HIL testing

As repeatedly remarked in this thesis, nowadays power systems have reached a rather high level of complexity, due to the presence of several control systems and fast actuators. Power electronics led to an increase in performance, together with an increase in system integration.

Indeed, having a controllable fast actuator in the system, as power converters are, it is common to demand more and more functions to it. The pros of such a practice are: reduction of the number of separate components to install, thus reducing technical spaces; improved performance, thanks to the use of a single component in spite of several coordinated ones;

possibility to add functions in a later moment, by reprogramming converter’s control system.

These advantages, however, are offset by a significant drawback: the integration of several functions in a single system led to the issue of guaranteeing its correct operation. Ensure correct operation is a primary need for such systems, because no fallback devices are present, and because the integration of many functions in the same system requires that each of them is able to operate without impairing the operation of the other functions.

In this regard, the modern Hardware-In-the-Loop (HIL) test benches allow testing control systems, demonstrating their correct operation before their installation on the field. In fact, HIL tests imply modeling on a suitable hardware and software system the system of interest, up to the desired detail level, with all input/output interfaces necessary to interact with the real world. The system to be tested is connected to these interfaces and act reading measures provided by the HIL simulator and sending to it the appropriate control signals. This allows testing the component as if it is connected to the real system, with the advantage of not risking damaging anything if the control system being tested does not behave as desired [101]. The HIL testing may be carried out at two different power levels, depending on the possibilities given by the hardware/software simulating system and the component being tested. When the test system and the simulator exchange data only at the signal level it is called HIL testing (or CHIL, Control Hardware-In-the-Loop, see Figure 36). Conversely, if the simulation system is capable of working at power level (thus providing also controllable loads and power sources) and the tested component is capable of providing/absorbing power the testing is named PHIL (Power Hardware-In-the-Loop, see Figure 37) [105] [108] [110].

An additional way of applying the HIL testing can be interfacing more HIL systems together via real data buses. In this case, the only hardware part is constituted by the communication interfaces between the systems, removing the need of a real system prototype. Doing that it is possible to apply inputs with real characteristics to simulated systems, such as delays, noise, and disturbances, and assess their impact. This application can be seen as a middle ground between simulations and HIL testing, allowing improving simulation results while avoiding the production of a test prototype. Examples of such an application can be found in [106] and [107], where an HIL hardware is used to emulate the response of an entire MVDC power system while three external FPGA are used to simulate the control system of the converters

supplying the main MVDC bus (see Figure 38). The communications between FPGA and HIL hardware are made through real data buses, both for measures and command signals.

Analyzing the results the non-ideal behavior of the system is clearly visible, as shown in Figure 39.

Figure 36 – Conceptual Control Hardware-In-the-Loop system scheme [105].

Figure 37 – Conceptual Power Hardware-In-the-Loop system scheme [105].

Figure 38 – HIL setup overview [107].

Figure 39 – HIL emulation of the response of a MVDC bus to a load increase (blue – measured voltage, red – averaged measured voltage) [107].

HIL testing commonly need a physical hardware to test, so they should not be considered part of system design. However, its aid in system design could be relevant if proper approach is applied. Indeed, HIL testing is commonly applied in prototyping new systems to demonstrate their applicability in real environment. Such a practice may take place before system design or even in the middle between design and production. In the former case, testing innovative systems will allow proving their correct operation, thus enabling the designers to include them

in system design as a viable alternative to conventional components. In the latter case, innovative system design can be tested before commercialization, using HIL test as a de-risking tool (thus allowing identifying critical issues before putting system on the market).

Finally, the use of HIL systems connected through real interfaces can be seen as the obvious step to be taken after software simulations, thus complementing them. Indeed, although software simulations offer the opportunity to help in the design of innovative systems, they remain approximations of the reality. Once the system has been simulated, implementing it in an HIL environment allows to get closer to reality even more, due to the possibility to consider the impact of real signals on it.

New design process

5.1 Introduction

The previous chapters have given all the information needed to answer to the following questions:

• what are AESs and how are designed nowadays;

• what is the IPS and why it is so significant for an AES;

• which are the issues that may arise from the conventional design process;

• what are the new trends which will increase the difficulty in designing an IPS;

• which new tools are nowadays available to help designers during design process.

In this chapter, an innovative design process will be presented, integrating the new design tools previously depicted, able to solve (or at least mitigate) the issues coming from conventional design and to aid in designing new generation integrated power systems. Such a design process, conceived during the PhD research activity, will be deiscussed focusing on the IPS’s design, but it is generally applicable to each sub-system's design, also outside shipboard applications.