The key to smart electricity meter anti-stealing

At present, most of the civilian and commercial buildings in the United States still use old-fashioned electromechanical meters to track the use of electricity. These meters are reliable and inexpensive, but are not suitable for power distribution systems that require precise, repeatable metering and wired or wireless communications, in other words, are not suitable for the upcoming smart grid power distribution system.

Smart grids rely on smart meters with full communication capabilities to monitor energy usage, enabling residents and business consumers to make informed decisions about energy usage and time of use. However, smart grids also face difficulties. Although in the federal government, Washington has passed relevant legislation, such as the 2007 Energy Act and the 2009 stimulus plan, utilities are actually distributing electricity on a state-by-state basis. In preparation for the smart grid, California and Texas are the two states that are most active in smart metering.

Regardless of whether the smart grid can proceed smoothly and seamlessly in some form at the (United States) national level, sufficient independent utilities are already purchasing and installing electronic meters, making it an important market. In the next 10 years, global utility companies may have to replace 500 million meters.

The smart meter consists of a microcontroller with an onboard ADC and DAC, a voltage and current sensing element, an AC/DC power converter, a backup battery, and wireless or wired communication functions.

In addition to intelligence and communication capabilities, smart meters need to be able to prevent tampering.

Several companies provide meter ICs, including Texas Instruments, ON Semiconductor Corporation, Maxim, Analog Devices, Teridian, and e2v. These ICs are basically microcontrollers. They track the usage information of the electricity and the information returned from the utility. They can perform some DSP operations on the voltage waveform and quality, send the information to a display, and store the information to be sent. .

Although the term “smart meter” indicates that the microprocessor is the central component, tamper-evident measures make the choice of current sensing components and power backups an important decision for smart meters.

Three types of current detection techniques are mainly used in electricity meters: current transformers, Rogowski coils, and resistive shunts. Which technique is used depends on whether the power distribution is multiphase or single phase. Most homes in the world only use single-phase power supplies that are routed from the generator into the home. The US civilian market uses a split-phase power distribution, which provides residents with 120VAC and up to 240VAC, and requires a current transformer as an isolation voltage/current sensor. Commercial and some civilian power in Northern Europe is a three-phase power source, but the home is still generally single-phase. In general, a single shunt resistor is used in single-phase power distribution systems when measuring voltage and current to determine the amount of electricity used for metering, and current transformers or Rogowski coils are used in split-phase or three-phase systems because of the different phases The measured voltage will exceed the voltage tolerance of the semiconductor device.

The most commonly used isolated sensors are: current transformers, Rogowski coils, and Hall effect sensors. The current transformer has a core that is easily affected by a strong permanent magnetic field. A magnet near the transformer will saturate the magnetic core so that the detection coil cannot pick up the AC electric field on the AC line. Rogowski coils are air cores that include a coil of wire that surrounds the power line. Unlike iron cores, air cores do not saturate under strong permanent magnetic fields. However, it is susceptible to other tamper methods. If there is a large inductance field, it will couple to the coil and suppress the AC current to be detected by the coil.

Hall effect sensors use a semiconductor device that detects the magnetic field generated by a varying current in a wire. However, their readings vary with temperature and cannot remain linear forever over a wide current range. This problem is even more serious in the U.S. market because the meter is rated up to 200A, while in other countries (such as India) the meter measures 20A or 40A. According to Kourosh Boutorabi, vice president and general manager of the Meter Product Business Unit of Teridian, “The market for linearity in the United States is important. The error requirement for utility meters is 0.2%. The rest of the world is close to 1%. If you use electricity, More often, more loss is more important in the measurement, and the measurement accuracy is more important."

Hall-effect sensors also have the advantage, Kourosh Boutorabi said: “Although they are not (as current transformers) as accurate, but they are cheaper. Hall-effect sensors can be connected directly without the physical connection (power line).” If you use a current transformer, You must manually assemble the power line and detection coil.

Most of the world's electric meters are single-phase meters, using a simple shunt resistor as current sensing element. Cathal Sheehan, Product Marketing Manager at Bourns' Resistive Products Division, describes the characteristics of the shunt as simple, inexpensive, and unaffected by magnetic and inductive fields, which often confuse magnetic and inductive sensors.

Mark Strzegowski, marketing engineer for energy metering products at Analog Devices, acknowledges that splitting does not provide isolation between multiple phases, so you need to provide isolation elsewhere. The company's solution is to use a shunt resistor with an isolated planar transformer that uses the company's iCoupler technology for isolating components: the sum of the iCoupler and the shunt resistor is still lower than the cost of the current transformer or Rogowski coil and still remains The linearity of the measurement. Strzegowski said: “One of the important issues to consider when selecting current detection technology is the performance required by the customer. The market has a tendency to move towards a wider dynamic range, and this also increases the number of measurements. You need to have the same accuracy Accurately measure current from 100A to 100mA because 0.1% is a typical accuracy specification."

