Medium voltage solid-state transformers: a study on standards compliance

The energy transition is impacting medium voltage distribution grids, leading to voltage fluctuations and harmonic distortions. One way to address this is through the use of a solid-state transformer (SST).

In an SST, a conventional distribution transformer is replaced by a power electronics-based voltage transformer. In networks with SSTs, voltages in networks can be regulated much faster and more dynamically. In addition, other functionalities can be realized with an SST, such as the filtering of harmonics, removing imbalance, or limiting short-circuit currents.

Over the past 16 years, at least 13 prototypes have been built in various countries. However, none of these prototypes address the importance of SST compliance with relevant standards that must be satisfied so that an SST can handle all possible conditions that can occur in a grid.

Sub-contributions for compliance

For his PhD research, Bram van Dam identified compliance to the IEC60076-3: Insulation levels, dielectric tests, and external clearances in air -standard as a requirement for SSTs.

In his work, two sub-contributions were derived. First, conventional transformers (and by extension, SSTs), are to be subjected to double the input voltages (compared to normal operation) for 60 seconds. This requires the power electronics to be fully dimensioned for that voltage, as opposed to the existing prototypes, which only limit themselves to the nominal 10 kVRMS.

The second sub-contribution is that the standard states that the primary and secondary windings of the transformer/SST are to be subjected to 40 kVPEAK, 50Hz for 1 minute. Along with the transformer core requiring connection to earth, this effectively presents a galvanic isolation test for SST power electronics. The consequence of this test is that both the medium voltage side, as well as the low voltage side, need to be carefully designed with respect to geometry, isolation, creepage, and clearance.

Prototype design

An SST prototype was designed, tested, and built as a research platform. The power architecture was selected, heavily influenced by available switching devices and their losses. The firmware and control loops have been implemented using MATLAB Simulink to soften the learning curve for future researchers. The SST research platform has been tested and is functioning as expected.

The IEC60076-3 standard is written for conventional power transformers. However, power electronics operate fundamentally different, and the required tests result in high-cost and complexity. The necessity of the stated test requirements has been investigated and the conclusion is that these are, unfortunately, necessary.

Failure to meet the first requirement of the double input voltage results in grid phenomena potentially exceeding the breakdown limits of the SSTs switching devices, resulting in a cascading total loss of the SST.

Failure to meet the necessary isolation requirements means an SST could be the single point in which the distribution grid becomes connected to the low voltage grid, again resulting in a destructive loss of an SST.

Title of PhD thesis: Medium Voltage Solid-State Transformer: An IEC60076-3 based design. Supervisors: Guus Pemen and Erik de Jong.