We propose a semi-microscopic model of YBa_2Cu_3O_{6+y} to investigate the origin of the 60 K plateau in its phase diagram. Our model is a "phase only" approach to the high-temperature superconducting system in terms of collective variables. It is able to capture characteristic energy scales present in YBa_2Cu_3O_{6+y} by using adjustable parameters representing phase stiffnesses and allows for strong anisotropy within basal planes to simulate oxygen ordering. We solve the model calculated T_c for chosen system parameters investigating the influence of oxygen ordering and doping imbalance on the shape of YBa_2Cu_3O_{6+y} phase diagram. Our results suggest that the oxygen ordering alone does not seem to be responsible for the existence of the 60 K plateau. However, relying on experimental data unveiling that oxygen doping of YBa_2Cu_3O_{6+y} may introduce a significant charge imbalance between CuO_2 planes and other sites, we show that simultaneously the former are underdoped, while the latter - strongly overdoped almost in the whole region of oxygen doping in which YBa_2Cu_3O_{6+y} is superconducting. This provides two natural counter acting factors, which possibly lead to rise the 60 K plateau with increasing oxygen doping.