Steve Drumm, Business Development Manager – Energy, Omron Electronic Components Europe BV
The growing markets for smart meters and solar panels have very specific design challenges and there are some distinctive considerations that apply when selecting relays and indeed switches for inclusion in these designs. This article explores the main areas in smart meters and solar panels where relays are used, and considers the relevant issues in the selection of a suitable relay.
New Standards for Smart Meters
Each successive generation of meters is typically smaller than earlier units and also draws less power. They also offer new ways of controlling for example the charging of hybrid electric vehicles, allowing this kind of non-critical function to be carried out at times of excess capacity. New functions to manage small scale local generators such as solar panels are also included. Some also disconnect the AC supply to the smart meter by PLC or by wireless M2M communication such as SMS, GSM or LW. Implementation of these functions normally entail the use of a relay. Relay manufacturers are maximising the benefits of smart metering by continually raising standards of accuracy, safety and reliability. By offering cost-effective relays, switches, photomicrosensors and connectors, they help to save power in electricity metering systems. What are the considerations for designers working in this field?
Relays in Smart Meter applications are now subject to a new standard: IEC 62052-31:2015(E) launched in September 2015. This specifies product safety requirements for equipment for electrical energy measurement and control and replaces IEC 62055-31. It applies to newly manufactured metering equipment designed to measure and control electrical energy on 50 Hz or 60 Hz networks with a voltage up to 600 V, where all functional elements, including add-on modules are enclosed in or form a single. It also applies to metering equipment containing supply and load control switches, but only those which are electromechanical in operation and is applicable to auxiliary input and output circuits.
Most smart meter manufacturers now require Utilization Category 3 (UC3) compliance for latching relays whereas previously UC1 or UC2 was seen as sufficient. UC3 requirements entail that the relay should withstand higher short circuit or over-current events.
Forward looking relay manufacturers are extending their latching relay portfolios to address the opportunity represented by the new generation of smart meters. They are reducing the profile of their latching relay devices and offering solutions capable of handling the potentially high inrush currents. Much effort is being made to improve the efficiency of relays where the key characteristics of switching capacity, switching endurance, coil type, load isolation and physical construction can have a notable impact on the overall energy efficiency.
Design features that can facilitate this application include the use of twin contacts to improve overcurrent capability, enhancing safety. Twin contacts can also reduce power loss and suppress temperature rises within the component. Changing the orientation of the coil layout can also reduce contact welding, improving over-current capability and electrical life. Features like this not only ensure true UC3 overcurrent protection but also reduce the energy consumed in the smart meter.
Clearly, the power consumption of the smart meter itself is a concern to designers, and latching relays which are only energised while switching and use less power than non-latching designs are therefore popular. Although the wattage is normally quite low, if you multiply this small number up across the millions of meters in use on the grid, it does add up. A self-latching relay is recommended, which only uses power during the switching process and doesn’t have to be electro-magnetically held open.
Smart Meter AC Disconnect
The two key areas for relays in a smart meter are AC disconnect, for disconnecting the supply, and auxiliary load management, allowing the consumer or the utility to switch in and out non time critical loads depending on the time of day and the level of load on the grid.
AC disconnect relays can have a particular impact on power consumption, as the high load currents can bring about self-heating which is inefficient. They can also be open or closed for extended periods, so really a latching design should always be used here. Designers should consider a relay such as the Omron G9T which has been specifically engineered for this application. It employs a proprietary double contact architecture which minimises self-heating.
Smart Meter Auxiliary Load Management
For local load switching and time of use functions, the relay is there to make rather than to break the circuit. A latching, single pole electro-mechanical relay with a maximum current rating of up to 120A at 250V AC would meet the needs of a smart meter employed in a commercial, residential or light industrial environment.
For higher loads up to 16A, solutions like Omron’s G2RL which is just 15.7mm high, are a popular solution. Despite its low profile, the G2RL meets VDE0700 requirements for household products according to VDE0110. Tracking resistance conforms to CTI>250, and the relay is available with a UL1446 Class F Coil Insulation. Dielectric strength between the coil and the contacts is 5000VAC. Designed for PCB mounting, the G2RL is offered with a flux protected or a fully-sealed case. These sensitive relays have a coil power consumption of just 400mW.
Latching designs such as Omron’s G5RL-U and the G5RL-K can switch up to 16A and are capable of dealing with high inrush currents of up to 150A. Clearly the utility has no control over the type of load that the consumer connects through the meter, and it needs to be capable of dealing with high inrush currents associated with inductive loads such as lighting circuits.
For lower loads even more compact solutions are available. Advanced construction makes the 10A G5Q-EU smaller than comparable designs, with an overall case size of just 20.3mm x 10.3mm x 15.8mm. Designed to switch 10A loads at the prevailing European 250VAC mains, Omron’s G5Q-EU compact single pole relay features a low power coil to save power supply size and cost, and tracking resistance that meets or exceeds CTI250. If the load is just 5A, there are other options. The G5NB is just 15.3mm high and 7.2mm deep and the G6B has a height of just 10mm.
Solar PV grid protection
Micro-generation of electricity from solar, wind and other sources can provide a great contribution to a sustainable green economy and relay manufactures have been developing components to meet the associated high-tech demands. In the interests of energy efficiency, non-latching relays are utilised for both bypass and DC disconnect functions from individual panel to complete DC circuit coverage.
The high power DC relays required in renewable energy installations like solar panels have specific challenges. With loads as high as 200A at 400VDC to be switched, arcing during switching can be a significant issue, resulting in deterioration in the contacts and a reduction in relay life. Components designed for this application, such as the Omron G9E series, employ a unique gas-filled construction, with magnets to contain the arc, eliminating the arc space normally required. The relays are therefore compact compared with alternatives offering similar performance, with overall dimensions of 36mm x 72mm x 67mm.
Grid tied solar PV inverters use pairs of monostable grid protection relays whose operation is essential to ensure system and user safety. The key challenges in the design of these relays are that their power consumption has to be extremely low yet they need to operate very quickly when required under hot start conditions. Omron has re-engineered its G7L-PV specifically to meet these requirements, with a fast operate time of typically 20ms, long life and a low power coil. It addresses the applicable standards which are IEC62109-1, 2 (July 2011) and VDE0126-1. The new Omron G7L-PV features a large contact gap of over 3mm to ensure working voltage and over-voltage standards compliance. Contact rating has been increased to 280VAC with a 30A, AC7a Inductive load. Other changes made to the G7L-PV include re-profiling of both the stationary and wiping contacts, slightly harder contact material, stronger coil spring, changing magnet wire from Class B (132 deg.C) to higher temperature tolerant F (150 deg.C) and spool material enhancement.
Solar panel disconnection
An issue of growing importance is the challenge of de-energizing solar PV panels and other renewable energy sources in the event of a fire in the building or other emergency. To do this safely requires a high power DC relay capable of rapidly interrupting 25A at 1000V DC quickly in the event of an emergency. In response, Omron has launched the G7L-X relay, capable of handling this load with a release time of just 30ms. This relay can be installed close to a PV panel or other renewable source, and disconnect it quickly in an emergency, to permit maintenance or in response to utility requests. The G7L-X conforms to UK and IEC solar inverter standards, as well as applicable UL and VDE electrical standards. Features of the design include an assisting magnetism circuit, to help achieve a very quick switchover. The relay contacts have been specifically designed to handle the very high DC loads that are a feature of this application, and have a contact gap of 6.0mm for safety.
Price hikes in energy costs are leading those with solar PV arrays to look at storing the excess electricity generated by their systems. Relay designs developed to enable battery management in electric vehicles can be applied to these designs too. This application entails interrupting high currents without the contacts deteriorating, and Omron has addressed this issue with a proprietary high efficiency magnetic circuit that suppresses arcing. The result is the G9EJ-1-E which is capable of interrupting 15A at 400VDC yet draws only 1.2W while switching. It also benefits from Omron’s proprietary contact driving system, which improves inrush current withstand performance and enhances component life.
Smart meters and control electronics for renewable energy present significant market opportunities for the electronic industry. Manufacturers of relays and switches have responded with innovative designs, which use less energy and have carefully designed contacts to prolong life in these very specific applications. We hope that this review of the available solutions will help designers in these markets understand the issues involved, and make the best selection from the wide range of solutions available.