The increase in variable renewable energy sources requires more flexibility in the power system. This could be provided by small distributed energy sources located at end-users. Aggregators can potentially exploit this flexibility by providing Primary Control Reserve (PCR) services to the Transmission System Operator, and congestion management services to Distribution System Operators. A simulation model based on Linear Programming is used to assess, for a case study in the Netherlands, whether such congestion management services are efficient. The results show that an aggregator can reduce the total system costs with 10-15% relative to traditional grid reinforcements, depending on price and depreciation of batteries, PCR remuneration, and wholesale electricity price.