PT. PLN (Persero) has to develop renewable energy which has a minimum energy mix of 23% in 2025 and 31% in 2030. An intermittent renewable-energy power plant like wind and solar power plant, which is uncontrollable and unpredictable, will be massively penetrated. For that reason, sufficient system flexibility is necessary. The effects of intermittent wind and solar power plants in Java-Bali are conducted in this research. The intermittency of wind and solar power plant are modelled from the historical weather data in some areas with the total capacity 1,930 MW, system peak load is 25,325 MW and the total generation capacity before penetration is 35,517 MW. Then the ramping rate difference before and after penetration for every station is evaluated, which can be increases or decreases. Nevertheless, the observations in this study are the increment of maximum ramp up requirement in 30 minutes resolution in Bekasi 4 EHV station become 125.46 MW (before 65 MW) and maximum ramp down requirement in Bekasi 3 EHV station as become 90.36 MW (before 45 MW). For ensuring the ramping rate requirement, Java-Bali system has to increase the flexibility.