STELM: DC electricity meter - DC electricity meters

The required voltage range is determined by the range of voltages occurring in the measured network. Especially for photovoltaic applications, the maximum system voltage without load must also be considered.

In some applications, the instrument is powered from the measured voltage in the same way as is customary for AC instruments. In addition to the advantage of saving a separate power supply, this solution has a significant disadvantage for some applications: there is a lower voltage limit in the system, determined by the characteristics of the meter’s power supply, and if the voltage drops below this limit, the measurement does not run. This problem is encountered, for example, when measuring the consumption of photovoltaic-powered heaters, when the voltage in the system drops under low light conditions, e.g. to 40 V from the nominal 300 V, but the panels still transmit measurable power and it is a shame not to read it with the meter. In this case, we then use instruments with a separate power supply, capable of measuring from zero input voltage.

The design of the instrument with a separate power supply has two variants: with direct voltage measurement, where the measured voltages are fed directly to the terminals of the instrument, and a variant with indirect voltage measurement, where an external isolation voltage converter is used for voltage measurement. The measuring range is then determined by the input range of this isolation transducer; we can supply an isolation transducer or any commercially available type can be used.

The connected (separate) power supply is of course galvanically isolated from the measuring and communication terminals of the instrument.

The meters are designed for external shunt connection. As standard, we supply German Weigel TP 0.5 shunts with a marking to the default shunt resistance accuracy corresponding to TP 0.2 – these shunts are included in the price of the instrument. The voltage drop across the shunt at full current load is 0.1 V, so at a current of 100 A there is a 10 W loss across the shunt.

Between the device and the shunt we insert protective DC fuses, which are also included in the delivery of the device.

For special cases, it is to supply an instrument for indirect current measurement through an external isolation transducer, which is mainly used in high-voltage applications. Another case where this type of instrument is used is measurement with non-contact DC transducers, which allow lossless current measurement (no heat loss at the shunt) at a significantly higher cost and lower accuracy.

The instrument is equipped with only a pulse (S0) output in the basic version. It is a galvanically isolated open-collector type output that emits a number of pulses corresponding to the measured energy; the conversion constant is indicated on the instrument label and is e.g. 1000 imp/kWh.

For more advanced systems, we offer a version of the instrument with a MODBUS-RTU communication module or with an RS-485/ASCII communication module.

Both communication modules allow the transmission of voltage, current, power and energy readings from the start of the meter power supply. Both types of communication modules use the RS-485 bus and are galvanically isolated. The MODBUS-RTU communication module works with the standard MODBUS-RTU bus, to which up to 32 meters can be connected; the RS-485/ASCII communication module is designed for systems with a single meter – this module transmits data in the form of a comma or a line. semicolon separated data fields, readable in a normal terminal. This data output is also used to coordinate with the load controller for photovoltaics.

The letter X in the type designation of the STELM-UIKX is replaced by a list of special modifications according to the following table: