The China Electrotechnical Society will hold the "11th China Electrical Equipment Innovation and Development Forum 2016" at the Beijing Railway Building on July 10-11, 2016, with the theme of "Research and Interpretation of the 13th Five-Year Plan for the Electrical Industry".
Researchers Li Jing and Huang Chenxi from the Guangxi Power Grid Co., Ltd. Electric Power Research Institute and Xi'an Jiaotong University published an article in the December 2015 issue of the journal *Electrical Technology*, designing a high-voltage semiconductor switch suitable for damped AC oscillating wave testing systems. This switch consists of two switching modules and an auxiliary power supply system, with each module composed of 10 identical switching units connected in series.
Each switching unit can be divided into three parts: the IGBT chip and its voltage equalization circuit, the IGBT gate driver, and the isolated power supply. A single switching module can withstand 20kV DC voltage when operating independently; two switching panels connected in series can withstand up to 40kV; the turn-on time is less than 250ns. A novel loosely coupled flyback voltage converter provides independent power to each switching module, achieving high voltage isolation between multiple outputs while simplifying space.
Finally, it was applied to a 30kV oscillation wave test system. The experimental results show that the switch has excellent performance and can be applied to the oscillation wave test of samples below 30kV.
Partial discharge testing of power cables is an effective method for assessing cable insulation condition. Damped AC Oscillating Wave Test System (DOVTS) is widely used in field testing of power cables due to its advantages such as small size, light weight, and good equivalence to AC voltage testing.
As shown in Figure 1, the damped AC oscillating wave test system consists of a high-voltage DC power supply, a high-voltage switch, an air-core reactor, a capacitive test specimen, and a high-voltage measurement unit. Among these components, the high-voltage switch is the most important part of the DOVTS.
At the start of the test, the capacitive test specimen (such as an XLPE cable) is first charged to a preset voltage using a high-voltage DC power supply. Then, the high-voltage switch is closed while the high-voltage DC power supply is simultaneously disconnected. At this point, the capacitive test specimen discharges through an air-core reactor and the high-voltage switch. The reactor and the capacitive test specimen form a series resonant circuit, generating a damped oscillating AC voltage on the specimen. This oscillating voltage typically lasts for tens of cycles, approximately hundreds of milliseconds. Once the voltage on the specimen drops to zero, the high-voltage switch is disconnected, and the DC high-voltage power supply is connected, allowing the next test to proceed.
Figure 1 Schematic diagram of the AC voltage test circuit for damped oscillation
Currently, several commonly used high-voltage switches are not suitable for damped AC oscillation wave testing systems due to their low power, slow switching speed, unstable breaking process, or excessively large size. With the continuous development of semiconductor technology, power electronic switches based on components such as thyristors, MOSFETs, and IGBTs are developing rapidly.
Over the past 20 years, various types of semiconductor switches have been widely used in pulsed power, nuclear fusion, static var compensators (STATCOM), and traction applications. However, these high-voltage switches are large in size and weight, and therefore cannot meet the requirements for field testing of damped AC oscillating waves.
This article focuses on the design process and application of a novel high-voltage electronic switch. This high-voltage switch consists of two switching modules and an auxiliary power supply. Each switching module comprises ten isolated switching units connected in series. Compared to traditional high-voltage switches, this switch achieves isolated power supply between IGBTs through loosely coupled transformers. Ultimately, the resulting switch boasts advantages such as small size, light weight, and low cost, facilitating field testing.
1. System Structure Design
Considering the application of the damped AC oscillating wave test system, this switch should meet the following requirements: the switch withstand voltage should reach 30kV, the conduction current should not be less than 40A; the turn-on time should be less than 1μs; and it should be small in size, light in weight, and easy to transport.
The structure and connection method of the high-voltage switch described in this article are shown in Figure 2. It mainly consists of two identical switch modules and a separate power supply system. When the maximum test voltage is less than 20kV, one switch module can work alone. When the maximum test voltage is 30kV, the two switch modules need to be connected in series.
Each switching module consists of multiple IGBTs and their auxiliary circuits. To achieve reliable power supply for each switching unit while reducing size, this switch is designed with a multi-channel isolated power supply system based on a flyback switching converter. This system transfers energy from the front stage to the back stage through loose coupling, providing independent power supply for each IGBT unit and is easily expandable.
