Conference: World Meter Design Congress
Location: San Jose, CA, USA
Presenter: Andreas Beck
Abstract: Presented by Andreas Beck at World Meter Design Congress
During the recent decades, smart homes, energy management and prepayment developed to a stage, where it is known to nearly everybody.
The limited availability and also high prices for electricity created the need for reducing peak demands and accurate billing of consumers. Consumers refusing to pay for energy consumed lead to the development of prepayment systems. A switch was required, which is able to meet the requirements in such systems.
The disconnection and reconnection of loads requires a special switch (relay). It has to meet the specifications defined in the IEC 61037. The switch must be able to withstand over currents, until the circuit breaker responds with disconnection. The amount of switching operations of the relay under rated conditions must ensure a lifetime of the equipment of more than 15 years.
Relays, which are employed in metering equipment or disconnection and reconnection of consumers, are exposed to more severe conditions.
IEC 61036 and ANSI C12 specify the fault current conditions for an electricity meter. The specifications have been compiled long before the switch was introduced in metering equipment.
Experiences gained from the introduction of prepayment meters created the need for additional requirements. Specifically, where consumers are disconnected in case of non payment, there is a high potential for tempering. Changing the meter constant, keeping the switch into a closed position, partial bypass or complete bypassing of the meter are a few examples. However, the switch is usually the first item, the consumer tries to temper with.
Welding the contacts by creating shorts on the load side or controlling the status of the switch with a strong permanent magnet are again only a few examples.
IEC 62055-31 contains the latest requirements for the load switch in payment meters.
High contact currents create high forces, which would open the contacts and lead to a destruction. State of the art relays use high contact pressures for the driving system. In addition, the current loops through the switch itself are used to increase the contact pressure with increasing current.
The design of the bus bar arrangement in the equipment must not influence the contact forces within the relay. Following some basic guidelines will help to prevent unforeseen problems and surprises.
The contact resistance together with the load current results in a power consumption. The power consumption increased with the square of the current (P = I2*R). Special contact materials ensure low contact resistance and losses in the current circuit of the meter. All parts carrying the load current are made of high conductive copper. The cross section area is large enough to minimize impedances.
For very high switching currents, two contacts are arranged in parallel to meet the required performance. The total power consumption and self heating of the metering equipment can be estimated by taking all conductors and joints into account.
The sensitivity to shock and vibration as well as the generation of forces during switching is minimized thanks to the rotary actuator, which operates the switch. This reduces the constraints on the mounting of the switch into the equipment.
The different parts in the current loop of the switch are riveted together. Special riveting processes ensure good long term stability in difficult environmental conditions.
The manufacturers of metering equipment can benefit from this technology by procuring customized switches, which have the complete bus bar arrangement already attached to the latching relay.
All production steps for the latching relay are closely checked. This ensures a high quality level of the final product.