Counters for electric energy

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 1 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:

        (1)

         (2)

                                              
for mono phase ac systems.