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Posted on 02 August 2019

More Reliability by High Integration

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The advantages of integrated filter technology

Today, power quality (PQ) filters are critical components in modern power conversion applications. PQ filters are used in traditional motor drives and increasingly in renewable energy applications. These applications often need power converters to adjust the power demand and convert different energy sources for use in existing grid lines.

By Jarkko Salomaki, General Manager Nidecon, jarkko.salomaki@nidecon.com

 

The purpose of PQ filters is to guarantee the quality of grid voltages without increasing the harmonic content of grid lines, as well as to store energy. Converter performance can be upgraded using technically advanced power quality filters.

PQ filters include LC, LCL and dUdT filters. LCL filters are used between the grid-side converter and grid in order to filter current according to the given grid requirements. Dudt filters are used between the generator/motor and converter in order to reduce overvoltages and common mode currents. Nidecon filters come as aircooled or liquid-cooled LC, LCL and dUdT filters for currents of up to 2000A.

Increased power density

In any area of electronics, the trend is to deliver better performance yet reduced size. Converters are no exception. Since inductive components are energy storage devices, their volume takes up a significant portion of the converter cabinet. In order to increase the power density of converters, the size of power quality filters must be reduced. Size reduction is achieved by increasing system packaging density, optimizing filter component form factors, and using higher switching frequencies, leading to reduced inductance requirements in the filters. Reduced inductance automatically means smaller filter dimensions.

The power density increase in Nidecon filters is down to the integrated cooling technology: active hidden hot spot removal in air-cooled windings and integrated liquid cooling channels inside the core. Nidecon’s novel winding concept enables the height of the core to be reduced. The smaller core also means a significant weight reduction compared to conventional filters. The reduced weight also results in cost savings thanks to reduced material and logistics costs. In Nidecon’s air-cooled solutions, the innovative winding concept enables air to flow through the filter, effectively eliminating hotspots (Figure 1). Even if the conventional component surfaces are cooled with liquid or air, local hotspots may still be generated inside the component. For example, a foil winding may have hidden hotspots inside the structure, leading to insulator failure. The core air gaps may have high local loss concentrations due to fringing flux which hits core laminates from the wrong direction, resulting in high local temperature rise and ultimately lamination insulation failure. None of these problems occur with Nidecon solutions.

Figure 1 - Part 1 - Traditional filter

Nidecon forced air cooling concept and comparison to a conventional filter

Integrated liquid cooling reduces filter size because cooling pipes go through the core instead of being on the core surface (Figure 2). There is no need for additional fans because the cooling system is efficient, meaning space is saved. Even though Nidecon filters are smaller and lighter than conventional filters, the output power is the same or even greater. In other words, size is reduced without compromising output power.

Nidecon filter is so compact it can be assembled into the same stack as the converter

Losses dissipated via the cooling liquid

In addition to a decrease in size, a further benefit of integrated liquid cooling is the fact that filter losses can be transferred to the cooling liquid. Usually, energy lost in the form of heat has to be reduced to a minimum in converters. Losses in LC-part and LCL-part typically amount to 0.2 - 0.65% and 0.5 - 0.8% of the throughput power, respectively. In general, the losses that occur in filters are core and winding losses.

Winding losses consist of both DC losses and additional high-frequency losses caused by skin and proximity effects in the windings. Additional high-frequency losses can be reduced to a minimum by way of novel winding concepts and careful design optimization. Core losses generally consist of load and switching frequency component losses in the core material and can be minimized through suitable material selection and careful design optimization. In Nidecon filters, the core is far smaller than in conventional filters owing to the aforementioned stacked winding system.

As power silicon improves, it becomes increasingly attractive for use in higher switching frequency converter applications. In conventional filters the losses increase as the switching frequency goes up. The new filter concept, in contrast, has reduced losses at higher frequencies. To some extent, the trend towards increased switching frequencies in power converters has been hindered by the limits posed by higher switching frequency operation of traditional choke technology.

With conventional filters, high throughput power and loss densities may easily lead to high temperature rises inside the components. The intention is to use liquid cooled filters to capture these losses in the liquid. The problem, however, is that typically only 60% of the losses will be transferred to the liquid, while the remaining 40% is transferred to the cabinet air. For this reason, additional cooling means are needed for the cabinet air and the power converter suffers derating at high ambient conditions. By using novel integrated liquid cooling concepts, 90 – 95% of losses can be transferred to the liquid, which means that heat convection to the cabinet is kept to a minimum. The resultant lower temperature increases filter lifetime and improves reliability. Lower temperatures also result in increased lifetime for insulating materials and reduce insulation hotspots. Typically Nidecon filters are in class F. Nidecon’s innovative cooling concepts enable the use of filters in places where temperatures need both to be controlled and to be low.

