In a series circuit of resistors, capacitors, and inductors, the phenomenon of power, voltage, current, and phase occurring is called series resonance. Its characteristics are: a pure resistance circuit, where power, voltage, and current have phases, reactance X is equal to zero, impedance Z is equal to resistance R, and the minimum impedance, current, maximum inductance, and capacitance of the circuit may generate voltages many times greater than the power supply voltage and high voltage. Therefore, series resonance is also known as voltage resonance.
The resonance voltage is superimposed with the original voltage, and parallel resonance: In a parallel circuit, resistors, capacitors, and inductors exhibit a phase phenomenon between the voltage and the total current of the circuit, which is called parallel resonance. Its characteristic is that parallel resonance is a fully compensated method that does not require reactive power, but only provides the power required by the active power resistor, generating resonance to minimize the total current of the circuit, and the branch current is usually greater than the total current in the circuit. Therefore, parallel resonance is also known as current resonance.
Characteristics and differences between series resonance and parallel resonance:
1. The load resonance mode can be divided into two types: parallel inverter and series inverter. The main technical characteristics and differences between series inverter and parallel inverter are as follows:
The difference between series inverters and parallel inverters lies in the different oscillation circuits they use, with the former in series with L, R, and C and the latter in parallel with L, R, and C.
(1) The load circuit of the series inverter has low impedance to the power supply and needs to be powered by a voltage source. Therefore, the rectified and filtered DC power supply terminal must be connected to a large filtering capacitor. When the inverter malfunctions, there is a large surge current, making protection difficult.
The load circuit of a parallel inverter presents high impedance to the power supply and requires a current source to provide it. However, in the event of inverter failure, the current will be limited by high reactance, which has a small impact and is easy to protect.
(2) The input voltage of a series inverter is constant, and the output voltage is a rectangular wave. The output current is approximately sine wave, and the converter always leads the angle voltage after the thyristor current is zero.
The input current of a parallel inverter is constant, the output voltage is approximately a sine wave, and the output current is a rectangular wave. Before the resonant capacitor voltage crosses zero, the load current of the converter is always in front of the voltage angle. In other words, both operate under capacitive loads.
(3) A series inverter is a constant voltage source power supply. To avoid the simultaneous conduction of thyristors on the upper and lower bridge arms of the inverter, which may cause a short circuit in the inverter power supply, it is necessary to ensure that it is turned off first and then turned on. That is to say, all thyristors (other power electronic devices) should be turned off for a period of time (t). Stray inductance, which refers to the induced potential generated by the inductance from the DC terminal to the equipment lead, may damage the equipment, so it is necessary to choose an appropriate surge voltage absorption circuit for the equipment. In addition, to ensure that the load current is continuous and the thyristor is not affected by the high voltage on the converter capacitor during the thyristor turn off period, there must be an anti parallel fast diode at both ends of the thyristor.
A parallel inverter is a constant current source power supply. In order to avoid the large induced potential generated by the filter reactance Ld, the current must be continuous. In other words, it is necessary to ensure that the thyristors on the upper and lower bridge arms of the inverter are first turned on and then off during the converter period, that is, all thyristors are in a conducting state during the converter (t) period. At this point, although the inverter bridge arm is directly connected, Ld is large enough not to cause a short circuit in the DC power supply. However, a long commutation time will reduce system efficiency, so it is necessary to shorten the t-gamma, that is, reduce the value of Lk.
(4) The operating frequency of the series inverter must be lower than the natural oscillation frequency of the load circuit, that is, to ensure an appropriate time. Otherwise, due to the direct connection between the upper and lower parts, the inverter will malfunction. The bridge arm of the inverter.
The operating frequency of a parallel inverter must be slightly higher than the natural oscillation frequency of the load circuit to ensure an appropriate reverse voltage time t, otherwise it will cause faults in the thyristor converter. However, if it is too high, the reverse voltage of the thyristor during the converter will be too high, which is not allowed.
(5) There are two power regulation methods for series inverters: changing the DC power supply voltage Ud or changing the thyristor trigger frequency, that is, changing the load power factor cos.
The power regulation mode of parallel inverters can only change the DC power supply voltage Ud, and changing cos phi will also increase the output voltage and power of the inverter, but the allowable adjustment range is very small.
(6) In the converter of the series inverter, the thyristor naturally turns off. Before turning off, the current gradually decreases to zero, so the turn off time is short and the loss is small. During the commutation period, the thyristor has a longer turn off time (t+t -).
In the converter of a parallel inverter, the thyristor is forced to turn off during full current operation. After the current is forced to drop to zero, a period of reverse voltage time is required, so the turn off time is longer. In contrast, series inverters are more suitable for induction heating equipment with higher operating frequencies.
(7) The thyristors of the series inverter need to withstand lower voltages. When using a 380V power grid for power supply, 1200V thyristors should be used. However, all currents in the load circuit, including active and reactive power components, should flow through the thyristor. If the inverter thyristor loses its pulse, it will only stop the oscillation and will not cause the inverter to overturn.
The crystal gate of a parallel inverter needs to withstand high voltage, and its value increases rapidly with the increase of power factor Angle. However, the load itself will form an oscillating current loop. Only active current flows through the inverter thyristor, and when the inverter thyristor occasionally loses the trigger pulse, it can still maintain oscillation and work relatively stably.
(8) Series inverters can be self-excited or self-excited. The output power can be adjusted by changing the trigger pulse frequency of the inverter. Parallel inverters can only operate in self excited state.
(9) In a series inverter, the trigger pulse of the thyristor is asymmetric and does not introduce DC component current to affect normal operation. However, in parallel inverters, the triggering pulse of the inverter thyristor is asymmetric, which can introduce DC component current and cause faults.
(10) The series frequency converter is easy to start and suitable for frequent starting work environments; Parallel inverters require additional starting circuits, which are difficult to start.
(11) Due to the rectangular wave voltage borne by the thyristors in the series inverter, the du/dt value is relatively large, and the absorption circuit plays a key role, while the di/dt requirement is relatively low. In parallel inverters, the current flowing through the inverter thyristors is a rectangular wave, so a larger di/dt and a lower du/dt are required.
(12) When the distance between the induction heating coil of the series inverter and the inverter power (including channel capacitors) is far, the impact on the output power is small. If coaxial cables are used or the spiral wires are placed as close as possible (better twisted together), the effect is not significant. For parallel inverters, the induction heating coil should be placed as close as possible to the power source (especially the channel capacitor), otherwise it will greatly reduce power output and efficiency.
(13) The voltage on the induction coil of the series inverter and the voltage on the gap capacitor are both Q times the output voltage of the inverter, and the current flowing through the induction coil is equal to the output current of the inverter.
The voltage on the induction coil and gap capacitor of a parallel inverter is equal to the output voltage of the inverter, and the current flowing through them is Q times the output current of the inverter.
In summary, parallel inverters and series inverters (usually referred to as parallel or series inverter power supplies) have their own technical characteristics and application fields. From the perspective of industrial heating applications, parallel inverters are widely used in smelting, insulation, heat transfer, induction heating and other fields, with a power range from several kilowatts to tens of thousands of kilowatts. Series inverters are widely used in insulation and high Q value and high-frequency induction heating applications in smelting, one to two furnaces, with a power range from several kilowatts to several thousand kilowatts. Currently, more than 90% of the variable frequency power supplies used in China's industrial sector are parallel variable frequency power supplies.