Electrical energy is not unlimited and prices continue to increase. The most efficient way to preserve energy and reduce cost is demand side management (DSM). DSM includes reduction of peak demand, energy saving through individual load switching, accurate billing of consumers and the disconnection of consumers refusing to pay for electricity.
Efficient DSM has become possible now that modern electronics and latching relays are available. The electronics are important for measurements, time keeping, intelligent load switching and many other features in modern DSM systems. The latching relay is the key component for the disconnection and reconnection of loads and consumers.
STANDARDS AND REQUIREMENTS FOR THE LATCHING RELAYS
The latching relays have to comply with several international standards. Some countries and utilities specify additional requirements to cater for their specific needs
IEC6180 covers basic parameters and definitions.
Part 1 specifies the general and safety requirements; and part 2 covers reliability of the contact system with different loads. This standard is applicable to basic relays (all-or-nothing relays).
IEC 62052 part 21 defines the requirements for tariff and load control, and IEC 62054 part 11 covers ripple control receivers (formerly defined in the IEC 61037). The specifications ensure a lifetime of the equipment of more than 15 years and the switch must withstand over currents until the specified breaker or fuse interrupts the current. IEC 62055 was introduced recently. Part 31 contains the latest particular requirements for the load switch in payment meters. It defines the payment meter load switching utilisation categories UC1 to UC4. UC1 are the basic requirements as defined in the IEC 62052 part 11. For UC2 and higher the switch shall also be capable of switching into fault currents as this may be experienced under fault conditions in a payment meter installation. The prospective test currents are 2.5kA (UC2), 3.0kA (UC3) and 4.5kA (UC4). The test is repeated three times on the same sample where the contacts shall open on the first attempt after each make cycle.
THE OPERATING ENVIRONMENT
The latching relays for DSM are often employed in harsh environments.
Over currents introduced by accident or deliberately are quite common. Faulty equipment, shorts in the wiring and connection of additional loads may result in switching currents higher than the permissible rated current of the relay. In such cases, the relay must withstand these currents until the breaker or fuse responds.
Experiences gained from the introduction of prepayment meters created the need for additional specification requirements. Specifically, where consumers are disconnected in case of non payment, there is a high potential for tampering. Changing the meter constant, keeping the switch in 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 tamper with. There are examples of keeping the switch closed with magnets and of consumers trying to weld the relay contacts by creating unprotected short-circuits over currents. The switch must be able to withstand the fault currents in such environments.
Many prepayment installations are located in informal areas. The meters are mounted on poles or on outside walls, exposing them to extreme environmental conditions. On winter nights, the temperature can be below zero; during hot days, the equipment can heat up in the sun to temperatures above 50ºC.
Any additional self-heating adds to the already extreme conditions. The impedance of the current circuit including the switch must be as low as possible. The mechanical and electrical characteristics of the switch must ensure reliable operation.
The equipment can be exposed to aggressive atmospheric conditions, specifically in coastal regions. All materials and joints must endure many years without failing or falling outside their specification as a result of corrosion.
THE LATCHING RELAY TECHNOLOGY
The relay contacts are made of specific material. This delivers a good compromise between lifetime (switching operations) and over current capabilities. The shape of the contacts is important as it defines the start and characteristics of wear over lifetime.
During switching, any bouncing has to be minimised to avoid welding and premature wear. The contact force has to be defined very carefully. Too much force can lead to sticking contacts – too little force results in excessive contact resistance.
State of the art latching relays employ a technique that adjusts the contact force at higher currents using the magnetic fields generated in the conductors inside the relay.
The contact arrangement must withstand tamper attempts with over currents. Double (parallel) contacts are used to increase fault current withstand and to reduce contact impedance.
The driving system usually consists of an actuator with a permanent magnet and an electromagnetic system with a single coil or a double coil winding. The driving system must be insensitive to tamper attempts with strong magnets or shock and vibration.
BUS BARS CONNECTIONS
The relay has to be connected to the meter terminals. The current loop should have as little impedance as possible. High conductive copper with sufficient cross sections is required. Any joints must be reliable over the lifetime of the meter. Poor joints may result in punctual self heating, material migration, oxidation and quite possibly premature failure.
In many cases, the connection is made with screws – specifically, where meter manufacturers use the lowest cost relay versions with small bus bar connectors. The materials for screws, washers and nuts have to be chosen correctly to avoid corrosion. The correct torque has to be applied during tightening.
The ideal situation would be for the complete bus bar configuration to be part of the relay. However, in many cases, this would not be economical as the material wasted during the punching of the bus bars adds unacceptable costs.
Welding customer specific bus bars onto the relay is another option. However, the welding process and the heat may result in deformation and change in material properties.
Riveting appears to be a good compromise. The different sections of the bus bars arrangement can be optimised regarding its material and surface. The different parts can then be riveted together to form the complete current loop including the switch.
Characteristics of good riveting joints are:
- The rivet fills perfectly the internal surface of the punched bore. It’s a so-called gas tight connection.
- Bus bars and contact spring are not deformed.
- The connection assures a laminar current flow and therefore avoids punctual heat rise.
Excellent results are only received with the right production equipment. For the riveting process special eccentric or toggle presses must be used as well as presses with controlled press force sensors. The machines have a fine adjustment to assure reproducibility. The tool is exactly adapted to the structure of the rivet and all pieces are pushed together while applying the forces.
This ensures good long-term stability in difficult environmental conditions.
THE METER DESIGN
It is recommended that equipment manufacturers contact the relay supplier at an early stage in their developments. The bus bar configuration influences cost, self-heating and reliability. The location of the switch inside the equipment can improve resistance to magnetic fields. The selection of the coil impedance may have implications for the driving electronics. A close cooperation ensures optimised performance, quality, reliability and cost.