Datasheet

Theory of operation STPMC1
20/77 Doc ID 15728 Rev 7
These four multiplexed signals are separated, by a digital de-multiplexer, back into eight ΔΣ
signals, called streams. The signal coming from the voltage channel of the STPMSx is
named with the suffix V, while the stream coming from the current channel is named with the
suffix C. For example, the voltage stream of the S-phase is named DAS-V.
Then, each pair of phase the voltage and current stream coming from DAR, DAS and DAT is
connected to a dual-channel RDSP, SDSP, TDSP unit (i.e. DAR-V and DAR-C are
connected to RDSP).
Each phase voltage input stream is proportional to phase voltage u. Each phase current
input stream is proportional to derivation of phase current di/dt, when it originates from
Rogowski coil, or to phase current i, when it originates from Shunt or CT or Hall sensor. In
this case a derivative is inserted into the voltage channel to get a stream proportional to
du/dt. The sensors differ from each other for sensitivity, phase error and susceptibility to
external EM fields.
Each of these DSP units performs the following:
● checks the integrity of the streams
● calibrates streams
● filters both streams with a dedicated decimation filter
● computes active and reactive energies, momentary and RMS values for voltage and
current, period of power line voltage signal.
In each DSP there are calibrators capable of adjusting the readings ±12.5%.
The power computer does the final calculations of the value and direction of the power and
checks for no-load condition.
Another dual DSP unit, called NDSP, processes the streams coming from DAN and DAH. In
fact, using the ENH
bit (see Tabl e 3 3 ), the user can select either the voltage stream of the
DAN pin (DAN-V) or the DAH stream as the input of the NDSP unit, while the current stream
DAN-C is always processed as neutral current.
In its voltage channel, the NDSP unit uses a 2 s time multiplex to process two streams.
During the first half of the interval the voltage input stream is processed (which can be DAN-
V or DAH, according to the ENH
bit), while during the second half a stream constituted by
the sum of all four calibrated currents (i.e. DAR-C + DAS-C + DAT-C + DAN-C).
In its current channel the NDSP unit process the current stream of the neutral conductor as
follows:
● checks the integrity of stream
● calibrates the stream
● filters the stream with a dedicated decimation filter
● computes momentary and RMS values of the stream
● if no errors have been detected in the phase timing, computes phase frequency,
integrates the phase powers by means of 3-input integrators of energies and generates
all pulse output signals.
When the DAH input stream is selected, it is checked to detect an external magnetic
influence (EMI) to the meter.