The small dimensions of the new Nidecon filter are also favourable with regard to corrosion prevention since there is less surface area that could corrode. In addition, the modular design of the Nidecon filter means that each module in itself is anti-corrosive. Thus, even if a surface does somehow start to corrode, this cannot go any deeper into the filter. Conventional filters that feature laminated cores have problems with corrosion which can penetrate along the laminate between the insulation layers. With the innovative core materials in Nidecon filters, in contrast, corrosion can be prevented.

Mechanical integration

Normally, conventional LC filters require own section inside the converter cabinet. The other common situation is that the L-part of the LC is in one stack, while the capacitors and the other L-part are fitted elsewhere in the cabinet wherever they fit best. These options require feedthroughs between the sections for the liquid cooling and the connections. In addition, more floor space is required since the cabinet needs to be larger.

Owing to the small size and flexible dimensions of Nidecon filters (LC, LCL & dUdT), they can be installed in the same stack, meaning the connections can be kept short and space saved. Their compact dimensions also mean they can be assembled inside a single converter module/cubicle. The filters can be connected to the same liquid cooling as for the converter module, meaning there is no need for feedthroughs. The result is an overall simpler solution.

The Nidecon filter dimensions are flexible thanks to the modular design concept of Nidecon filters. The filter can be fitted in the available space inside the converter cabinet. Depending on the space allocated to the filter, the filter can be designed to be assembled either horizontally or vertically. Thanks to this geometrical flexibility, the converter cabinet can be designed without having to worry about filter space requirements. By integrating Nidecon filters as a standard in Semikron power assemblies, the exact filter space requirements are known and the cabinet size can be optimized. Besides, the customer then has to factor in space for the cabinet only, not both the filter and the cabinet.

Nidecon filter is so compact it can be assembled into the same stack as the converter 2

Intelligent connections

Nidecon filters are designed so that they can be fitted directly into the converter module, resulting in cost savings because the copper rails can be kept short. There is no need for complicated horizontal rails, the number of connections can be kept low, and the mechanical assembly of the rails is simpler, too. Rail insulation will be easier, too, since the rails are shorter and no feedthroughs are used. With shorter rails there will be less vibration and, consequently, less mechanical stress acting on the filter. The result of this is increased filter reliability.

A typical problem that occurs with long rails is heat build-up which is then transferred to the cabinet. With shorter rails, this heat transfer is reduced. Another advantage of shorter rails is that there is less area for harmonic currents to spread to and cause heat losses and heat transfer to the cabinet. In drive applications, harmonic currents cannot be avoided but the harm can at least be reduced to a minimum. With long rails also the parasitic components cause possible problems such as EMC noise and overvoltage spikes with fast switching currents. Smaller parasitic parameters are crucial when power density is increased. In addition, longer rails would need fans to cool them down, which would mean more space would be needed. With the compact Nidecon filter and the short connections, there will be less heat development, reduced losses for the overall system, and fewer EMC problems.

Increased reliability thanks to integration

The issues described here affect the reliability of the filter. Two separate components that are reliable on their own are not necessarily reliable when used together. Semikron and Nidecon have solved this problem by integrating the filter into the power stack. The reliability of the entire package is improved because the filter and converter are specifically designed to be used together. It is also easier for the customer to procure the whole package from one supplier, reducing the customer’s responsibility and speeding up overall project time on the customer side. Since the filter is integrated into the power stack and the package performance has been tested before delivery to the customer, the customer can take the package straight to the field. The filter is already connected to the power stack, and the cabinet is easy to install in the final application: all the customer has to do is connect the input and output. Integrating the Nidecon filter into a Semikron power assembly module is the next logical development step.

Integration is also important with regard to knowledge transfer: converter designers obtain better knowledge about the ways filters can be used, and filter designers learn more about the conditions in which filters are used. IGBT modules are usually designed for certain switching frequencies. Thanks to this knowledge transfer, the power rating can be optimized more easily since there is more knowledge about how much more power can be generated before a given component reaches its maximum rating. For the customer this means an optimized package with optimized losses or temperature rise for the given load situations.

Co-operation helps expand the knowledge about the performance of the whole package, and the customer can be provided with more specific parameters for optimum package function. The losses of the inductive component alone are difficult to measure, but package losses are easier to identify. As a result, the customer gets more precise data, and Semikron can predict the need for maintenance and ultimately improve package lifetime.

The advantage of integration is that the customer gets an entire package from a one-stop supplier - a package in which the constituent components are designed specifically to work together. Finally, a standardized package comprising power stack with integrated filter also brings about efficiency gains resulting from volume production.

 

 

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