are connected to the distribution grid. In terms of measurement, compliance with the limit values is hardly ever checked, or only in a simplified manner. In a pilot and demonstration program (P+D) of the Swiss Federal Office of Energy (SFOE), the optimal method for continuous monitoring of the contribution of individual plants to voltage distortion has now been determined.

The growth of distributed generation and new device technologies (e.g., electric vehicles) is driving profound changes in power grids. In addition, future grids are characterized by lower and more volatile short-circuit power, making it more likely that degradation of power and voltage quality will have a significantly greater impact on efficient and stable operation than in the past. However, many studies and field trials are limited to efficiency and stability at grid frequency and do not consider the impact of grid feedback, or consider it inadequately.

Almost all installations for the generation, consumption or storage of electrical energy cause grid perturbations. Devices with power electronics, for example, lead to harmonics, and single- or two-phase connected devices lead to asymmetries. This reduces power and voltage quality and can interfere with the operation of other devices and equipment. Therefore, it is important to reliably quantify the grid compatibility of an installation to ensure reliable and efficient grid operation and to comply with electromagnetic compatibility requirements.

Today, the feedback effect of installations on the grid voltage is calculated before they are connected to the grid based on empirical values and guidelines. Metrological proof of compliance with the limit values is either not provided at all or only by means of procedures based on simplifying assumptions. This can lead to unexpected disturbances occurring during the operation of a plant, which affect the operation of the grid. On the other hand, possibly expensive remedial measures (e.g. filters) are already demanded during the planning stage, which would actually not have been necessary during operation of the plant.

By applying new methods and indices, the iREF Grid project determined a method for continuously monitoring the contribution of a single plant to voltage distortion. Through comprehensive and systematic measurements in cooperation with nine Swiss grid operators, different methods were evaluated and the optimal method was identified. As a result, interactions (e.g. compensation effects) can be determined more precisely in order to reliably identify plants with unacceptably high grid perturbations or to avoid investments in unnecessary remedial measures. This will make a lasting contribution to more efficient use of the grid infrastructure, more effective integration of new technologies and more reliable operation of future grids. The results also make an important contribution to the improvement of guidelines and standards.

Project goals and questions

Interference emission limits for a plant are calculated in the planning phase using methods from relevant guidelines and standards (e.g. DACHCZ guidelines for the assessment of grid perturbations). These methods are based on assumptions and simplifications, which usually lead to rather conservative limit values that do not efficiently utilize the absorption capacity of the grids with respect to grid back-impacts and are often difficult to meet by customers and only with higher investments. Emission limits can be expressed as voltages or currents. The voltage limits are often so low that they cannot be verified at all or only with difficulty by measurement after commissioning of the plant. For this reason, today compliance with specified limit values is often assessed by measuring the corresponding current at the point of connection (PCC). In practice, this measurement is often not carried out at all or only for a short period (e.g. several days to a week) directly after commissioning of the plant.

Due to interactions between the grid and the plant, the measured current consists of two components: one caused by the grid (responsibility of the grid operator) and one caused by the plant (responsibility of the customer). Intentional or accidental compensation effects as well as low simultaneity of the disturbance emission of several plants may lead to a smaller negative or even positive influence on the corresponding voltage quality parameter than the measured current suggests. An overly conservative assessment of the contribution by one plant may lead to inefficient use of the available absorption capacity or to unnecessary installation of mitigation measures. Especially in the case of random compensation, the contribution by the plant may change significantly over time due to changes on the grid side (e.g., change in switching condition or replacement of a transformer) or the customer side (e.g., replacement of equipment or expansion of the plant).

A device for the continuous metrological quantification of the contribution of a plant to the voltage quality, which takes into account the challenges described, did not exist so far, but is urgently needed for a proper determination of the grid repercussions of a plant. This is also confirmed by the ongoing discussions in international working groups and between customers and grid operators. Especially when the measured power output exceeds specified limits, there is no clear framework on how to determine the actual contribution to the reduction of power quality in a transparent way for both sides.

The objective of the iREF-Grid project was to develop and validate a method for continuous assessment of a plant’s contribution to voltage harmonics and its proof-of-concept implementation in an existing power quality instrument (Class A). The applicability of some promising methods and indices was tested and evaluated in terms of simplicity, transparency and effectiveness, among others, through extensive field tests with several Swiss grid operators. The results of the measurements were analyzed and discussed with the grid operators in order to identify the best method to accurately determine the contribution of a plant to the reduction of voltage quality. This will lead to a more efficient utilization of the hosting capacity of future grids in terms of harmonics and asymmetry, also taking into account an increasing temporal variability of the hosting capacity. In addition, guidelines for assessing the contribution of a plant to harmonics in the grid have been developed and have been incorporated into the revision of the DACHCZ guidelines for assessing grid feedback.

In principle, the measuring device can be applied not only to systems with several devices, but also to individual devices such as chargers for electric vehicles or PV inverters. It can be used in all grid levels, whereby the accuracy of the current and voltage transformers used must be ensured for the measurement of harmonics (frequency-dependent transmission behavior).

Metrological verification in the DACHCZ

Up to now, metrological proof of compliance with limit values has not been provided at all or only by means of simple procedures based on a number of simplifying assumptions. In the current DACHCZ (3rd edition 2021), reference is made to this topic in chapter 2.9 Measurement and the type and scope of the measurement as well as the measurement procedure according to IEC 61000-4-30 are discussed. Figure 1 serves for a better understanding of the classification of the metrological verification in the connection assessment.