Smart Grids Data Processing Analysis [Step 4]
Intellectual meters : Architecture and implementation
Early architectures of solid-state meters supposed the use of many information systems. At such architecture the microcontroller controls the whole system and the display, and several analogue-digital converters and signal processing processor perform metrological tasks. Subsequently, to combine the analog-digital conversion and digital signal processing, special information systems were used, made to order by the manufacturers of metrological equipment. It is obvious that such architectures do not possess sufficient flexibility required in the conditions of the modern market, as the execution of the information system to order requires considerable time and own investments for additional research. It should also be noted that custom information systems were suitable only for certain networks with specific architecture, which reduced the effectiveness of the solution.
In classic architecture, where several inverters are used, the following disadvantages can be highlighted: low accuracy due to interchannel crosstalk and high component costs. Interchannel crosstalk, in turn, requires additional protection of the technical components and the embedded software. It should also be noted that to implement a wide range of analogue 2000:1 input signals, organizations have to switch to differential mode, which is an expensive solution.
As one of the optimal solutions can be considered the method Single Converter Technology (developed by Teridian), which belongs to the class of integrated measuring solutions of crystal-based systems. The architecture, which uses this method as a basis, rationally uses metrological functionality by combining a single sigma-delta ADC with multiplexed inputs and a programmable computer (Computation Engine - CE) to process data in real time. With this solution, developers can flexibly customize the calculator to meet the measurement and processing requirements of utility organizations, minimizing changes at the hardware level.
Multiplexed systems are a cheaper alternative to classical systems, where the architecture provides a separate analog-to-digital converter for each channel. Solutions based on multiplexed systems reduce interchannel crosstalk by using switching circuits that allow input channels to be scanned by selecting each channel in a circle for processing with the same analog-to-digital converter.
This approach is particularly effective for applications such as power management with several signals that are similar in nature. When using such solutions, the main condition is to store phase information between channels. This allows the CE calculator in a multiplexed system to perform measurements on different channels simultaneously. Multiplexed crystal-based systems with a single converter allow the matching of gain and compensation offsets, reduction of interchannel crosstalk and design flexibility. Together, these benefits provide a relatively low-cost solution for high accuracy measurements with a wide dynamic range (2000:1).
The crystal-based systems have the ability to quickly reprogram so that the CE calculator's embedded software can be updated on a simplified basis and engineers have the ability to configure metering equipment with various current sensors such as current transformers, Rogowski coils and current shunt. This simplifies the introduction of anti-intruder techniques.
There are two main methods to implement an automatic meter reading system. The choice of method depends on the specific legislation of the particular country or region.
The first method allows extensive metrological functionality in the final point of measurement. Regulatory authorities in certain regions impose strict requirements on utility companies to reduce the risk of data loss and to obtain accurate energy consumption data for billing purposes. According to the regulations, meter reading is carried out at fixed intervals (every 15 minutes), and the received information is sent to the operator every 8 hours. However, to ensure protection against possible failures when sending information via the communication channel, the regulations require that at least two samples of data are always stored at the measuring point. Thus, it is necessary that the metrological chip can store consumption data for 16 hours.
The second method assumes less functionality, but more favorable price conditions. In case regional authorities impose more loyal requirements to the process of collecting and interpreting meter readings, utility companies reduce the cost of implementing metrological functions in the meter (unless a broader meter function is needed to ensure the payback of the system, to prevent unauthorized actions). According to long-term forecasts of experts, by combining the functions of AMR with the metrological system on the crystal, it is possible to achieve even greater savings. At the moment, the main difficulty in implementation is mainly the heterogeneity of AMR communication methods. For example, information can be transmitted via conventional modems (in fixed or cellular networks) or using power lines (PLC). Also note that in this case the price for additional equipment may vary from USD 3 (when using PLC modems) to USD 20 or more (when using cellular modems).
The possibility of system reprogramming in conditions of active network usage gives utility organizations flexibility to change tariff plans depending on changes in the volume of energy consumption within the network. Thus, when the majority of customers change the hours of peak energy consumption, as well as when the value of demand depends on the season, there is a need to change the period of the day for which there are maximum electricity prices. By tracking electricity fluctuations and changes in peak load values, it is possible to change the tariff policy.
