How to install an energy meter ?
The proper installation of an energy meter is an essential element for the accurate indication of the measured values: electric power or electric energy. Normally, differences can occur between mono-phase and three-phase systems. We will present two possible connections, one for mono-phase systems and another for three phase systems.
In order to guarantee the house security, the metering system is equipped with different kind of seals, hardware and software implemented. Only the distribution companies can install and modify the parameterization and the tariffs for the energy counter (metering system).
Figure 1 Connections of mono and three phases energy counters
If the house installed power or demand power are higher than maximum allowed by the counter, adequate current transformers must be inserted in the measuring circuit for each phase. For special application at medium voltage (6 KV usually) the counter can include voltage transformer too.
In each situation, two corrections must be realized: scale correction for the voltage and measured power, and phase correction, as only the air transformer does not shift the input signal phase.
After installing the energy meter it is compulsory to seal all the elements that could influence the measuring values, in order to prevent possible changes of wiring by unauthorized persons.
Details regarding integrated circuit meter
The last few years, the IC meters have developed and now represent a solid-state solution for the classical induction counters, increasing the accuracy.
An example of these IC meters is presented in figure 2, which brings into attention an IC meter produced by Analogue Devices.
Figure 2. Three phases IC meter produced by Analogue Devices (source Analog Devices)
The ADE7758 has a total of six analog inputs divided into two channels: current and voltage. The current channel consists of three pairs of fully differential voltage inputs: IAP and IAN, IBP and IBN, and ICP and ICN. These fully differential voltage input pairs have a maximum differential signal of ±0.5 V.
The anti aliasing filter prevents the artifacts. Input signals with frequency components higher than half the ADC sampling rate distort the sampled signal at a frequency below half the sampling rate. This will happen with all ADCs, regardless of the architecture. The combination of the high sampling rate ?-? ADC used in the ADE7758 with the relatively low bandwidth of the energy meter allows a very simple low-pass filter (LPF) to be used as an anti aliasing filter. A simple RC filter (single pole) with a corner frequency of 10 kHz produces an attenuation of approximately 40 dB at 833 kHz. This is usually sufficient to eliminate the effects of aliasing.
The digital values obtained by sampling all current and voltage signals using an adequate frequency (resolution of ADC 16 bits), are employed to calculate the instantaneously power that is cumulated in special registers. The current sense detection circuit can correct the zero cross moment in narrow limits +1.36° to 2.72°, increment 0.104° at 50Hz. Compensation of offset errors will improve the accuracy of circuit. The high pass filter eliminates the DC offset.
The circuit supports the standards IEC 60687, IEC 61036, IEC 61268, IEC 62053-21, IEC 62053-22, IEC 62053-23 requirements. Some of the circuit feats are the following: assures less than 0.1% error in active energy measurement over wide current dynamic range, supplies active energy, reactive energy, apparent energy, provides voltage and current form of signals at request.
The SPI interface allows the initial settings parameters and the instantaneously values for energy, power, voltage and current to be transferred per each phase.
In figure 3 it is presented a possible energy counter structure.
Figure 3. Block diagram of an energy counter based on ADE7758 IC meter
In a first phase the micro controller program would initialize the IC meter; in the second one the data sent by IC meter through SPI bus would be received.
The controller will manage the following aspects: the operator display, the commands to the circuit breakers acting in the energetic circuits and the interface used for communicating with mobile devices that collect accounting data from the counter.
The system can remotely communicate to the central billing station by implementing an Automated Measure Reading (AMR) system and using an adequate interface.