Where do I start measuring?
Electricity meter lexicon - Lesson 2: Step by step to energy management with the right metering concept
In our blog series "Electricity meter lexicon", we would like to share our experiences. In this second lesson, we explain how to take the right approach to arrive at a suitable metering concept.
Unnecessary or incorrect meters can quickly drive up the costs of the energy management system by €5,000 or more for no reason. We therefore recommend starting measurement at the transfer meter and the main distribution boards. Only after an initial data-based analysis should the system be extended to specific installations.
Three meter levels: Transfer meter, main distribution board and sub-distribution board
The electrotechnical topology is the same in almost every industrial company (picture above): The energy is supplied from the electricity grid at the transfer point and recorded there by the transfer meter. This meter is the relevant energy meter for billing purposes. Industrial companies receive the 15-minute load profile from their supplier as basic information. In fact, the meter contains significantly more parameters and fundamental errors can already be detected with regard to load peaks, standby consumption, voltage fluctuations or power asymmetries. We recommend starting with such an analysis in order to decide whether there is any potential for savings.
The transfer meter (see images below) is often the only meter at 20 kV level and is therefore necessary for determining transformer or distribution losses. From an energy management perspective, "submetering" begins after the transfer meter.
Transfer meter
The 20 kV distribution board with the transformers is located directly behind the transfer meter. The transformers feed the busbars of the low-voltage main distribution board (LVMD). At this point on the "transformer underside" there is usually some kind of measurement (examples in the three pictures below).
Depending on the year of construction and equipment, you will find power quality measuring devices, multifunctional devices or analog current and voltage measuring devices at the bottom of the transformer. In most companies, we recommend normal universal measuring devices, which have the best price-performance ratio and can be read out via Modbus TCP, for example, with very high data quality using any evaluation software. High-quality power quality measuring devices should be used if there are frequent disturbances in the power quality (e.g. screen flickering, UPS).
In older systems, there are usually four analog display devices. One is for the voltages and the other three for the phase currents. Although the devices cannot be read out, they considerably reduce the work for the electrician: both the current transformers and the voltage tap can simply be reconnected to a modern universal measuring device (caution when converting: current transformers should always be short-circuited). The holes in the switch cabinet door can also continue to be used, as the installation dimensions are identical. The new universal measuring device replaces all four displays. The three holes in the switch cabinet door that become free provide space for additional sub-meters.
The low-voltage main distribution board (LVMD) has the task of distributing the current coming from the transformer via the busbar. The main distribution boards are usually ideal for measuring devices: the cables are easily accessible. There is also less cabling required for communication lines, as several meters can be installed in one place. This is a major advantage over metering in the sub-distribution board or directly at the installation.
The costs of the metering system can be optimized if not all systems are equipped with meters straight away. The best result is achieved by proceeding step by step along the energy distribution (see Sankey diagram below). First, only the transfer meter is recorded in order to determine from the load profile whether energy management is worthwhile at all. Then the bottom of the transformer is connected to the system at the feed-in of the low-voltage main distribution board.
Practical tip:
1. Meter selection: Universal meters have the best price-performance ratio. A variant with Modbus TCP should be used. These are compatible with all evaluation systems. Manufacturer-specific protocols or pulses should always be avoided!
2. Meter location: The low-voltage main distribution board is the ideal location for meters. Several meters can be installed here at the inlets and outlets, thus reducing the amount of cabling required for the communication lines. Only if the individual system cannot be tapped at the low-voltage main distribution board does it make sense to install a meter at the system. The unmeasured remainder is always decisive for the expansion of data acquisition. If this can be fully explained both in the Sankey diagram and in the stacked chart over time, no additional meter is necessary. If it is not clear which system is responsible for the consumption characteristic, additional meters should be installed.
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