Abstract This paper briefly introduces the working principle of a non-circulating current AC-AC frequency converter and designs a novel zero-current detection circuit. This circuit features a simple structure, no circulating current, short dead time, and the ability to eliminate false current signals. Through careful analysis of the problems encountered in the actual use of this circuit, the causes of the problems are identified and solutions are proposed. Keywords: Vector control; AC-AC frequency conversion; Zero current detection. After incorporating vector control technology, variable frequency speed regulation has been widely used due to its excellent performance. AC-AC frequency conversion systems have low losses due to the absence of intermediate links, and even lower losses and costs when using a non-circulating current method. Therefore, non-circulating current AC-AC frequency conversion has become an important frequency conversion method. However, a non-circulating current AC-AC frequency converter consists of two reversible rectifier circuits (each output phase) connected in anti-parallel. To prevent power supply short circuits, only one circuit can be allowed to operate at any time, while the other waits. When switching between working groups and waiting groups, the original working group is only allowed to be engaged when it is completely turned off. Therefore, a method to determine whether the working group is turned off is needed, i.e., a zero current detection method. This paper presents a novel zero current detection method with a simple structure, small non-circulating current dead time, and the ability to eliminate "false" current signals. 1. Working Principle of Non-Circulating Current Thyristor AC-AC Frequency Converter An AC-AC frequency converter is a device that directly converts a fixed-frequency AC power supply into another frequency without passing through an intermediate rectified DC link (Figure 1). Figure 1(a) shows a three-phase fully controlled bridge rectifier circuit with six thyristors connected together. Figure 1(b) shows the trigger pulse forming circuit of a three-phase fully controlled bridge rectifier circuit. The phase-controlled pulse is generated by comparing the desired signal with a sawtooth wave. High-frequency modulation can reduce the size of the pulse transformer. After the pulse is driven by the power, it is coupled to the thyristors through the pulse transformer. All six trigger pulses are controlled by the pulse blocking signal. In the non-circulating current operation mode, this signal can be used to block the working group pulse and open the waiting group pulse to achieve reliable non-circulating current switching. When the system is abnormal, it can also be used to conveniently block all pulse protection main circuits. Figure 1(c) is a full diagram of the frequency converter. Each box in the figure is a three-phase fully controlled bridge rectifier unit, which includes a three-phase fully controlled bridge rectifier circuit as shown in Figure 1(a) and a trigger pulse forming circuit as shown in Figure 1(b). It can be seen that the three phases are connected in a star configuration, and each phase is generated by two bridge rectifier units connected in anti-parallel. The two bridge rectifier units are divided into a positive group (thyristor symbols pointing upwards) and a negative group (thyristor symbols pointing downwards), controlled by the aforementioned pulse blocking signal. When the current is positive (current flows from the inverter to the load), the positive group operates and the negative group is blocked; conversely, when the current is negative (current flows from the load to the inverter), the negative group operates and the positive group is blocked. 2. Common Zero Current Detection Method and Elimination of "False" Current When the thyristor is on, the voltage between the anode and cathode is approximately 1V; when the thyristor is off, the voltage between the anode and cathode is approximately equal to the power supply voltage. Therefore, by detecting the voltage between the anode and cathode of the thyristor, it can be determined whether it is off. Analyzing the single-phase circuit structure of the AC-AC inverter shown in Figure 2, it can be seen that all thyristors are connected in anti-parallel pairs. When detecting the voltage drop, a pair of thyristors is used as a group for detection. A common zero current detection circuit is shown in Figure 3. The voltage across thyristors K1 and K2 is rectified by a diode rectifier bridge and applied to the optocoupler. When either K1 or K2 is turned on, the voltage across them is close to zero, the phototransistor in the optocoupler is cut off, and the output is low-level L=0. Conversely, when both K1 and K2 are turned off, their voltage drop is equal to the power line voltage, which turns on the phototransistor in the optocoupler, and the output is high-level L=1. In Figure 3, R1 and R2 are current-limiting resistors, and the Zener diode DW sets a threshold voltage to suppress interference. For a three-phase AC-AC converter, 18 such detection circuits are needed, making the circuit complex. Furthermore, the aforementioned ordinary zero-current detection circuit exhibits a phenomenon of "false" current. During the thyristor's blocking period, the voltage drop across its terminals is a sinusoidal AC voltage. Since AC voltages naturally cross zero, the thyristor voltage drop is zero, and the zero-current detection circuit outputs a low level – this is the "false" current signal. Because it's not due to the thyristor being on, it's not a genuine current signal. A trigger pulse signal is needed to eliminate this "false" current signal, requiring a relatively complex circuit diagram. 3. Novel Zero-Current Detection Circuit The novel zero-current detection circuit differs from the ordinary zero-current detection circuit. When detecting the voltage drop, it doesn't just detect the voltage across a pair of thyristors, but rather detects all the voltage drops required for a single phase output at once. Figure 4 shows the zero-current detection circuit for the output of phase a of the frequency converter. As shown in Figure 1, the specific structure of the output section of phase a of the AC/AC frequency converter is shown in Figure 2. Six of the twelve thyristors in phase a are connected to the output terminal of phase a. If the current through these six thyristors is zero, then the current in phase a must be zero. Therefore, zero-current detection only needs to detect the current of these six thyristors. As shown in Figure 2, the six thyristors are connected in anti-parallel pairs. Therefore, during testing, three pairs of thyristors are tested (it is easy to see that if the AC-AC converter is composed of three-phase zero-type rectifier circuits connected in anti-parallel, the testing circuit is exactly the same as this circuit). The voltage drop of each pair of thyristors is applied to the input terminals of a pair of anti-parallel optocouplers through a current-limiting resistor. To prevent false identification due to interference, a Zener diode is connected in series at the input terminal of the optocoupler to provide the minimum threshold for identification output. The output terminals of the optocouplers are directly connected in parallel. Therefore, as long as one of the two thyristors is conducting, the voltage drop will be very small, and the output terminals of both optocouplers will be in a blocked state. However, if both thyristors are cut off and the voltage drop exceeds the threshold, then only one output terminal of the optocoupler will be in a conducting state. The outputs of three pairs of optocouplers are connected in series, with one end connected to the power supply and the other end grounded through a resistor R. This structure implements an AND logic: if any one pair of optocoupler outputs is blocked, the path from the power supply to resistor R is cut off, and the voltage drop across R is zero; only when all three pairs of optocoupler outputs are on is the path from the power supply to resistor R connected, and the voltage drop across R is the power supply voltage minus the saturation conduction voltage drop of the three optocouplers, which is close to the power supply voltage. In other words, as long as one thyristor is on (current flows), the output of resistor R is low; only when all thyristors are off (no current flows) will resistor R output a high level. Therefore, the moment the voltage across resistor R jumps is when the voltage across the last thyristor to be on changes from positive to negative and exceeds the threshold voltage. At this moment, the thyristor has been reliably turned off due to the large reverse voltage, while the other thyristors are in the off state. At this moment, the switching between the forward and reverse groups can be safely performed without setting a delay, resulting in no circulating current, small dead time, and low harmonic content in the output current. When the voltage level on resistor R goes high, the non-circulating current switching logic circuit blocks the pulses of the original working group and opens the pulses of the original waiting group. The original waiting group then becomes the new working group. When a thyristor in the new working group is triggered and conducts, the voltage level on resistor R immediately goes low until the current crosses zero again. Therefore, this circuit structure ensures that the circuit outputs a high level (indicating no current) only for a very short period between the current crossing zero and the thyristor in the new working group conducting. During this period, there will be no "false" current output due to the natural zero-crossing of the transistor voltage drop. At other times, the circuit outputs a low level (indicating current). The "false" current is submerged in the "true" current and will not interfere with the normal operation of the circuit. Therefore, no additional circuit is needed to remove the "false" current signal. Thus, this zero-current detection circuit only requires three detection units as shown in Figure 4 to reliably detect the three-phase current. The circuit structure is simple, has no circulating current, short dead time, and can automatically eliminate "false" current. 