In our blog series „Electricity Meter Lexicon“ we would like to share our experiences.
In this second lesson, we’ll teach you how to take the right approach to get the right metering concept, and how unnecessary or incorrect meters can quickly increase the cost of an energy management system by €5,000 or more for no reason. Therefore we recommend the start of measurement at the transfer meter and the main distribution boards. Only after an initial data-based analysis should the system be specifically extended to systems.
Three counter levels: Transfer meter, main distribution and sub-distribution
The electrotechnical topology is the same in almost every industrial company (top picture):
The energy is supplied from the power grid at the transfer point and recorded there by the transfer meter / transfer counter. This meter is the relevant energy meter for billing purposes. Industrial companies receive the 15-minute load curve from their supplier as basic information. In fact, there are considerably more parameters in the meter and fundamental errors can already be detected with regard to peak loads, standby consumption, voltage fluctuations or current asymmetries. We recommend starting with such an analysis in order to decide at all whether there is potential for savings.
Often the transfer meter (see pictures below) is the only meter on 20kV level and therefore necessary for the determination of transformer or distribution losses. From the point of view of the energy industry, „submetering“ begins behind the transfer meter.
Directly behind the transfer meter is the 20kV distribution with the transformers. The transformers feed the busbars of the low-voltage main distribution boards. Exactly at this point on the „transformer underside“ there is usually some kind of measurement (examples on the three pictures below).
Depending on the year of construction and the equipment, the transformer underside contains mains quality measuring devices, multifunction devices or analogue current and voltage measuring devices. In most companies we recommend normal universal measuring instruments. These have the best price-performance ratio and can, for example, be read out via Modbus TCP with very high data quality using any evaluation software. High-quality mains quality measuring instruments should be used if there are frequent disturbances in the mains quality (e.g. screen flickering, UPS).
In older systems you will usually find the four analogue display devices (top right picture). One is for the voltages and the other three for the phase currents.
Although the devices cannot be read out, they reduce the work for the electrician considerably: Both the current transformers and the voltage tap can simply be connected to a modern universal measuring device (caution during conversion: current transformers should always be short-circuited). The holes in the switch cabinet door can also still be used, as the installation dimensions are identical.
The new universal measuring instrument replaces all four displays. The three free holes in the switch cabinet door create space for further sub-meters.
The low-voltage main distribution boards have the task of distributing the current coming from the transformer via the busbar. The main distributors are usually ideally suited for measuring instruments: the cables are easily accessible. In addition, cabling for communication lines is saved as several meters can be installed at one location. This is a great advantage over measuring in the sub-distributor or directly at the system.
The costs of the measuring system can be optimized if not all systems are equipped with measuring devices. The best result can be achieved by proceeding step-by-step along the energy distribution (see Sankey diagram below). Initially, only the transfer meter is recorded in order to determine from the load profile whether energy management is worthwhile at all. Then the underside of the transformer is connected to the system when the low-voltage main distribution boards are fed in.
The decisive factor for the expansion of data acquisition is always the unmeasured remainder. If this can be fully explained both in the Sankey diagram and in the time course of the stacked chart, no further counter is necessary. If it is not clear which system is responsible for the consumption characteristics, additional meters should be installed.