Small and intelligent medium voltage circuit breaker

World renowned switchgear companies have had small intelligent circuit breakers for a long time, such as Eaton's T-VAC vacuum circuit breaker, as shown in Figure 1. This vacuum circuit breaker integrates intelligent protection relays with a phase spacing of 150mm and a circuit breaker width of 431mm. It is equipped with Digitrip intelligent comprehensive relay protection, self powered digital trip protection, small current sensor size, long delay, short delay, instantaneous and grounding protection, power factor, voltage, current, harmonic distortion value and waveform recording, trip time recording, range selection interlocking to clear the fault of the circuit breaker closest to the fault without time lag, reducing fault damage, avoiding unnecessary and wasted downtime, high-precision measurement, measurement of electrical energy, power quantity and power quality, Modbus communication protocol.

General Electric (GE) uses the operating mechanism of the low voltage frame circuit breaker GLOBAL ACB for the medium voltage intelligent circuit breaker iVB. Based on the concept of integrated design, GE's iVB vacuum circuit breaker hopes to achieve the intelligence of the equipment by highly integrating the electronic sensing unit and protection system. The primary equipment, secondary signal, and protection are perfectly integrated into one, maximizing functional integration and creating maximum value for customers. It is a circuit breaker with a "brain" self tripping function.

IVB is also a miniaturized circuit breaker with a phase spacing of 150mm and a circuit breaker width of 480mm, which can be applied to 550mm switchgear. The intelligent protection system for circuit breaker protection communicates through Modubus, and the circuit breakers are connected using twisted pair cables. Real time monitoring and control of the parameters and status of all circuit breakers in the line are achieved through the touch screen of the switchgear. The iVB intelligent circuit breaker is a fully integrated product that replaces traditional switchgear discrete components such as current transformers, comprehensive protection relays, secondary wiring, etc. The total cost of the circuit breaker is lower than the total price of traditional discrete components. A circuit breaker is like an outgoing cabinet.

The latest generation compact iVD4 vacuum circuit breaker from ABB, as shown in Figure 2, uses recyclable thermoplastic material PT integral casting solid sealed pole, which has greatly improved mechanical and low-temperature performance. Its insulation capacity greatly exceeds the requirements of international and domestic power industry standards, and is environmentally friendly with a phase spacing of only 135mm and a handcart width of 414mm. It is an intelligent solution and a comprehensive switch equipment asset health management solution,

Real time temperature monitoring of thoughtful switchgear, accurate measurement of main circuit temperature, and identification of safety hazards; Intelligent motor drive control and protection system, supporting "one click sequential control" programmatic operation, achieving safe and efficient operation and maintenance; Combining user monitoring with ABB cloud technology for real-time data analysis; By accurately predicting early faults in circuit breakers, one can control their health status; Mobile clients or browsers can access operational data anytime and anywhere, supporting Android, iOS, and Windows operating systems. Complete experimental verification has been conducted in accordance with NB/T42044 "3.6 kV~40.5 kV Intelligent AC Metal Enclosed Switchgear and Control Equipment".

The insulation problem of miniaturized circuit breakers

The biggest problem with miniaturization design is to solve the insulation problem, especially the insulation problem between phases. When the phase spacing is around 150mm, an arc extinguishing chamber insulation cover structure or a fixed sealed pole form can be used. Even if the phase spacing is small, it is necessary to seal the pole columns, and a clear air distance of more than 20mm needs to be maintained between the sealed pole columns. Based on capacitance voltage division calculation, if the air spacing is too small, it will cause the electric field strength between the air to be too high, making it difficult to withstand power frequency withstand and lightning impulse withstand voltage, resulting in insulation failure.

Operating mechanism of miniaturized circuit breaker

The size of the general operating mechanism is too large, and due to the miniaturization of circuit breakers, it is recommended to use vacuum arc extinguishing chambers with light contact weight, low opening speed, and small stroke. This reduces the operating power required by the mechanism, effectively reducing the wear and tear of the operating system, and thus reducing the frequent maintenance required for the circuit breaker. If using the operating mechanism of low-voltage frame circuit breakers, Eaton's T-VAC, GE's iVB, and ABB's VD4 vacuum circuit breakers all use the same operating mechanism as low-voltage frame circuit breakers. VD4 adopts a modular EL type operating mechanism with spring energy storage and free tripping function. The main spring is included in the module, and the mechanism body comes with a manual energy storage handle, which gives the mechanism complete driving function, precise action characteristics, and stable expected life. Optional secondary accessories that can be easily and quickly installed. The simple design concept brings high reliability to the components.