The trigger control unit controls the IGBT units via multiple optical fibers, employing synchronous triggering to avoid dynamic voltage unevenness caused by gate signal delay in series-connected IGBTs. When the trigger circuit generates a "conduction" signal, the multiple fiber optic triggers simultaneously generate optical signals. Upon receiving these optical signals, the fiber optic receiver synchronously and effectively controls the multiple IGBTs.
At the same time, an optical signal is transmitted to the control circuit of the high-voltage DC source, cutting off the high-voltage DC power supply. After the switch is turned on for a certain period of time (usually several hundred milliseconds, determined by the duration of the damped oscillation voltage), the system resets, and IBGT returns to the off state.
Figure 2 Schematic diagram of the overall structure of the high-voltage switch
1.1 Drive Circuit
The gate drive circuit of the IGBT chip is shown in Figure 3. VH and VL are +15V and -8V respectively, and they have the same neutral point. The +15V voltage can make the IGBT turn on quickly, and the -8V voltage can make the IGBT turn off without being affected by the Miller effect, thus ensuring reliable turn-off.
To enhance the current-source and current-sinking capabilities of the drive circuit, a push-pull amplifier circuit is needed for power amplification on the control unit output side. The experimental results are shown in Figure 4. When the gate resistance is too small, ringing occurs in the gate drive voltage VGE; when the gate resistance is too large, the gate drive voltage VGE rises slowly, significantly affecting the IGBT's operating speed. After comprehensive consideration, a gate resistance of 5.1Ω was ultimately selected.
Figure 3. Drive circuit diagram
To reduce the dispersion of gate signals, this paper employs a multi-path fiber optic transmission system to ensure that the arrival time of each trigger signal at the IGBT gate is essentially the same. Devices of the same model, from the same production line, and from the same time period were selected to minimize the inherent dispersion of the devices themselves. Testing revealed that the delay times of the gate signals of each IGBT are essentially the same, with an error within 20ns. Figure 4 shows the waveforms of the trigger signal and the IGBT gate signal for one of the switching units, indicating that the time between the trigger signal activation and the rise of the gate voltage is approximately 460ns.
Figure 4. Test waveform of the drive circuit
1.2 Power Supply Unit Design
Each switch panel has 10 switching units. When two switch panels are connected in series, a total of 20 IGBTs operate simultaneously, requiring 20 isolated power supplies. Some isolated power supply methods have been described in the literature, but these methods are difficult to miniaturize. Therefore, this paper proposes a simple topology and a new control strategy. The circuit uses only one semiconductor switch M1, which reduces size and improves reliability.
As shown in Figure 5, the 220V AC mains power is stepped down to an appropriate voltage range by the step-down transformer Tr1, then rectified and passed through two ferrite cores. By controlling the on/off state of the MOSFETs, the current in the circuit is changed, thereby altering the magnetic field strength in the ferrite cores. This induces a voltage in the secondary coil, allowing energy to be transferred from the primary side to the secondary side.
The ferrite core is the core component of this power supply system. As the key to energy transmission, it employs loose coupling to achieve input-output isolation within a narrow range and has strong scalability. Simultaneously, power factor control technology is used to ensure that the primary side current always follows the rectified and filtered voltage waveform, improving energy transmission efficiency while reducing current peaks, preventing core saturation in the loosely coupled transformer, and enhancing the stability of the power supply system.
Figure 5 Power supply unit circuit diagram
Figure 6 shows the theoretical waveforms at key points in the circuit. UT is the output voltage waveform of the rectifier bridge, iP is the primary-side current waveform, iS is the secondary-side current waveform, uP is the primary-side voltage waveform, and ug is the gate drive signal of the MOSFET. As can be seen from Figure 6, when the secondary-side current drops to 0, the semiconductor switch is closed, meaning the circuit operates in transient mode.
The controller employs a dual closed-loop control system. The inner loop is a current loop, which samples the primary-side current using a current sensor and the primary-side voltage using a resistor divider. The outer loop is a voltage loop, consisting of a feedback circuit and a PI compensator. The inner and outer loop circuits work together to ensure a constant secondary-side output voltage.
