1 Introduction
Medium-voltage high-power drive systems have been widely used in industrial production, such as pipeline pumps in the petrochemical industry, fans in the cement industry, water supply pumps in pumping stations, traction machinery in the transportation industry, and rolling mills in the metallurgical industry. Compared with low-voltage drives, medium-voltage drives have higher technical requirements and challenges in many aspects. Some problems that are insignificant or even non-existent in low-voltage drives are problems that must be solved in medium-voltage drives. These technical difficulties generally include: technical requirements related to the power quality of the grid-side rectifier, technical difficulties related to the design of the motor-side inverter, limitations of switching devices, and overall requirements of the drive system. Traditional two-level inverters have many problems in high-power applications: they require bulky, energy-consuming, and expensive transformers; increasing the switching frequency to obtain a high-quality output waveform results in high switching losses; and to meet the requirements of high voltage, devices need to be connected in series, thus requiring complex dynamic voltage equalization circuits. To address this, German scholar Holtz proposed a three-level inverter circuit topology in 1977, in which each phase arm has a pair of switching transistors to assist the neutral point position. Later, in 1980, Japanese scholar Nabae further developed this circuit, replacing the auxiliary switches with a pair of diodes connected in series with the midpoint of the main pipe in the upper and lower bridge arms to assist in the positioning of the midpoint. This circuit is easier to control than the former, and when the main pipe is turned off, it only bears half the voltage of the DC bus, making it more practical.
2. Design of the main circuit of a three-level frequency converter
Figure 1 shows the main circuit diagram of a three-level frequency converter. The phase-shifting transformer has a delta-connected primary winding and two symmetrical windings connected in a Y-connection and a delta connection, respectively, on the secondary side. The outputs of the two secondary windings are respectively processed by two identical 6-pulse rectifier units to form a 12-pulse rectifier. The 12-pulse rectifier allows the low-order harmonics generated by the 6-pulse diode rectifiers to cancel each other out, thereby reducing the harmonic distortion of the grid-side current and improving the power factor of the grid side. Generally speaking, the more pulses a diode rectifier has, the smaller the harmonic distortion of the output grid-side current. However, in actual products, diode rectifiers with more than 30 pulses are rarely used, mainly because the cost of the transformer increases significantly while the performance improvement is not obvious. Therefore, this paper uses a 12-pulse diode rectifier.
Figure 1. Main circuit of a diode-clamped three-level frequency converter.
As shown in Figure 1, each bridge arm has 4 IGBTs, 2 clamping diodes, and 4 reverse recovery diodes. Taking phase A as an example, when the body diodes of IGBTs and IGBTs are conducting, the stator phase A voltage is 0; when the body diodes of IGBTs and IGBTs are conducting, the stator phase A voltage is 0; when IGBTs and IGBTs are conducting, the stator phase A voltage is 0. IGBTs and IGBTs cannot conduct simultaneously; which one conducts depends on the direction of the phase A load current. Therefore, for a three-level inverter, its AC side voltage has 3 states: 0, 0, and 1. Combining the 3 bridge arms, there are a total of 27 switching states, i.e., 27 space voltage vectors. The disadvantages of this topology are: three-level and higher inverters require a large number of devices, significantly increasing control complexity and causing fluctuations in the neutral point voltage.
3. Design of Control Circuit for Three-Level Frequency Converter
The block diagram of the three-level frequency converter control system is shown in Figure 2. It includes 12 IGBT drive circuits, voltage and current detection circuits, interface processing circuits, DSP control circuits, and human-machine interface.
Figure 2. Block diagram of the three-level frequency converter control system
The DSP control circuit outputs corresponding PWM drive signals according to the motor parameters and operating modes input from the human-machine interface. These drive signals are processed by the interface circuit and then transmitted to the drive circuit via optical fiber. The drive circuit receives the signals from the optical fiber and drives the IGBTs using a dedicated drive chip, 2SD315AI. High-precision voltage and current Hall effect sensors manufactured by LEM are used for voltage and current detection. Voltage detection includes positive and negative bus voltage detection and midpoint voltage detection. The voltage and current values returned by the detection circuit are processed and calibrated by the interface circuit before being sent to the DSP control circuit for calculation. When the bus voltage or midpoint voltage fluctuation exceeds the allowable range, the protection circuit activates, blocking the IGBT signal and displaying the current alarm on the human-machine interface.
4. Three-level voltage space vector pulse width modulation technology
Like two-level methods, multi-level voltage space vector pulse width modulation (SVPWM) originates from motor flux tracking technology. It employs a voltage vector equivalent synthesis approximation method, aiming to make the motor flux trajectory as close to a circle as possible for specific switching and control. This control method has advantages such as high voltage utilization, fewer switching operations, and ease of digital implementation. Therefore, it is currently the most widely used control method in medium and high power frequency converters both domestically and internationally.
Assume the asynchronous motor is powered by an ideal three-phase symmetrical voltage source, i.e.
(1) In the formula, is the effective value of the power supply voltage, and is the angular frequency of the power supply voltage. Because the three phases U, V, and W are spatially different, the following expression can be made using the concept of voltage space vector:
Figure 3. Three-level voltage space vector diagram
Figure 4. Voltage space vector of the first sector
Due to the symmetry of the spatial vector diagram, only one region needs to be analyzed, as shown by the large triangle in Figure 4. When the reference vector falls within a certain triangular region, in order to obtain a sinusoidal voltage of the corresponding frequency at the output and reduce the harmonic content of the output voltage, the reference vector is synthesized from the three vectors that make up the triangle. The interaction time of each vector and the relationship between the reference vector should satisfy the following formula:
Table 1. Vector Action Time Table for Voltage Space Vector Modulation Method
The action time of each vector in regions 1, 2, 3, 4, etc. can be obtained, as shown in Table 1.
In the actual implementation of the voltage space vector modulation (SVPWM) method, it is first necessary to determine which sector and region the top of the reference voltage vector falls into based on the magnitude and phase angle of the reference voltage space vector. Then, based on the detected DC bus voltage value, the voltage space vector in that region, and the constraint equation of that region, the action time of each voltage space vector is calculated. Finally, the voltage space vector is output according to the time sequence, which can realize SVPWM modulation and achieve the approximation of circular magnetic flux.
5. Conclusion
Multilevel inverters are a new type of high-voltage, high-capacity power converter. Their main advantages are: a higher number of voltage levels results in lower harmonic content in the output voltage; lower switching frequency leads to lower switching losses; and lower device stress eliminates the need for dynamic voltage equalization. Three-level inverters have a relatively simple structure, and their circuit topology can be considered a special case of multilevel inverters. Issues that need to be addressed in multilevel inverter control include midpoint clamping and maintaining dynamic balance of the midpoint potential, power device spike absorption buffer circuits, PWM algorithm simplification and control strategies, high-voltage power device drive, and system power supply. Given the current development level of power switching devices, devices with voltage withstand capabilities of tens of thousands of volts are unlikely to emerge in the short term. Multilevel technology is an effective way to solve high-voltage, high-power variable frequency speed regulation problems. Furthermore, under the current trend of high-voltage direct current transmission in power systems, multilevel technology also plays an important role in power transmission and distribution.