The ability to protect the power supply network from unauthorized actions is one of the key advantages of smart grids. Classic actions by intruders include breaking into a meter's structure by opening its case and locking the mechanism, placing magnets near the meters to saturate its magnetic components, adding capacitance, loads with a single-period rectifier or high instantaneous current. It is not uncommon to bypass the meter, resulting in an increase in AC current through the Counter Neutral Pins. Modern solid-state metrology systems allow designers to prevent intruders by obtaining data from the common power consumption network about such characteristics as unbalanced load, current through the neutral wire, direct currents caused by single-period rectifiers, detection of external magnetic fields. Node (substation) meters can also calculate the difference between the total energy generated and the total energy billable and report any deviations over the AMR network. In cases where a forensic investigation is assigned to recover stolen energy, smart meters help to obtain reliable evidence: accurate data on time of theft and amount of stolen energy.
Basic features for smart meters:
Multi-port communications with flexible architecture to support AMR channels; ability to interact with local network devices such as thermostats; development of a topology that includes subnet measurements
Multi-channel readout of input data with high processing speed, such as on the basis of SCT (Single Converter Technology), discussed above, to reduce the cost of the system I thank the multiplexing of inputs using sigma-delta ADC in combination with a programmable computer CE
Real-time processing of various input signals from sensors with minimal use of hardware components; correction for temperature or other environmental parameters to improve efficiency and avoid calculation errors
Firmware that can be upgraded during active use to extend the life of the smart metering system and dynamically adjust tariff plans for the best use of resources
Multiphase load monitoring and processing tools to manage power consumption, analyze network load and optimize the real-time monitoring process.
Visualization of the received data on the general screen, with support of different supply voltages and screen resolutions
Different volumes of internal flash memory for storing consumption data, tools to interact with external memory
Set of tools and mechanisms to detect the actions of intruders to prevent theft of energy resources; support for current transformers, Rogowski coils and current shunt, as well as combined current sensing mechanisms; circuit break sensor
Full-featured operation with unipolar power supply in special safe mode, which provides for detection of actions of intruders, with single and multiphase power readings
Real Time Clock (RTC) embedded in the overall topology
A range of analog interface chips for meters contain integrated real-time clocks with an accuracy of 60 minutes per year. In case the meter is connected to an intelligent power supply which synchronizes the real-time clock at certain intervals, this error can be ignored. In the case of classic meters, consumers, at a certain time interval, will start to encounter significant discrepancies between the invoices submitted for payment and the data taken from their own meters, unless the classic meters are equipped with expensive high-precision real-time clocks.
The functionality of the integrated real-time clock includes measuring and tracking the integrated temperature sensor and optimizing the capacitive load of the integrated resonator on the basis of the received data, in order to compensate for the natural temperature loss of the frequency of the reference resonator. Due to the fact that the frequency may vary depending on external conditions, at each operating temperature, the resonator and chip are calibrated in a single module to ensure greater frequency stability. In this case, there is no need for engineers to perform calibration to obtain highly accurate data. Some of the latest integrated real-time watches use MEMS technology to improve the accuracy and stability of the design itself. The all-silicon resonator used in this watch provides the same low frequency and low current as large information systems based on a quartz resonator, while maintaining a compact design. Moreover, solutions based on this technology are highly resistant to high-temperature assembly processes, are able to operate after shocks and vibrations exceeding 20g, and use an offset scheme to compensate for aging.
Some solutions use a temperature-compensated silicon generator as a basis. Using the measurement data from the integrated temperature sensor, the temperature compensation algorithm optimizes the resonance frequency to take into account temperature effects on an automatic basis. With this solution, the system can guarantee high accuracy when measuring temperature. Unlike large information systems based on a quartz resonator, such solutions have a frequency loss of less than ± 0.5 ppm after high-temperature reflow soldering, and also provide stability of frequency characteristics (< ± 5 ppm) at all operating temperatures from -40 to +85°C.
Flexible on-chip measurement systems, both multiphase and single-phase, are used in residential applications (monitoring, saving data from the last few sessions, etc.), which typically contain a 5-MHz 8051-compatible microprocessor core, a 32-bit programmable computer, a real-time clock, integrated flash memory with up to 64KB and RAM up to 5KB.
The characteristics of intelligent systems considered above show that developers, engineers and architects of power supply systems use them to develop a variety of metrological devices - from the cheapest solutions with limited functionality to high-quality devices with a large amount of integrated internal memory, the ability to quickly reprogram and high-precision meter readings. Systems based on smart meters are very flexible. This allows developers and utility companies to adapt to new market conditions and growing customer needs as the smart meter market develops, and to flexibly change their policies to meet the rules and standards of regulatory organizations, while maintaining high cost-effectiveness and efficiency in use.
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To be continued