The subject of communication entails introduction of some hardware and digital course of actions. Hence, in this post, we shall talk more about the world of Electronics and Hardware Programming (Firmware). We shall talk about the PCB of the Meter and will try to understand different components of the same broadly.
The Panel Meters essentially use Microcontroller, whereas the Utility Meters are built using Microprocessor as such meters are expected to perform more precise and multiple kinds of processes of data reading, recording, storing and communicating. Nowadays many Panel Meter manufacturers have started using the Microprocessor to provide enhanced meter functionalities. Here we shall talk about a typical configuration of Panel Meters.
Since we covered the basics of energy meter connection here, we shall continue this with what’s inside the Energy Meter.
Basically, the current and voltage being sensed (directly or via CT/PT), are processed by the Signal processors and provides output such as
- Sampled Data
- Reading values of current and voltage
- Reading the phase-angle
- Reading the frequency
- Deriving power (KW/KVA/KVAr) and energy (KWh/KVAh/KVArh) parameters
- Deriving any such parameters which are configured from reading
- Communicating such values to the Microcontroller
A high-pass filter in the current channel removes any dc component from the current signal. This eliminates any inaccuracies in the real power calculation due to offsets in the voltage or current signals. The real power calculation is derived from the instantaneous power signal. The instantaneous power signal is generated by a direct multiplication of the current and voltage signals. In order to extract the real power component (i.e., the dc component), the instantaneous power signal is low-pass filtered. This scheme correctly calculates real power for sinusoidal current and voltage waveforms at all power factors. All signal processing is carried out in the digital domain for superior stability over temperature and time.
The real power calculation method also holds true for non-sinusoidal current and voltage waveforms. All voltage and current waveforms in practical applications will have some harmonic content. Using the Fourier Transform, instantaneous voltage and current waveforms can be expressed in terms of their harmonic content.
Signal Processor is a kind of IC that conducts said functions.
Microcontroller reads/stores/communicates the data being read/derived by the Signal Processor. The outcome of the function of converting actual readings being read by the signal processor is stored, processed and communicated to the communication ports / LCD Display by the Microcontroller. It embeds the time stamp to the processed signals.
The Microcontroller has provision to get connected with the LCD display. Depending upon the functionalities the display could be of a different type in nature.
Advancement in the technology has reduced cost of production and many Panel Meter manufacturers provide advanced kind of Meter Display, having more functionality and ease of taking readings.
Meter reading can be noted/configured by given flat press button. A user can also set what all fixed parameter values one wants to see by choosing the parameters; and also can set rotating view of the set parameter values. This functionality provides the ease of reading the data without touching the meter.
Broadly, three kind of communication ports being used by the Energy Meters.
Panel Meters provide RS-485 and/or RS-232; whereas Utility Meters provide RS-232 and/or Optical Port. Optical port is used very rarely (once in a month to collect billing related data).
Both RS-485 and RS-232 are serial communication ports. The whole purpose of a serial interface is to provide a single path for data transmission wirelessly or over a cable. Parallel buses are still used in some applications. But with high-speed data so common today, a serial interface is the only practical option for communications over any distance greater than several feet. Serial interfaces can be used to provide standardised logic levels from transmitters to receivers, define the transmission medium and connectors, and specify timing and data rates.
- RS-232 (Ref. http://www.electronicdesign.com)
The standard defines a logic 1 as a voltage between –3 and –25 V; and a logic 0 as a voltage level between +3 and + 25 V. Signal levels are commonly referred to as a mark for logic 1 and a space for logic 0. Voltages between ±3 V are invalid, providing a huge noise margin for the interface. Noise voltages in this range are rejected. In
common practice, logic 0 and 1 levels are typically as low as ±5 V and as high as ±12 or ±15 V.
The transmitter and receiver configurations are single ended (not differential) with a ground reference. The cable medium can be simple parallel wires or twisted pair. The length of the cable determines the upper data rate and generally should not exceed 50 feet. However, much longer cable lengths can be used with low data rate conditions. The popular DB9 connector carries the signals shown. The numbers are the pin numbers on the connector.
Although not formally part of the RS-232 standard, most serial devices using the interface also use what is called a Universal Asynchronous Receiver Transmitter (UART). This IC, usually is separate from the line driver and receiver circuits. It implements a basic communications protocol that involves transmitting up to 8 bits at a time. It performs serial-to-parallel and parallel-to-serial conversion. The data is often ASCII characters, but any data word up to 8 bits can be transmitted.
- RS-485 (Ref. http://www.electronicdesign.com)
Its configuration and specifications extend the range and data rate beyond that of the RS-232 interface capabilities. The RS-485 standard specifies differential signalling on two lines rather than single-ended with a voltage referenced to ground. A logic 1 is a level greater than –200 mV, and a logic 0 is a level greater than +200 mV. Typical line voltage levels from the line drivers are a minimum of ±1.5 V to a maximum of about ±6 V. Receiver input sensitivity is ±200 mV. Noise in the range of ±200 mV is essentially blocked. The differential format produces effective common-mode noise cancellation.
The standard transmission medium is twisted-pair cable of either #22 or #24 AWG solid wire. Two lines are minimum but a third reference wire can be used. Four-wire cables can also be used if full-duplex operation is desired. The cables may be shielded or unshielded, with unshielded the most common. The nominal characteristic impedance is 100 or 120 Ω. Terminating load resistors are required to ensure a matched line condition, which prevents reflections that introduce data errors.
Cable length defines the upper data rate. But because of the lower logic voltage levels and the differential connection, data rates can exceed 10 Mbits/s depending on cable length. Maximum cable length is commonly defined as 1200 meters or about 4000 feet. The typical maximum data rate at 4000 feet is 100 kbits/s. A general guideline is that the product of the length of the line in meters and the data rate in bits per second should not exceed 10^8. A 20-meter cable, for example, would allow a maximum data rate of 5 Mbits/s.
There are further components of Real Time Clock, Noise Filters, RAM, ROM, etc. – which are not discussed here. However, the functionality of the meter and application of such components are very subjective to the meter manufacturer.
In the subsequent post, we shall discuss on how the Energy Meter communicates data tp the external world using different protocols, such as ModBus, DLMS, etc.