Electronic Design
The Need for Current Sense, Circuit Protection and Communications in Smart Meters

The Need for Current Sense, Circuit Protection and Communications in Smart Meters

by Kelly Casey, Market Director, Industrial/Consumer for Bourns, Inc.

The smart grid is the next evolution of the electricity grid with the convergence of electric power, telecommunications infrastructure and information technology. The ultimate goal is to allow seamless communication between utility companies and the end user for a true “end-to- end” smart grid (see fig. 1).  The key elements of a smart grid will provide energy awareness (i.e., providing cost signals to the end user), energy response (i.e., allowing the end user to respond to the price signals) and energy emergency (providing immediate response to power shortages). This advanced metering infrastructure will allow utility companies to level load the system and moderate against the shortcomings of peak demand.

One of the key application elements of the smart grid is the smart meter located at the end user that will allow two-way, real time communication between the power companies’ substation or headquarters and the user.  A smart meter looks very similar to a traditional electricity meter located in a residence or business but identifies consumption in more detail.  Smart meters use one of several communication protocols to transmit energy consumption information via a network back to the local utility for monitoring and billing purposes (telemetering). Smart meters are hybrid devices combining measurement, processing, and recording with communication functions.  Thus they utilize proven elements common to either type of system: shunt resistors for current measurement and multilayered varistors (MLV), transient voltage suppression (TVS) diodes, gas discharge tubes (GDT), fuses, and positive temperature coefficient (PTC) devices to provide circuit protection for internal signals and communications interfaces.  This article will discuss the smart meter, current sense design, and components necessary for circuit protection in a smart meter and various communications environments. 

A Look Inside Smart Meters
Three main sections comprise the internal smart meter design.  The power system includes a switched mode power supply and battery backup to ensure the metering electronics remain powered even when the main line is disabled.  A microcontroller with an onboard analog-to-digital converter (ADC) and a digital-to-analog converter (DAC) along with sense components for voltage and current provide the intelligence.  Finally, a wired or wireless communication interface allows the meter to interact with the rest of the grid, and in some cases the end user’s network.  A block diagram (see fig.2) of the key components internal to the smart meter is shown in figure two.

Power System
A switched mode power supply provides power to the electronics in the meter, converting from the main line alternating current (AC) voltage to the direct current (DC) voltages required.  A switch will turn on the battery backup AC/DC when there is no power from the main line.  Using a MOSFET or similar technology will ensure that current flows from the battery backup AC/DC converter only when the battery backup system is activated, remaining isolated from the power system during normal operation.

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Current Sense
Current sensing and current feedback are required for safety purposes in many high power systems, which shunt resistors can provide inside a smart meter. This solution is ideal for residential smart meters as these precise low-value resistors are considerably lower cost than other options such as a current transformer and require little effort to incorporate in the system.  A shunt resistor simply is placed in series with the high current electric bus bar, and the current flowing through it is calculated. The calculation can be performed based on the proportional relationship of voltage and current in a resistor of known value.  This current value and the instantaneous voltage are multiplied to get the power consumed at any instant, and is continually monitored in by the microcontroller.  Shunt resistors for use in smart meters require specific construction using a copper-manganin material that is e-beam welded, which is a sophisticated process not all manufacturers provide.  It is important to perform due diligence in finding a shunt resistor that meets this e-beam welding requirement. 

Protecting Intelligence and Communications
Smart meters contain more sensitive electronics than traditional meters and require circuit protection components such as TVS diodes, fuses, PTCs and MLVs to protect against overcurrent and overvoltage.  For example, the microcontroller will require resettable protection with fast reaction time as a defense against surges.  A fast-acting PTC resettable fuse can handle this task.  In other situations that do not require reset capability, a thin film chip fuse may be employed.  Diodes can provide suppression of transient voltage on the input and output signals of the microcontroller.  Electrostatic discharge (ESD) protection may be required as well, which can be met with MLVs presently offered with ultra-low capacitance and in package sizes that fit the tightest of circuit board designs. 

No single solution is in place for communication between the smart meter and the utility or end user.  Communication protocols are widely varied based on factors such as geographical regions, location of an individual meter, what is supported by the utility servicing the area, and the maturity and longevity of those supported technologies.  Whether wired or wireless, the components that would be necessary for protecting the communications interface include TVS diodes, MLVs, PTCs, fast-acting fuses, GDTs, and magnetics.   

Smart Meter Meets Smart Grid
Given the internal functionality and design of a smart meter, the information it collects must be provided to the utility through the network components that make up the smart grid.  There are several categories that envelop most of the solutions available. Wired options include RS485 or the plain old telephone system (POTS).  Wireless protocols quickly are emerging as the preferred method of connecting the networks within the grid. 