Figure 6. Theoretical waveforms at key points
2. Actual design of the switch module
The final high-voltage power electronic switch design consists of three parts: two switching modules and an auxiliary circuit board. The auxiliary circuit board comprises two parts: the power supply unit preamplifier and the fiber optic transmission section. The fiber optic transmission section includes multiple optical transmitters and their auxiliary circuitry, enabling control of the switching modules. The power supply circuit mainly includes a step-down transformer, a rectifier circuit, and a power factor correction circuit. It achieves isolated power supply between multiple IGBTs by passing a high-voltage silicone rubber insulated wire through the magnetic core in the middle of the switching module.
As shown in Figure 7, each switch panel consists of 10 IGBT units. Each IGBT unit can be further subdivided into a power module, a driver module, the IGBT itself, and its auxiliary voltage equalization circuit. The IGBT auxiliary voltage equalization circuit is composed of a static voltage equalization circuit and a dynamic voltage equalization circuit. The dynamic voltage equalization circuit consists of seven identical TVS diodes connected in series to suppress transient overvoltages that may occur across the IGBTs. The static voltage equalization circuit consists of multiple megohm-level resistors connected in series. To ensure sufficient electrical distance between different potentials, grooves are etched in the switch panel at locations requiring high-voltage isolation to increase creepage distance.
Figure 7. Physical image of the high-voltage electronic switch
The power supply was tested under full load, and its waveform is shown in Figure 8. Figure 8 shows that the circuit output voltage is approximately 18V, consistent with the design value. Figure 8 also shows that the envelope of the primary-side current waveform is sinusoidal and in phase with the rectifier bridge output voltage. Figure 8 illustrates the relationship between the primary-side current waveform and the MOSFET gate drive signal. When the MOSFET is turned on, the primary-side current rises linearly to its maximum value of 7.5A , and when the MOSFET is turned off, it rapidly drops to 0. Figure 8 shows that at the instant the MOSFET is turned off, a spike appears at its terminals, caused by the ringing effect resulting from the stray inductance and parasitic capacitance of the primary side.
Figure 8. Test results of the power supply unit
3. Application and Experimental Results
As shown in Figure 9, this switch was applied to a 30kV damped AC oscillating wave test system to test its performance. The test circuit parameters are as follows: the inductor used is a 760mH partial discharge-free air-core reactor, and a 500nF partial discharge-free high-voltage capacitor is used to equivalently test a cable of a certain length.
Calculations show that the resonant frequency of the circuit is...
Product | Development of a Novel Power Electronic Switch for a 30kV Damped AC Oscillating Wave Testing System
As shown in Figure 10, the IGBT collector voltage increases linearly before the trigger signal arrives. After the trigger action is given, the tested switching unit turns on almost simultaneously, indicating that the static and dynamic voltage equalization effects of the IGBT have achieved the expected goals, and the switching action is fast and consistent. The final oscillation voltage on the test capacitor is shown in Figure 10. The decaying oscillation voltage decays to half its peak value after 10-11 cycles, indicating that the impedance in the oscillation circuit is very small. In subsequent cable partial discharge tests, it can excite potential defects in the cable, enhancing the effectiveness of this oscillation test system.
Figure 9 Experimental Circuit Diagram
Figure 10 Experimental results
Based on the modular design concept, this system is designed as a lightweight power electronic switch consisting of two switch modules and a power supply unit.
The power supply system achieves input-output isolation within a limited area through loose coupling, ensuring a 50kV isolation voltage while reducing the overall size, significantly improving the integration of the multi-output isolated power supply system. Furthermore, it possesses strong scalability, capable of simultaneously powering multiple IGBTs operating in series. This paper tests this multi-output isolated power supply system under full load conditions, and the experimental results show that the power supply scheme fully meets the requirements of this device.
Finally, the high-voltage switch was connected to a damped AC oscillation wave generator system for overall testing. At a test voltage of 30kV, all switching units within the high-voltage switch operated normally, and their voltage equalization effect was good. The test results show that the high-voltage power electronic switch designed in this paper is suitable for oscillation wave testing of distribution cables below 35kV.
