Scheme Design of External Miniature Circuit Breaker for Smart Meter

With the advancement of the construction of smart grids, the smart meter charge control system, as a basis for a wide range of quantities, has been valued by the State Grid Corporation of China and China Southern Power Grid Corporation. In order to further improve the reliability of the electric energy meter charge control system, strengthen the quality control of the external circuit breaker of the electric energy meter, and ensure the safe and stable operation of the intelligent management and intelligent service of the electric power system, the State Grid Corporation of China and China Southern Grid Corporation respectively formulated the "Electric Energy Table External Circuit Breaker Technical Specifications", China Quality Center (CQC) developed and launched the business of external circuit breakers for electric energy meters in May 2016.

The external miniature circuit breaker of the electric energy meter (hereinafter referred to as: miniature circuit breaker) is the key device that cooperates with the intelligent electric energy meter to realize the intelligent cost control function. The intelligent electric energy meter with relatively large rated current (>60A) generally adopts the miniature circuit breaker. Realize various fee control functions.

Figure 1 is a schematic diagram of a miniature circuit breaker. The miniature circuit breaker is used in conjunction with the smart meter, which can realize the functions of automatic opening and power failure for arrears, and automatic closing and power renewal for renewal. There are two wire connections between the miniature circuit breaker and the electric meter, which are the control signal wire and the feedback wire. The control line adopts the AC220V level mode. The power supply of the control unit is taken from the control signal line, and the phase line can be used for a short time when the switch is opened. The phase line leakage current needs to be less than 0.2mA after the action is completed. The feedback signal also adopts the AC220V level mode, and the internal series resistance is output after 100kΩ. The control unit contains a DC motor used to drive the actuator, which drives the circuit breaker to open and close through the action of the motor, and the actual position of the motor is detected by the position sensor.

Scheme Design of External Miniature Circuit Breaker for Smart Meter

figure 1. Schematic diagram of external circuit breaker for electric energy meter

According to the "Technical Specifications for External Circuit Breakers for Electric Energy Meters", miniature circuit breakers need to meet the following technical requirements:

1) Rated control level voltage: AC220V;

2) Rated control level current: ≤ 1mA;

3) Phase line leakage current: After the control unit is closed, the steady-state current consumed by each phase line is less than 0.2mA;

4) Rated frequency: 50Hz or 60Hz;

5) Automatic closing time: ≤3s;

6) Power-on delay: ≥ 4s.

The external miniature circuit breaker is a fully automatic circuit breaker, which has functions such as opening of arrears, closing of a payment, status indication of cost control, talk-keeping of arrears, and status feedback. According to the design requirements, Figure 2 shows the block diagram of the recommended miniature circuit breaker system.

Scheme Design of External Miniature Circuit Breaker for Smart Meter

figure 2. Block Diagram of Miniature Circuit Breaker System

The technical parameters of the miniature circuit breaker require that the phase line leakage current after the control unit is closed, the steady-state current consumed by each phase line is less than 0.2mA, so an ultra-low power MCU must be used. According to the function and performance of the miniature circuit breaker control unit, summarize the specific technical requirements of the MCU:

1) Low power consumption: no more than 150uA;

2) Flash: about 4KB;

3) RAM: about 1KB;

4) With on-chip EEPROM;

5) Package: 20-pin, TSSOP package;

6) ADC: 10-bit or 12-bit, 1 channel.

According to the specific technical requirements of the above MCU, we chose the MSP430FR2111 of the MSP430FR ferroelectric series, which is an ultra-low power MCU tailored by TI for miniature circuit breakers, power banks, electronic cigarettes and other fields.

The closing and opening commands issued by the MCU need to be executed by the DC motor, and finally drive the actuator to complete the action. The DRV8848 solution is used in this system. One set of control outputs directly drives the DC motor to execute instructions, and the other set of outputs is used to control the power supply of the position hall sensor DRV5023 to reduce the power consumption of the system after closing, so as to achieve zero ．Requirement of 2mA input current.

The miniature circuit breaker needs to take power from the AC power supply line, and the solution has high volume requirements, so the solution uses the UCC28910 primary-side control flyback solution with integrated MOSFET. UCC28910 is specially designed for isolated flyback power supplies, providing stable output voltage and current without the need for optocouplers. Its built-in 700V startup and intelligent power management module can ensure that the device achieves high-voltage startup. During normal operation, the internal high-voltage startup current source is turned off, so the program can achieve extremely low standby power to meet the 0.2mA input current requirements. The UCC28910 output voltage is 12V, which supplies power to the DC motor of the miniature circuit breaker and the position Hall sensor, and at the same time converts 12V to 3.3V through TLV70433 to supply power to the MCU.