Radio frequency (RF) based technology is changing the way that customers and the utility company interact with the use and sale of electricity.  In some cases smart meters act as the gateways for home automation, using RF signaling technology such as Zigbee, to communicate the energy pricing and consumption to the end user.  This allows the user to intelligently use electricity.  Use of prepaid RFID smartcards can indicate to the utility how many electricity credits a given user has purchased and available, and thus determine whether to provide power to this user.  Devices operating with 6LoWPAN, Zigbee, and RFID are examples of RF based technology connected with RF mesh networks. 

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Cellular solutions are possible as well, including GSM/GPRS and ISM-Band 868/900 MHz radio.  Wireless Mbus, the EN 13757-4:2000 standard for utility company communications, is an example of an emerging protocol that operates at 868 MHz.  These cellular solutions make use of the existing cellular network to transmit metering information.  Broadband over Power Line has been viewed as a potential replacement of POTS and cable networks for providing broadband, though is it not in service in all areas.  WiMax networks are capable of connecting a variety of device types, increasing the interoperability of equipment from different manufacturers following numerous protocols.  It specifically excels in last mile links and in connecting equipment over large areas. 

With this variety of protocols, the equipment that supports communication in the grid will be quite diverse.  The table outlines a selection of the types of equipment one would expect to find in these networks.

This access type telecommunication equipment will be located outdoors, and must be capable of withstanding severe environmental conditions. In order for the equipment to be reliable in all types of weather, circuit protection solutions must be designed to withstand such conditions as lightning strikes and induced voltages from nearby lightning strikes. GDTs, thyristors, TVS diodes, PTCs, MOVs and fuses are some of the devices that can be included in these circuit protection schemes. 

A Review of Circuit Protection Components
ith such a variety of components proven for use in circuit protection solutions for similar environments, it is worth reviewing some of the mentioned circuit protection components beginning with voltage protection devices.  TVS diodes are small silicon-based components that do exactly what the name indicates: protect electronics from transient voltages by suppressing the transient voltage.  TVS diodes are used for surge and ESD protection.  Another popular overvoltage and ESD protection device is the MLV, which is a multi-layer varistor connected between a data line and ground.  When an overvoltage condition is present the resistance decreases and the current rises exponentially as it is diverted through this path away from the electronics it protects.  As the first line of protection against overvoltage conditions, a GDT will shunt the surge to ground or source when its sparkover value is reached, creating a virtual short.  It returns to its normal high impedance state when the event has cleared.

Several overcurrent devices have been mentioned as well.  Resettable PTC fuses are used for different environments or levels of protection based on the material.  A ceramic PTC generally is well-suited for protection of electronics that would be subject to lightning strikes whereas a polymer PTC generally would protect general electronics from less extreme operating conditions.  Either material operates in a similar manner by changing from a very low resistive state to an extremely high resistive state when an overcurrent event such as a surge or fault occurs.  As a positive temperature coefficient device it is sensitive to temperature, and the change in resistance is the result of the device heating to a switching point at a rate of I2R in the presence of overcurrent.  Following the event the device cools, and finally returns to its rated resistance.   Other fuses that may be used include thin-film chip fuses, fast-acting fuses and single blow telecommunications fuses.  Thin-film fuses and fast-acting resettable fuses are suitable for protecting sensitive electronics.  Thin-film fuses may not be resettable, and often are found in power circuits.  Fast-acting resettable fuses frequently are used in high-speed interfaces and as secondary circuit protection.  Telecommunications fuses are capable of handling much larger current levels and are suitable for primary protection in telecommunications applications where the environmental conditions are a crucial factor in protection.

Protection in Action
A proven solution with the discussed components is shown below in the example of protection in the popular RS-485 wired serial interface.  A low capacitance TVS diode array and a fast-acting resettable fuse adorn each line of the differential pair for secondary protection between the RS485 chip and the GDT, which provides primary overvoltage protection.  The fast-acting resettable fuse (see fig. 3) is inserted in series and is essentially invisible to the circuit under normal operation.

Conclusion
As the smart meter market builds momentum, it’s important for design engineers to remember the main building blocks with proven components that provide a safe haven for the smart meter in the smart grid.  Current sensing feeds directly to the processing portion of the design, and precision in this portion of the design is of paramount importance.  Shunt resistors are virtually tamper-proof compared to the meter-slowing schemes possible with magnetic components, adding value to the utility.  Finally, it is important to determine the circuit protection necessary, which will depend on the types of processors and communications protocols for a given installation.  A smart meter generally will contain several connectivity options for maximum versatility in installation.  Just as the example RS-485 circuit protection solution has been proven in existing equipment, there are solutions available for other communications standards as well.  Wireless smart meters interface with a variety of equipment, so care must be taken to ensure the level of circuit protection from the end user to the utility company meets or exceeds the involved standards to provide the desired “end-to-end” solution.

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