3.1. Terms and structure definition
Measuring the energy consumption it is important to be done every house.
In some cases, the energy is produced by little and individual generators like: windmill generators, water generators, solar and photovoltaic (PV) cells, fuel cells, and bio-fuel cells.
The role of counters is to measure and cash the energy consumption.
Electric energy meters can be classified in several different ways:
- According to the kind of electric supply they are fitted to measure (continuous current or alternating current);
- Mono phase or poly phase;
- According to whether they record intermittently or continuously;
- According to the principle of their action, whether mechanical or electronic;
- According to the nature of the measurement, whether quantity or energy meters.
Animation 4 Structure of an energy counter
The first energy counters were designed based on the principle of electromagnetic induction, but nowadays-electronic devices, named IC meters, have replaced the old counters and present the following advantages:
- Accuracy (the majority offers a precision class better that 1% on variation of input current, this is more that 1:1000)
- Increased quantity of measured parameters, namely RMS (Root Mean Square) values for current and voltage, reactive and apparent power, power quality, like total distortion factor, etc.;
- Reduced size;
- Low cost;
- Good manufacturability;
- Good reliability.
The Reset circuit defines the initial status of energy counter. It starts the programs of microprocessor in case of “power on phase” (cold reset) or when a reset condition occurred -press the reset button or enables the watchdog timer - (warm reset).
Special circuitry assure change of data according with the month and year – is the case of leap years or the transition between summer/winter times that is stored in the internal memory. Few galvanic isolated inputs are use for the external selection of tariffs or for modifying internal counter's time constants of the electric metering system.
Analog signal processing means the current - voltage conversion for line currents, filtering the signals using a low-pass filter (cut-through frequency minimum is 250 Hz) and simultaneous sampling for all inputs.
A group of 6 Analog/Digital Converters (ADC) converts the current and voltage signals into numeric values (the sampling frequency is much bigger than the network frequency, approx. 1000Hz/ input).
The digitized signal is then used to calculate the active and reactive quasi-instantaneous power. Some electric meters have reference sources attached to their inputs, which realize by switching on the ADC's inputs the automatic internal calibration.
The three-phase electric meter power supply is designed to perform even if one or two phases are missing (the maximum power is 2 W). The meters must record every outage voltage. The supervision system for the power supply sends toward the electric meter processor an interrupt signal in order to initiate the current state memory process and record the event handler.
The microprocessor is usually a DSP, MSP or microcontroller that manages the acquisition, storage, calculation, display and transfer the data power and energy (active/reactive, consumed /produced), weighing average of the measured values for active/reactive power and the billing time range. This value is stored in the memory profile.
By simulating a classic electric meter's voltage pulses for active/reactive power the metering system issue frequency pulses proportional with the measured power.
A simple keyboard allows to select on display values. Two categories of buttons are presented on the counter:
- External accessed buttons
- Internal accessed buttons, located beneath seals, they protect against changing or overwriting the profiles or scale coefficients, different from standard.
Measured/ computed values for power, energy or any other item are on display and can be selected through buttons.
The optical interface (IrDA) is used for setting/selecting the billing profile and for reading the measured values registered by the electric meter. This interface connects the electric meter to an acquisition network (it's easy to be changed or replaced).
Most frequently RS232C (voltage - reading of the electric meter by PC), RS485 (reading the electric meter from a fully balanced network), Bluetooth or radio frequency (for example 2.4 GHz - for remote reading), radio waves in one of the available bands 300, 430, 900 MHZ are used using a specific protocol.
They are used to command equipments or systems directly and allow implementation of the local control loop (e.g. setting a threshold for the users).
For example we can consider the ADE775x family from Analog Devices, integrated circuit meters. These circuits ensure the acquisition of analog signals, for current and voltage, which are adapted and filtered by a low band pass filter, after they are applied on the inputs of analog to digital converters.
Each signal is converted separately (the sampling rate is in accordance with the Shannon theory), but all signals are sampled at the same time. After the conversion, the numerical values are stored in the registers of a DSP (Digital Signal Processing) or a MSP (Mixed Signal Processing), and the main values are calculated using the formula:
(3.1)
(3.2)
for mono phase ac systems.
Represents the power factor defined that the ratio of true (active) power to apparent (total) power. 
- represents the active power, 
-represents the reactive power and S represents the apparent power. 
is the line voltage, respectively 
phase voltage, and 
represents the current line or current phase.
Animation 5 Structure of a three phases system
For the three phase systems, in the particular case of balanced consumption, we have:
(3.3)
(3.4)
(3.5)
By integrating the power along time, the counter calculates the energy –active, reactive and total. These values and other information, such as voltage and current on each phase, are stored into a controller register.
By request of an external controller, the IC meter provides the requested values using the serial SPI interface and, at the same time, the IC meter issues pulses with a frequency in a linear dependence with the electrical energy. So, the counter can provide on outputs, the active, reactive or total energy:
(3.6.1)
(3.6.2)
(3.6.3)
Many new counters can provide other important information, such as: instantaneous voltage and current, total distortion factor, different harmonics of input signals.
3.2. 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 3.9 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.
3.3 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 3.10, which brings into attention an IC meter produced by Analogue Devices.
Figure 3.10Three 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 3.11 it is presented a possible energy counter structure.
Figure 3.11 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.