Latching relays and Current transformers for energy meters
How to design a proper latching relay for electricity energy meters.
Compared to other electric control switches, latching relay has the ability to save amounts of energy significantly and is a low-cost component. Furthermore, the performance of the latching relay has been tested in market applications for many years and is the preferred control component for energy meters, and they are increasingly being used in the smart meters as well. In practice, it is often the case that the energy meter designer and the manufacturer don’t understand the structure and characteristic of latching relay very well, leading to misunderstandings in the requirements and choosing of standards which impacts the quality control of the payment energy meter to some extent.
The technical specification of the latching relay means that a series of technical requirements need to be applied to the design of the relay. The latching relay can only be used in an energy meter if it meets the technical requirements, however, if the technical specification is too strict and there is too much redundant ability, the cost will be unnecessarily high.
Currently, most specifications of latching relay are based on part 1 of the IEC61810-1“Electromechanical Elementary Relays–Part 1: General and safety requirements”, however, not many manufacturers have considered the special requirements of this standard. IEC 61810-1 only states the base requirement of a relay, and some items only state the principle of testing method. As to the performance indicator of a relay, it only states a series of preferential parameters. The relay performance levels are very different, although they are all according to the IEC61810-1. So to determine the performance index properly is critical to specifying a latching relay. In light of this fact, the relevant standards of the electric energy meter should be studied before the specification of latching relay is made, and the parts are selected. The specification should cover the relevant requirements; further, the relay manufacturer should investigate the special requirements from the energy meter manufacturer who will apply the magnetic latching relay, and should communicate and cooperate with the meter manufacturer to find a solution which satisfies both parties.
The main items of magnetic latching relay specifications are as following:
1. Performance requirement: the contact switching load of latching relay during normal operating, including voltage value, current, the nature of the power and the load characteristic;
2. Safe requirement: including the insulation level between the conductors which have different potentials, the level of lightning impulse withstand voltage, the clearances and creepage distance between the insulated conductor, the temperature rising and the fire resistance;
3. Reliability and durability: including electrical life mechanical life and the failure rate;
4. Fault resistance: such as over load, short circuit current, strong magnetic field, strong shock and vibration;
5. General performance: including rated input power (coil power) dimension, weight and operation time etc.
6. Environmental adaptability: including environmental temperature, humidity, atmospheric corrosion, and the degree of pollution etc.
7. Other features: like the performance required for meter assembly (solderability, solder heat resistant) and the logo etc.
Taking the above standards into account, it is important to consider the economic factor as well, in terms of cost reduction within the correct parameters. The importance of each performance indicator is not the same for a relay. Based on the level of its importance we are inclined to list the items in order of above. According to this principal, to specify a latching relay should follow these steps:
a) To determine the functional level to meet the requirements of the load switched;
b) To determine the security requirement according to IEC61810-1 and demand of energy meter standards;
c) Reliability and durability should be in accordance with the relevant stipulation of the energy meter standard and user requirements and failure rate could reference to IEC 61810-1;
d) The capability of fault resistance should be in accordance with the relevant stipulation of the standard energy meter (for example, the stipulation about the capability of short-circuit current withstanding in IEC 62055-31) and should be determined by conferring with users. The protection against the strong magnetic field, strong vibrations as well as strong shock is not necessary for the performance of an energy meter. Under normal circumstances, energy meters do not withstand these stresses. This is because of additional requirements to prevent the meter users from electricity stealing. Neither International standards or National standards for energy meters or relays have yet requested for this. At present, some energy meter manufacturers have raised the demand in this respect, but have not yet put forward specific level ;
e) General performance should be based on technological possibilities and determined in consultation with users. Because it is a magnetic latching relay and does not consume electricity after action, and the operational frequency is very low in energy meters, we suggested that the indicator should be relaxed. This should improve reliability and reduce the difficulty of designing and manufacturing, thereby reducing the cost of relay. Meanwhile it would not reduce the energy meter performance or increase the cost of energy meter significantly, in addition, because the meters are used in fixed equipment.
In order to design and manufacture conveniently and reduce the cost, the restriction of dimensions, weight and operational time should not be too strict;
f) Environmental adaptability could be determined according to the internal environment of an energy meter, also according to the recommended values of IEC 61810-1 and determined in consultation with the user;
g) Other requirements refer to other provisions of the IEC standards. Yueqing RunEasy Electronic (Topro Technology) developed a wide range of latching relays, conform to the relevant standards of energy meters in conjunction with latching relay characteristics and eliminates the defects that existed in previous latching relays, such as: phase lacking, the carrying ability to short-circuit current under standard IEC62055-32 UC 2 and UC3, insufficient vibration resistance, high temperature rise etc.
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