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Activating electrical prepayment systems in homes

20 Jul 2015  | Hamed Sanogo

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Since a customer profile was created when the customer first signed up with the utility company, the customer is known by the system. The back-office server now has the customer's personal information (e.g., name, address, etc.), MMV (initially zero for new customers), the HDU's ROM ID, and the selected memory-page data information from their profile. To purchase energy credits, the customer follows the transaction flow shown in figure 7.

1. The customer uses an NFC-enabled smartphone or tablet loaded with a custom app to prepay energy credits (e.g., 1000kWh). In this case the app buys these credits directly from the utility via the customers' cellular data plan or home Wi-Fi connection. The utility's app guides the customer to do this. Once the server has implemented the transaction and verified that the payment has processed successfully, the server will increment the MMV by the amount of energy credits prepaid by the customer.

2. The server then generates a data packet consisting of the new page data (i.e., a new MMV) and a corresponding computed message authentication code (MAC) to be used for an authenticated write action to the HDU's MAX66242. The server transmits the packet via the established cellular data or Wi-Fi connection to the smartphone.

3. The customer is now ready to transfer the purchased credits from the smartphone onto the HDU using the NFC interface. As explained in the nonfiction fable at the outset, the customer simply brings the smartphone near the HDU's antenna. The best and most reliable connection is when the smartphone's NFC antenna and the HDU's octagonal antenna are optimally aligned. The smartphone then writes the new MMV (e.g., 1000kWh) to the HDU (specifically, to the MAX66242 memory page) using an authenticated write along with the message authentication code (MAC) provided by the server. (Please note that the NFC smartphone is just a conduit for doing this authentication; it does not have the M-Secret nor does it have any knowledge of it.) Once this memory authentication is complete, the smartphone sends a short high-frequency (HF) message alerting the HDU that the transfer is complete. This short HF message toggles the PIO pin on the MAX66242, and the HDU's microcontroller (the MAXQ610) interprets this interrupt as an indication that a new set of energy credits is available. The microcontroller then reads the MAX66242 memory page and updates its own copy of the MMV. Only the server with the M-Secret can make this update, which is now displayed on the HDU's screen for the customer. The kilowatts remaining will be recalculated and displayed.

Field readings: Controlling fraud and electricity theft
Occasionally, to ensure that a security breach has not happened to the smart meters and/or HDU, the utility company needs to perform field readings. (Refer to step 7 in figure 5). Field readings will identify mismatches between the MMV value stored on the utility company's server and the one stored in the HDU. The HDU's MMV readings should never exceed the MMV stored on the server, so any mismatch between these two numbers indicates a tamper or even theft of electricity.

The field readings collect the total kilowatt hours currently stored in the HDU and upload it to the utility's server. There are two ways to collect field data: use PLC communication to link the smart energy meter to the HDU via the house's electrical lines, or let a service technician come and use a smartphone. The steps shown in figure 8 represent the field reading transaction implemented on the HDU demonstration platform.

1. The field technician uses a smartphone app to authenticate the meter by requesting a random challenge from the utility's server.

2. The smartphone instructs the HDU to perform a "Compute and Read Page MAC" for the memory page containing the MMV.

3. The smartphone sends the resulting MAC to the utility's server to validate the meter's authenticity. This method keeps the smartphone from knowing the M-Secret.

4. The utility's server computes its own MAC and compares it with the MAC received from the HDU.

5. If the two MACs match, the server responds with a "Good HDU" message to the smartphone. This means that the HDU is in good working order and has not been tampered with. If, however, the authentication process is not successful, this means that the HDU has been tampered with.

Figure 8: Service technician's checks for fraud. Any HDUs identified with this process would be removed from the field.

The electrical prepay system presented here allows customers to choose when they pay, how they pay, and how much they buy. Prepay lets customers take more control of their bills and utility usage. They will, of necessity, become more cognizant of their energy use over time.

One thing is sure—an advanced smart metering infrastructure (AMI) system outfitted with two-way communications, a remote disconnect switch, and an HDU with the DeepCover secure MAX66242 NFC tag authenticator provides an effective energy prepayment system. In areas like the U.K., Australia, and New Zealand where a prepay energy credit system exists today, customers know how much it costs to run certain appliances and they can relate that to a specific amount per day for electricity. This leads to the adoption of conservation measures, including turning off appliances, turning down a water heater, purchasing Green Star appliances, installing energy-efficient bulbs.

Such a framework will help utilities simplify business operational efforts, cut costs, reduce delinquent account risks, improve cash flow, provide immediate customer service, and encourage conservation. Simply put, this is a prepayment system which creates a win-win situation for the utilities, the consumers, and the world's resources.

[1] "Prepaid Electric Metering," Pike Research, March 2012,

About the author
Hamed M. Sanogo is an Executive Business Manager at Maxim Integrated and is responsible for the Secure Authenticator & NFC/RFID product lines. He graduated from the University of Alabama at Birmingham (UAB) with a BSEE, and earned an MSEE at the University of Michigan-Dearborn and an MBA in Technology Management at the University of Dallas.

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