1. Problems of using AC circuit breakers in DC circuits With the development of socialist construction, more and more areas and departments are using DC power, such as power generation and distribution systems, traction vehicles in cities and mines, electrified railways, chemical industry, metallurgy and defense industry, etc. Can plastic case circuit breakers (and universal circuit breakers) designed for AC 50Hz be used for DC circuit protection? Years of practice have shown that they can be used, but the current setting value and wiring need to be modified or corrected. 1.1 Protection characteristics 1.1.1 Overload long delay protection characteristics (1) Thermal type (bimetallic element trip unit) Direct heating by bimetallic element (direct heating type) or heating by heating element, the heat is transferred to the adjacent bimetallic element (side heating type). Since the heating effect of the average value of DC current is the same as that of AC effective value, there is basically no change between AC and DC. However, when the rated current is greater than 1500A, due to factors such as the skin effect, the heat generated by the effective value of the AC current is greater than that of the DC current, but the impact is not too great. Therefore, the thermal bimetallic element trip unit does not need to be modified when used in DC. However, some specifications of the thermal type, such as the rated current of 600A and above, use the CT type (current transformer type), which uses the secondary current of the transformer to supply the bimetallic element or heating element as a heat source, and therefore cannot be used in DC circuits. (2) Full electromagnetic type (hydraulic type, commonly known as oil cup type trip unit) When the AC (50Hz) circuit breaker (its trip unit is full electromagnetic type) is used in DC circuit, the operating characteristics will change. The range of change is 110%-140% of the original operating current (this is considering that most of the DC power supply is rectified and filtered by AC). Therefore, when using a full electromagnetic AC circuit breaker in a DC circuit, its trip unit needs to be redesigned. 1.1.2 Instantaneous Trip Units: For those using electromagnets as electromagnetic trip units (instantaneous trip units), the instantaneous current setting value will vary, and the rate of change depends on the DC waveform. For DC circuits that have undergone AC rectification and filtering, the instantaneous trip setting current is approximately 1.30 times that of AC. If the AC instantaneous trip is 10In (In is the rated current), then the circuit breaker used in a DC circuit should be adjusted to 13In during factory testing, and its setting error value (e.g., ±20%) can remain unchanged. 1.1.3 Electronic Trip Units: Whether for overload long-delay or short-circuit instantaneous trips, electronic trip units originally designed for AC 50Hz cannot be used in DC circuits. 1.2 Short-Circuit Current Flow: As is well known, 50Hz AC current crosses zero 100 times per second. At this instant, there is no current, and even if there is voltage between the moving and stationary contacts of the circuit breaker (for a purely inductive load, U is at its maximum when i=0; the same applies to a purely capacitive load; for a purely resistive load, i=0 and U=0), the arc is relatively easy to extinguish. However, the characteristic that DC current has no zero-crossing point makes arc interruption quite difficult. When using an AC 50Hz circuit breaker as a protective device in a DC circuit, a multi-break connection method must be adopted. There are two types of multi-break connection methods: series and parallel, as shown in Figures 1 and 2. The DC voltage used by the various circuit breaker models in Figure 1 should be 250V. For DC systems, as long as the voltage value is consistent, the undervoltage release and shunt release originally used for AC do not need any changes and can be used in DC circuits. 2. Circuit breakers designed for 50Hz are used in high-frequency circuits. In industrial applications, some sectors design drive motors at higher frequencies. For example, high-speed motors used in the textile industry have frequencies of 100-120Hz, while motors used in the woodworking industry have frequencies of up to 300Hz. Some equipment in the aerospace industry has a specified frequency of 400Hz, etc. (The motor speed n, n = 60f/p, n is proportional to the frequency f). If a circuit breaker designed for 50Hz is used in a high-frequency circuit, what changes need to be made to the circuit breaker's characteristics? 2.1 Issues with conductor heating and reduced load capacity. AC conductors have skin effect and proximity effect. The skin effect, etc., means that only a portion of the conductor's cross-section is used to carry current. Therefore, the AC resistance of the conductor will increase linearly with increasing frequency. For a 10mm diameter copper wire, the usable cross-section is: approximately 60% of 50Hz at 1kHz; and approximately 20% of 50Hz at 10kHz. In addition, this high frequency will also generate magnetic induction in the ferromagnetic materials adjacent to the conductor, causing "hysteresis loss". "Hysteresis loss" increases rapidly with the increase of frequency, and "eddy current loss" will also be generated in the adjacent metal materials. "Eddy current loss" also increases with the increase of frequency. Therefore, as the frequency increases, the resistance of the conductor increases due to the skin effect, and the hysteresis and eddy current losses of the metal materials of the adjacent components connected to the circuit breaker, such as the shaft, operating mechanism, and fasteners, increase. If the same load current is used, the temperature rise of the circuit breaker will inevitably exceed the specified value of its product, causing thermal aging of the product and reducing the service life of the circuit breaker. For the above reasons, the load capacity (rated current) of the circuit breaker will change according to formula (1): Where fx is the power grid frequency higher than 50Hz, In(fx) is the load capacity (rated current) when the frequency is 50Hz, and In(50Hz) is the load capacity (rated current) when the frequency is 50Hz. The relationship between the allowable operating current value and the frequency in Table 1 is calculated according to formula (1), as shown in Table 1. As shown in Table 1, from approximately 100Hz onwards, in order to maintain the allowable heating limit, the allowable operating current (rated current) must be reduced accordingly. 