Frequency-limited <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e338" altimg="si2.svg"> <mml:mrow> <mml:mi mathvariant="script">H</mml:mi> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> -optimal model order reduction based on the relative error criterion