The issue of temperature rise in miniature circuit breakers

The small phase spacing of circuit breakers results in significant additional heat loss due to skin and proximity effects, and the small size makes it difficult for heat to dissipate, especially in the form of fixed sealed poles. The heat generated by vacuum arc extinguishing chambers is concentrated inside the sealed poles, and the thermal conductivity of both engineering plastics and epoxy resins is low. Therefore, it is necessary to reduce the resistance of the arc extinguishing chamber contacts, increase the area of the upper and lower supports appropriately, and reduce the resistance of the handcart circuit breaker contacts to reduce heat generation. If the insulation problem can be solved, the use of insulation frame form can effectively solve the temperature rise problem. For example, T-VAC circuit breakers use DMC cast insulation brackets, and the basic frame meets the rated lightning impulse withstand voltage requirement of 75kV for products with a rated voltage of 12kV. By adding phase to phase insulation plates, the insulation requirement of 95kV for products with a rated voltage of 17.5kV can be met, making it highly versatile. Vacuum arc extinguishing chambers, flexible connections, etc. are exposed to air, and heat is directly dissipated, which is beneficial for temperature rise. The schematic diagram of the insulation frame of the miniature circuit breaker arc extinguishing chamber is shown in Figure 3.

Integration of intelligent protection terminals

The intelligent terminal of circuit breaker includes functions such as system measurement protection, mechanism status monitoring, monitoring of key performance, remote control, etc. The system measurement and protection functions mostly use intelligent digital trip units, which integrate the functions of switchgear comprehensive protection. For example, T-VAC and iVB circuit breakers use intelligent trip units similar to low-voltage frame circuit breakers to achieve real-time measurement of current, voltage, active power, harmonics, and fault diagnosis. The automatic trip of long delay, short delay, instantaneous action, etc. is the basis for realizing the intelligence of circuit breakers; The monitoring of the status and key performance of the mechanism is the monitoring of temperature rise, partial discharge, vacuum degree, operating mechanism coils and motors, opening and closing speed, etc. It can be said to be a supplement to the intelligence of circuit breakers. The electric operation of rocking in and out further enhances the operation and maintenance functions.

Therefore, intelligence should be based on the premise of system intelligent protection. Intelligent protection is based on digital microcomputer processing technology, which can achieve autonomous tripping without the need for external power supply. System calculation is achieved through digital processing of small current and voltage values. For example, the output voltage of the Roche coil current sensor is 150mV. Therefore, integrating current and voltage sensors and power transformers on the circuit breaker can reduce losses, improve protection speed, and increase reliability. Remote setting and control of power distribution system through communication interface to achieve automation.

Performance and status monitoring require monitoring of the primary and secondary circuits of the circuit breaker, which can be measured by invasive sensors or non invasive calculation analysis. From the perspective of safety and reliability, more and more intelligent products adopt non-contact monitoring, such as current monitoring of opening and closing coils and energy storage motors, which is achieved through intelligent monitoring units installed inside the switchgear instrument room without damaging the overall performance of the circuit breaker. Temperature rise measurement is carried out through wireless transmission of strap temperature measurement, which does not affect the operation system of the circuit breaker.

Installation issues of intelligent sensors

The intelligent circuit breaker adopts a Roche coil current sensor, which needs to be installed on the high voltage contact arm, as shown in Figure 4. Due to the small phase spacing of the circuit breaker, the sensor is a low voltage component and installed on the high voltage contact arm. It requires primary and secondary low voltage isolation and electrical insulation to ensure the withstand voltage level of the circuit breaker. Generally, it adopts a fixed sealed structure and is poured together with the arc extinguishing chamber pole, which is costly, has poor universality, and cannot be maintained or replaced separately; There are also current and voltage integrated sensors that use insulation support, and integrated control power transformers, which are small in size and easy to install.