2.2 Busbar load capacity When the frequency exceeds 50Hz, the allowable operating current (rated current) of the busbar can be obtained according to formula (2): Formula (2) reflects the utilization rate of the conductor cross section. At 1kHz, In (1000Hz) = 22%In (50Hz); at 10kHz, In (1000Hz) = 7%In (50Hz). 2.3 Short-circuit breaking capacity of circuit breakers at high frequencies When AC electrical appliances interrupt short-circuit current, the arc is extinguished at the zero-crossing point of the current. Only by adopting arc-extinguishing means after the zero-crossing point to prevent the arc from reigniting can the arc be effectively interrupted. At higher frequencies, the current rapidly crosses zero one after another, shortening the arc cycle of each half-wave. The degree of ionization in the break gap (between the open moving and stationary contacts) is less than that at 50Hz. Due to the short cycle, the degree of deionization at high frequencies is also less than that at 50Hz. Therefore, the extinction of the arc at high frequencies has both advantages and disadvantages compared to 50Hz. According to actual measurements, the short-circuit breaking capacity does not change much in the range of 50-100Hz. Even at 200-400Hz, the reduction in breaking capacity is only about 10%. 2.4 Changes in the tripping characteristics of circuit breakers at high frequencies (1) Overload long-delay inverse-time characteristic (thermal tripping, i.e., bimetallic element tripping), which is directly heated by the heating of the working current (or by the heating resistor element, which transfers the heat to the bimetallic element on the side) or by the secondary current of the current transformer, causing the bimetallic element to bend due to heat. After a certain no-travel delay, it finally pushes the tripping element to trip the circuit breaker. Within the frequency of 500Hz, the main heat comes from the normal heating of the conductor. The induced heat caused by high frequency is actually very small and can be ignored. The tripping time is only slightly faster than at 50Hz. When the frequency is greater than 500Hz, the induced heating cannot be ignored. The higher the frequency, the shorter the tripping time. Therefore, when used in power supplies of 50Hz or higher, users should place special orders with the manufacturer, who will use suitable materials and adjust the distance of the bimetallic elements during assembly to meet the needs of high-frequency circuits. (2) Instantaneous trip unit: The instantaneous trip unit used for short circuit protection with non-delayed electromagnet overcurrent trip unit is a current-type trip unit. Its action is directly proportional to the magnitude of the current passing through the electromagnet (strictly speaking, the generation of electromagnetic attraction is directly proportional to the product of the magnetomotive force passing through it, i.e., the current value and the number of turns of the coil wound around it). In addition to the current value, it is also closely related to the duration of the current. At a frequency of 50Hz, the electromagnetic trip unit can operate at about one half-wave peak current, that is, the attraction force generated within half-wave time is sufficient to attract the moving iron core (armature) of the trip unit to its terminal position, thus tripping the circuit breaker. At higher frequencies, the half-wave time becomes shorter, and the armature can only operate when the effective value is reached. Obviously, without increasing the current value, it is impossible to operate at high frequencies when the half-wave time is only 50% (for 100Hz), 25% (for 200Hz), or even 12.5% ​​(for 400Hz). Some large foreign manufacturers have conducted extensive tests on power supplies with frequencies ranging from the lowest 162/3Hz (used in the United States) to 500Hz. The tests show that the higher the frequency, the greater the current value required to be increased, and the linear increase ratio is approximately 1:√2. That is, if the current is 500Hz when the frequency is 16×2/3Hz, the current value needs to be increased to 500×√2＝707A when the frequency reaches 500Hz. (3) The national standard for undervoltage release and shunt trip release stipulates: Undervoltage release: 70%-35%Ue, release (circuit breaker opens) 85%-110%Ue, close (circuit breaker closes) Shunt trip release: 70%-110%Us, engage (circuit breaker opens) Ue is the power supply voltage and Us is the control voltage. Since both of the above release devices are voltage coils, their electromagnetic attraction depends on the Φ value, Φ＝BS. The magnetic flux Φ is designed according to the formula U/≈4.44 fBSW. BS= U/4.44 fW, where B is the magnetic induction intensity and S is the cross-sectional area of ​​the electromagnet core. Assuming the electromagnet's attraction force is FF∝B²·S, the proposed undervoltage or shunt trip unit designed for 50Hz for use in high-frequency circuits indicates that the electromagnet's structure (core type, cross-sectional area, magnetic circuit length, etc.) remains unchanged. The attraction force F is proportional to B and S, and the value of B is related to the applied voltage, frequency f, and number of coil turns W. After conversion, for an AC electromagnet (voltage coil), the attraction force F is: where lc is the magnetization length of the iron core, δ is the air gap between the iron core and the armature of the electromagnet, μ0 is the permeability of the air, and g is the leakage flux per unit length. From equation (3), it can be seen that if the attraction force F is kept constant, U, S, lc, δ, g, etc. are all unchanged, but if f increases, the number of coil turns must be changed so that the product of fW remains unchanged. If W is 10,000 turns at 50Hz, fW = 500,000 ampere-turns, and now f becomes 500Hz, then the number of turns should be reduced to one-tenth of the original number of turns. According to the above, the user should apply to the manufacturer for the supply of undervoltage or shunt trip units for high frequency. (When the manufacturer redesigns, it does not simply reduce the number of turns, but also has to consider the excitation current of the coil and many process measures to keep up.)