SHANGHAI (Reuters) - Police in southern China have detained seven people in connection with an underground banking scheme involving more than 20 billion yuan ($3 billion), the state news agency Xinhua reported. From a suspicious bank account in Shaoguan, a city in Guangdong province, the investigation snowballed to involve a suspected 10,000 people and 148 accounts across more than 20 provinces, Xinhua reported. The suspects allegedly profited from changes in the exchange rates for yuan and Hong Kong dollars, it said without giving details. The yuan, or renminbi, is not fully convertible and the government limits the amount of foreign currency to which individuals and businesses in China have access, which has given rise to networks of underground money changers and banks.  ($1 = 6.6086 Chinese yuan renminbi)  