Detailed Explanation of the Differences Between Series Resonance and Parallel Resonance

The variable-frequency series resonance device is widely used in industries such as power, railway electrification, steel, machinery, metallurgy, petroleum, and chemical engineering. It is suitable for AC withstand voltage tests on capacitive test samples with high voltage and large capacity.
The variable-frequency series resonance device is also widely used in AC withstand tests of 10kV, 35kV, 110kV, 220kV, and 500kV polyethylene power cables; it is applicable to AC withstand tests of 60kV, 220kV, and 500kV GIS. It is also applicable to the power frequency withstand tests of large transformers and generator sets. The difference between series resonance and parallel resonance lies in the different oscillation circuits they use; the former uses L, R, and C in series, while the latter uses L, R, and C in parallel. The detailed explanation of the differences between series resonance and parallel resonance is as follows:
(1) The load circuit in series resonance presents a low impedance to the power supply and requires a voltage source for power supply. Therefore, at the end of the rectified and filtered DC power supply, a large filter capacitor must be connected in parallel. When the inverter fails, the surge current is large and protection becomes difficult.
The parallel resonant load circuit presents a high impedance to the power supply and requires a current source for power supply. A large inductor needs to be connected in series at the end of the DC power supply. However, in the event of inverter failure, since the current is limited by the large inductor, the impact is not significant and easier to protect.
(2) In series resonance, the input voltage is constant, the output voltage is a rectangular wave, the output current is approximately a sine wave. The commutation occurs after the current in the thyristor reaches zero, so the current is always ahead of the voltage by an angle of φ.
In parallel resonance, the input current is constant, the output voltage is approximately a sine wave, the output current is a rectangular wave, and the commutation occurs before the voltage on the resonant capacitor crosses zero. The load current is always ahead of the voltage by an angle of φ. This means that both are operating in a capacitive load state.
(3) In series resonance, the power supply is a constant voltage source. To prevent the upper and lower bridge arms of the thyristors from conducting simultaneously, causing a short circuit of the power supply, during the switching process, it is necessary to ensure that the thyristors are turned off first and then turned on. That is, there should be a certain period (t) during which all thyristors (and other power electronic devices) are in the off state. At this time, the stray inductance, that is, the induced electromotive force generated on the leads from the DC terminal to the devices, may cause damage to the devices. Therefore, an appropriate surge voltage absorption circuit for the devices should be selected. In addition, during the thyristor turn-off period, to ensure the continuous current of the load and to protect the thyristors from the high voltage on the switching capacitor, fast diodes should be connected in reverse parallel across the thyristors.
The parallel resonance is powered by a constant current source. To prevent a large induced electromotive force from being generated on the filter inductance Ld, the current must be continuous. That is to say, it is necessary to ensure that the thyristors on the upper and lower bridge arms of the resonance are turned on first and then turned off during commutation, that is, all thyristors are in the conducting state during the commutation period (tγ). At this time, although the inverter bridge arms are directly connected, since the inductance Ld is large enough, it will not cause a short circuit of the DC power supply. However, the commutation time is long, which will reduce the system efficiency. Therefore, tγ needs to be shortened, that is, the value of Lk needs to be reduced.
(4) The operating frequency of the series resonance must be lower than the inherent oscillation frequency of the load circuit. That is, a suitable time t must be ensured. Otherwise, due to the direct connection of the upper and lower bridge arms during resonance, the commutation will fail. Wuhan Guodian Xi Gao Frequency Converter Series Resonance, intelligent frequency tuning, can automatically tune, scan the resonant frequency, and can also be manually searched.
The operating frequency of the parallel resonance must be slightly higher than the inherent oscillation frequency of the load circuit to ensure an appropriate reverse voltage holding time t; otherwise, the commutation between thyristors will fail. However, if it is too high, the reverse voltage that the thyristors will bear during commutation will be too high, which is not allowed.

(5) There are two ways to adjust the power in series resonance: either by changing the DC power supply voltage Ud or by altering the triggering frequency of the thyristor, which in turn changes the load power factor cosφ.
The power regulation method for parallel resonance is generally limited to changing the DC power supply voltage Ud. Although changing the cosφ can increase the output voltage of the inverter and increase the power, the adjustable range is quite limited.
(6) In series resonance during commutation, the thyristors are naturally turned off. Before they are turned off, their current has gradually decreased to zero, resulting in a short commutation time and low loss. During commutation, the thyristors that are turned off experience a longer period of reverse voltage (t + tγ).
In parallel resonance during the switching process, the thyristor is forced to turn off under full current operation. After the current is forced to drop to zero, an additional reverse voltage application time is required, resulting in a longer turn-off time. In contrast, series resonance is more suitable for use in induction heating devices with higher operating frequencies.
(7) The thyristors in series resonance require lower voltage. When powered by a 380V power grid, a 1200V thyristor is sufficient. However, all the current of the load circuit, including both active and reactive components, must flow through the thyristor. When the inverter thyristor loses the pulse, it will only cause the oscillation to stop, but will not lead to inverter disruption.
The voltage that the thyristors in parallel resonance need to withstand is high. Its value increases rapidly as the power factor angle φ increases. However, the load itself forms an oscillating current circuit. Only the active current flows through the inverter thyristors, and even when the inverter thyristors occasionally lose the triggering pulse, the oscillation can still be maintained, and the operation is relatively stable.
(8) Series resonance can operate both self-excitedly and externally excitedly. During external excitation, the output power can be adjusted simply by changing the frequency of the inverter trigger pulse; while in parallel resonance, the device generally operates only in the self-excited mode.
(9) In series resonance, the triggering pulses of the thyristors are asymmetric, which does not introduce DC component currents and thus does not affect normal operation; while in parallel resonance, the triggering pulses of the inverter thyristors are asymmetric, which will introduce DC component currents and cause faults.
(10) Series resonance start-up is easy and is suitable for applications where frequent starting is required; while parallel resonance requires an additional starting circuit, making the start-up process more difficult.
HV HIPOT Variable Frequency Series Resonance system, during the automatic scanning process of the system, the frequency is automatically swept from 30Hz to 300Hz. After the frequency sweep is completed, the system initially finds the resonant frequency point based on the sweep results, and then conducts a fine frequency sweep within a ±5Hz range with a resolution of 0.01Hz. Finally, the resonant frequency is precisely locked.