4. Problems and solutions encountered in actual use of the circuit These "no current" signals showed virtually no change, thus ruling out the analysis that the problem was caused by current discontinuity. After careful observation of the output signal waveform of the zero-current detection circuit, it was found that the appearance of these positive pulses was very regular, with completely equal intervals between them. There were three such pulses within a 20 ms timeframe. After further careful analysis of the entire circuit, the problem was finally discovered. It turned out that the above analysis was based on the assumption that the commutation between thyristors was completed instantaneously. Thus, during the entire positive half-cycle of the current, at least one thyristor was in the conducting state, and its voltage drop was very low. Correspondingly, the output terminal of one optocoupler was blocked, and the voltage drop across resistor R was low, resulting in a low-level output. However, due to the leakage reactance of the transformer, the conducting thyristors cannot be instantaneously cut off. The commutation process from one thyristor to another cannot be completed instantaneously. Therefore, during the commutation process, it is equivalent to two thyristors conducting simultaneously, sharing the line voltage between the two phases. This voltage is often quite high, sufficient to turn on the corresponding two sets of optocoupler outputs. At this time, the thyristor of the other phase is in the cutoff state, and its corresponding optocoupler output is also conducting. Thus, the path from the power supply to resistor R is established, and the detection circuit outputs a "false" no-current signal pulse. Once the cause of the problem is found, the solution is relatively easy. Since the "false" no-current pulse is generated during the commutation process, which generally ends quickly and lasts for a very short time, the positive pulses are very narrow. A slight modification to the circuit is needed to remove the narrow positive pulses. The modified part is shown in Figure 5. In the circuit of the AC-AC frequency converter developed by the author, the novel zero-current detection circuit shown in Figure 4 was used, but in actual use, the circuit did not initially work completely. Using an oscilloscope to observe the output waveform of the zero-current detection circuit, it was found that the circuit did not show a "false" presence of current but rather a "false" absence of current. According to the above analysis, during the positive half-cycle of the current (and similarly during the negative half-cycle), since the three thyristors in the positive group conduct in turn, the output of the detection circuit should always be low, indicating a continuous presence of current. However, the waveform observed on the oscilloscope shows a different situation. During the positive half-cycle, the output of the detection circuit is mainly low, but several very narrow positive pulses appear, meaning there is "no current." Since these "no current" signals occur during the period when there is continuous current, they are "false" absences of current. If, according to the above analysis, the circuit switches between the working group and the waiting group whenever the output signal of the detection circuit shows an upward jump, problems will obviously arise. Initially, the author believed that the appearance of these "no current" signals was due to discontinuous output current. While the "no current" signal does reflect current discontinuity, increasing the output voltage and thus the output current, based on the circuit in Figure 4, by connecting a small capacitor C in parallel across resistor R and another small resistor R in series between the optocoupler and the output, if a narrow positive pulse occurs during commutation, capacitor C charges through resistor R1. Because the positive pulse is very narrow, it passes before the voltage across capacitor C reaches a relatively high level, and the capacitor discharges through R, so the output is always low. If it is indeed a zero-current crossing, the high level from the optocoupler will persist, and capacitor C can charge to a high output level through resistor R. Therefore, by adding capacitor C and resistor R1, the circuit can eliminate the positive pulse formed during commutation without disrupting the normal zero-current signal output. Of course, capacitor C and resistor R1 will delay the zero-current signal output, increasing the dead time of the no-circulating current; therefore, their values ​​should not be too large. 5 Conclusion This novel zero-current detection circuit can be directly used for zero-current detection in AC-AC frequency converters formed by anti-parallel connection of three-phase bridge and three-phase zero-type fully controlled rectifier circuits. With expansion, it can also be used in AC-AC frequency converters formed by anti-parallel connection of 12-pulse and above controllable rectifier circuits. Using this zero-current detection circuit can reduce the cost of frequency converters and improve their reliability and performance. References [1] Ma Xiaoliang. High-power AC-AC frequency conversion speed regulation and vector control technology (3rd edition) [M]. Beijing: Machinery Industry Press, 2003. [2] Ren Yuying. Zero-current detection circuit in AC-AC frequency conversion speed regulation system of doubly fed motor [J]. Journal of Daqing Petroleum Institute, 2001 (2): 90-92. [3] Zhou Juan. Simulation and zero-current detection of AC-AC frequency converter [J]. Industrial Automation, 2004 (2): 13-15. [4] Qin Xiaoping. Doubly fed speed regulation and cascade speed regulation of induction motors [M]. Beijing: Machinery Industry Press, 1993.