3i Infrastructure said its investment portfolio continues to perform satisfactorily while continuing to deliver an attractive yield.In the six months to the end of September the infrastructure investment company invested £32m, with the main investment since its last update on 15 July being a further £5m pumped into a junior facility issued by Viridian, a company which operates both regulated and unregulated businesses within the Irish energy markets.Cash balances stood at £330m at the end of September; the company has no debt.The company’s junior debt portfolio, valued on a ‘mark to market’ basis, has increased in value by 20% since the end of March. On the downside, however, the company said valuation gains will be partially offset by adverse movements in exchange rates, particularly relating to the company’s holding in the 3i India Infrastructure Fund.