Smart meters have been touted to consumers as being integral in helping them lower their utility bills. Armed with accurate information – such as consumption pertaining to specific times of day – consumers can then adjust their behaviors to shrink their bills and their carbon emissions and, if regulations allow, even select from different pricing structures based on peak energy demands. The accurate information smart meters provide also benefit utilities, offering site-specific information from remote locations that can be used to monitor power quality, eliminate manual meter reading visits, respond to storm or natural disaster service disruptions and help them better align power consumption with power generation. These devices have two main goals: efficient, seamless two-way communication between the utility and customer, and remote connect/disconnect functionality that can help turn power on and off when necessary.
Robust and accurate switching is essential
A typical smart meter power architecture consists of three phases: AC/DC offline switching, DC/DC switching for communications needs, and a connect/disconnect drive circuit. Since power comes off the main AC line, which can be low single-phase input voltage or a high, 3-phase input voltage, a switched mode power supply converts the power to the appropriate DC voltage. At that point, the voltage can range anywhere from 12 V to 40 V depending on the power needed to drive the relay. The digital signal processors, microcontrollers and the circuits required for two-way communications typically use 3.3 V, so a DC/DC converter provides that second level of switching.
The smart meter’s third power function is its ability to safely connect and disconnect the remote customer’s power during a shortage of power sources. There are some other circumstances where this functionality is useful such as when a customer leaves home for a long period of time or a change of occupancy takes place. The customer does not need to arrange for a visit from the utility to disconnect service, thereby saving both time and effort. Smart meters are also able to remotely detect unusual activity, making their disconnection capabilities particularly desirable where electricity theft is common or when a storm disrupts power to a neighborhood. An additional benefit for the utility is the ease in which it can turn off electricity if bills are unpaid.
A perfect example of smart meters’ ability to connect and disconnect after a loss of power is how they handle loads associated with compressors. The compressors have very high switch currents and need a minimum off time before cycling or else overheating can occur. According to Aung Tu, product line director at Fairchild Semiconductor, “The surges from power restoration can be reduced by delayed turn on and sequencing of loads and reducing peak surge currents. This is where the disconnect function shines by doing it remotely.”
All these remote operations are conducted via a service disconnect switch. In most cases, a dual-coil polarized latching relay is used. The disconnect driver circuit takes the higher voltage, for example from a 20-V rail, and regulates the relay open-close duration time. It also ensures current is immediately available to actuate the solenoids, since it sometimes must overcome fused contacts from when the relay is in the closed position while connected to the grid.
Single-chip relay drivers present new disconnect switch options
Designers have a number of options when implementing a disconnect driver circuit. Discrete solutions are always a consideration, but implementing the necessary protection functions would result in a higher component count and in turn, higher cost. Standard relay drivers are constantly enhancing their load handling capabilities as well as their ability to withstand high impulses. These drivers can be used in combination with RC delay timing.
IC vendors are also integrating many of the traditional disconnect driver circuit functions into a single device. Meters get their “smart” label thanks to their two-way communications capabilities, but the single-chip, fully integrated relay drivers that have emerged to enable service connect/disconnect functions have earned their “smart” moniker largely because of their timing functions. Since an external filter or the RC delay timing circuit is necessary to handle the wide input pulse variation – such as when the relay contact takes a relatively long time to travel between its stationary On and Off positions -- a fully integrated smart relay driver offers built-in protection features which limit pulse width or make modifications to ensure the pulse is longer than the duration required by the relay specification. Designers can also work with the IC vendor to adjust the timing functions on the relay driver: “If they have an application that requires the initial filter time or qualification time of 200 us or 20 us, we can program it for them,” says Aung Tu of Fairchild Semiconductor, which recently introduced the FAN324x dual-coil smart relay driver with integrated switches and an onboard timer. “Likewise, maximum can be 1 ms to 350 ms.” The FAN324x smart meter driver also features a strong DC current to break through welded contacts without using the external switches and a single bias voltage design to allow for isolated and non-isolated designs.
Another issue is excessive currents drawn from the supply rail which can result in two relays being On at the same time and in turn cause relay damage. Smart relay drivers offer integrated XOR protection which do not allow output pulses when two qualified input signals are received at the same time, allowing only 1-0 or 0-1 relay operation.
Single-chip smart relay drivers also offer protection features that can save component count and space compared to discrete relay implementations, such as internal thermal shutdown protection which can help smart meter manufacturers enhance safety where the meter must work with an older, perhaps unreliable socket. In addition, smart meters present a particularly noisy environment for the relay driver circuit since they are directly connected to the incoming feed from the grid and controlled by a microcontroller, so integrated noise protection is an important feature for smart relay drivers. Also, a smart relay driver offers a wide operating voltage that can handle up to 60 V which should ensure protection against surges and the associated relay damage. The wide operating is desirable for use with relays that have several voltage ratings, sometimes as high as 40 V and which are used throughout the world.
A worry-free rival to discrete solutions
Smart meter lifetimes can run from 10 to as much as 20 years. During that time, the service connect/disconnect function may not get used often, but when it is needed – such as when safety is at stake – it must work as intended. The protection features that are integrated into a smart relay driver make it a worry-free rival to discrete relay solutions. And with the assurance that the right operation is always done correctly, the service connect/disconnect switch in a smart meter can manage the grid in ways that didn’t seem possible in the days of mechanical switches.