Low-inductance customized power resistors for energy technology
Release time:
2022-01-07
Concentrated power generation technology is undergoing a revolution. Traditional power plants (such as coal, gas, and nuclear power plants) are gradually being replaced by renewable, decentralized solutions (such as wind, solar, or biomass power plants). The latter type of power plants typically follows the same principles, namely using renewable energy (solar, wind, and biomass) to generate direct current or alternating current voltage or current through one or more conversion processes. As energy producers are decentralized, an increasing amount of electricity is no longer fed into high-voltage power systems but rather into medium and low-voltage power systems. Therefore, there will be an increasing need for smart power systems that adopt energy storage devices in the future.
Concentrated power generation technology is undergoing a revolution. Traditional power plants (such as coal, gas, and nuclear power plants) are gradually being replaced by renewable, decentralized solutions (such as wind, solar, or biomass power plants). The latter type of power plants typically follows the same principle, namely using renewable energy (solar, wind, and biomass) to generate direct current or alternating current voltage or current through one or more conversion processes. As energy producers are decentralized, an increasing amount of electrical energy is no longer fed into high-voltage power systems but instead into medium and low-voltage power systems. Therefore, in the future, intelligent power system management using energy storage devices will be increasingly needed to meet peak load demands in a variable manner. The core component of these power plants is the power converter, which can provide the generated electrical energy to the power system or consumers in a synchronous manner and with appropriate quality. These power plants not only impose extreme demands on various components but must also achieve a lifespan of about 20 years under harsh environmental conditions.
In recent years, the development focus of power converters has shifted towards higher power density and higher switching frequencies of semiconductors. The first development makes it possible to improve cost-effectiveness, as it allows for an increase in output power while keeping system costs nearly unchanged. The second development increases system efficiency, as system losses decrease due to the increase in switching frequency.
The power converter moves from the input side (left) in the circuit diagram to the consumer side (right), with a resistor (RI) used to limit the charging current, protecting the capacitor from interference. After power is applied, this resistor must accept a large amount of pulse energy. This energy flow or application of energy occurs within a very short time during capacitive charging and usually only occurs once (during startup and is not periodic). This places special requirements on the resistor technology used. Adiabatic boundary conditions exist due to the short duration of the pulse. Therefore, this energy is only applied to the effective material of the resistor (acTIve material) and does not propagate through the resistor via thermal conduction.
Compared to thick film or thin film technology, wire-wound resistors have a large effective mass and can therefore adapt to high pulse energy and continuous power. However, low-power converters utilize all resistor technologies. This includes SMD-MELF thin film resistors, LTO thick film resistors, and the aforementioned wire-wound resistors. If the power is higher, series or parallel wire-wound resistors can be combined as printed circuit board (PCB) components (G200, AC, RS, CW, FS, and Z300). When the power exceeds 50 W, many special wire-wound resistors (such as GWK, GWS, CSxx, FST, FSE, EDGx, RSO, and GBS series) or thick film resistors (such as LPS or RPS on heat sinks, which can be installed away from the PCB due to extremely diverse connection options) are available.
After the AC voltage is converted to the appropriate voltage level, the B6 bridge is used to adjust the AC voltage, and then an inductor is used to suppress interference. On one hand, the series connection of the DC link resistor (RDC) and the DC link capacitor is used to limit the charging current of the DC link capacitor. This current generates relatively high but infrequent pulse loads. On the other hand, this series connection is used to suppress harmonics in the DC link circuit, which is equivalent to a continuous load (due to the continuously repeating pulse sequence). Therefore, the resistors used must be specified to withstand continuous power and pulse power.
The latest power converters have the option to provide capacitive or inductive reactive power to the power system or power consumers. This is achieved by increasing or decreasing the DC link circuit voltage. This increase is implemented using an internal boost converter or directly at the input position of the power converter. Chopper resistors (RBR) are used to reduce the DC link circuit voltage by converting excess electrical energy into thermal energy. Power MOSFETs, IGBT modules, or thyristors provide the switching function for the resistors.
Power MOSFETs and IGBT modules can perform high-frequency switching operations, but thyristors only support low-frequency operation. When there is a risk of the DC link circuit voltage exceeding the specified maximum value, these power switches connect the chopper resistor. After the DC link circuit voltage drops due to this operation, the chopper resistor is disconnected again. These chopper resistors are easy to install, have numerous connection options, and their power dissipation ranges from 100 W to 1000 W. In this case, chopper resistors are often found in combination with crowbar resistors. In the event of a fault in downstream components, this combination allows for the complete dissipation of the energy from the DC link into the resistor (RBR), thereby preventing damage to the power converter. Here, steel grating or steel plate resistors with a primary component of steel can be selected. For these types of resistors, extremely diverse alloy steel plates have a herringbone structure, and by cascading them, resistance values, continuous power, pulse power, and maximum surface temperature can be set as needed. Insulation between the steel plates can use ceramic or mica materials. Additionally, these steel plate resistors also have very low inductance, so no additional voltage spikes occur during switching.
Steel grating resistors (such as Vishay GREx) are suitable for high continuous power due to their natural convection cooling provided by their large surface area or the use of fans. The maximum achievable heat dissipation in this case is only limited by the available installation space and fan output. However, if greater power is required with less insulation space, water-cooled resistors, such as the Vishay WCR series, can be used.
In addition to the transient input surge current, filtering, and the limitations of the described DC link circuit matching, the DC voltage is converted into AC voltage with variable frequency and pulse width through the shown H-bridge circuit. Filtering resistors (RHF) and series output capacitors are used to suppress harmonics at the output stage. Additionally, RHF also serves to limit the charging current of the filtering capacitor. Wire-wound resistors with end caps (such as GWK and FVT series) are suitable for filtering applications due to their ease of installation and high pulse power and continuous power. The difference in this series of products lies in their robust glass insulation, which has moisture resistance and chemical cleaning material properties. The GWK and FVT series can also easily achieve low inductance resistors. In this regard, two resistance wires are wound in a double-strand manner around the resistor body. According to the principle of superposition, the magnetic fields generated by the rotating currents cancel each other out.
Next is the crowbar resistor (RCR) - if they have not yet been placed in the DC link. As mentioned earlier, the role of this resistor is to prevent surrounding components from overloading in the event of a fault.
RDC filtering resistors, RBR chopper resistors, and RHF filtering resistors greatly assist in optimizing the efficiency of optimized systems. In the case of high-speed switching components (such as IGBT or power MOSFET), it is necessary to carefully design these resistors to achieve low inductance. Incorrect resistor selection may lead to resonant circuits due to parasitic inductance and capacitance. These circuits can generate peak voltages, causing semiconductor components to overload or become damaged. Additionally, parasitic inductance can 'round off' signal waveforms, making it impossible to generate rectangular pulses with steep rising edges. Furthermore, this also negatively impacts the power and efficiency of the entire circuit.
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