Diverting resistors inherently suffer from heating problems: Although their response linearity is good, they cannot handle some of the largest loads because of the self heating effect. Also, current transformers generally need to compensate for less phase distortion, especially when the application requires more precise measurements, such as measurements of reactance and harmonic energy. In general, a single-phase meter uses a shunt resistor as a sensor, and a three-phase meter uses a current transformer.

Silvestro Fimiani, product marketing manager of Power Integrations, believes that in addition to preventing tampering, meter efficiency is also very important, especially in the near future, there may be more than 100 million meters installed or replaced worldwide. He said: "During the life of an electricity meter, the energy cost that would be spent due to the inefficient use of the meter would be as much as $20. This is equivalent to the cost of an electricity meter." In the world of smart meters, Fimiani may be the rare person who emphasizes energy efficiency. Few other suppliers care about the efficiency of the meter, perhaps because the purchaser of the meter is generally a public utility rather than an organization that pays for the meter's power consumption. This cost is not visible but it will be passed on to consumers.

The components in the electricity meter should consume less energy. The reason is not only to save costs and improve efficiency, but also to ensure that the meter can operate efficiently under battery power. It seems a strange idea to use a battery to power the meter, but the smart meter must continue to work even when the power is off.

The importance of battery backup in an electricity meter is related to the area where it is located. For example, in the United States, if there is a power outage, there is no measurable thing, so there is almost no demand for the utility meter to be awake. Analog Devices's Strzegowski pointed out that the situation in India is different. Some regulations require that there must be two batteries in the system. A battery provides power to the meter to hold readings and information for 24 or 48 hours and keep the display lit. The other battery will keep the meter information for at least two years to prevent the meter from being tampered with. It removes the meter from the power line so that there is no voltage in the meter circuit, but power is still delivered in the cord.

The battery backup of the meter generally uses a lithium/thionyl chloride battery. Its self-discharge rate is on the order of nanoamperes, and the shelf life is more than 10 years. Tadiran is the largest U.S. manufacturer of such batteries. The battery itself is sufficient to complete the backup of data in the meter, but data cannot be sent out through wireless communication. One solution is to trickle a supercapacitor with a battery. Pierre Mars, vice president of application engineering at Cap-xx, an ultracapacitor manufacturer, explained how the government's regulations affect the meter: “Although the meter is connected to the main power line and has a redundant power source, the meter cannot use it to send data. For example, when I In Australia, the rule here is that the energy the meter receives from the customer's power line should not exceed 2 W, so they need to use a super capacitor to provide energy for GSM burst communication transmission. This requires a burst of 6 W for 0.6 ms, while the average The power is less than 0.75W." Mars pointed out that if the government sets the rules for energy use instead of limiting the maximum power, then this problem does not exist.

In addition, the super-capacitor also provides enough power for the transmission of “last words”. For example, when the power network is interrupted, the super capacitor backs up enough power to transmit one time, warning that the meter has lost power.

The power losses caused by power theft in various regions of the world are different: the United States may account for less than 4%, while India reports that the loss is greater than 10%, and this loss in Latin American countries can reach 20%. Research on anti-tampering is another reason for turning to smart meters. The simplest and most common way to tamper with a meter is to use a sensor.

Why is the anti-electricity method in the United States important? Because with the continuous increase in the amount of electricity, 4% of the loss is enough to attract the attention of utilities. But more importantly, stealing electricity is the most common source of energy for illegal "hemp-houses." In rural areas, electricity bills for these houses can easily reach 10,000 U.S. dollars per month, and stealing electricity from neighboring power lines is the easiest way to draw electricity. Neighbors and public utilities don't like the "honey house," and using smart meters may be an effective way to curb such activities. However, some people also worry that any activity involving large organizations (such as government or public utilities) is monitoring individual behavior.

Soaring energy prices have also prompted non-utility applications to adjust their power measurement efforts. Dave Heacock, senior vice president of TI's high-volume analog and logic business, said that server farms sometimes require their own front-end AC/DC power suppliers to provide power measurement capabilities so that farms can charge customers based on usage: such as during peak hours Electricity will be charged at a higher rate. Heacock said: "Users will think, 'I have to do all the credit card transactions when the electricity prices are low in the middle of the night'."

The tolerance for this application may not be as stringent as a utility meter: Heacock recommends that for loads greater than 20% of full load, the typical tolerance for such measuring equipment is 2%, and at 5% full, it drops to 5%. . He explained that the reason for relative laxity is that they do not want to increase the cost of the solution.