A method of operating a dual fed system comprising: a first busbar; a second busbar; at least one generator connected to the first busbar and having an associated prime mover that produces exhaust emissions; at least one generator connected to the second busbar and having an associated prime mover that produces exhaust emissions; and an electrical load connected to the first busbar by means of a first power converter and to the second busbar by means of a second power converter; the method characterised by the steps of: using a power allocation algorithm to minimise exhaust emissions from the prime movers (D1, D3) by finding the minimum value of a cost function Q, where Q = ∑ i = 1 N λf i P i + μ g i P i whilst observing the following constraint, ∑ i = 1 N P i = P tot where: f i ( P i) is a first set of functions estimating the nitrogen oxides emissions rate for the prime movers (D1, D3), g i ( P i) is a second set of functions estimating the carbon dioxide emissions rate for the prime movers (D1, D3), N is the number of generators (G1, G3) and associated prime movers (D1, D3), P i is the power delivered by generator i, where i = 1,..., N, λ and μ are weighting gains, one of which can be zero, and P tot is the total power requirement of the dual fed system; loading or unloading the generators (G1, G3) according to the power allocation algorithm; and supplying power to the electrical load from the first and second busbars (2a, 2b) at a ratio that minimises exhaust emissions from the prime movers (D1, D3). a first busbar; a second busbar; at least one generator connected to the first busbar and having an associated prime mover that produces exhaust emissions; at least one generator connected to the second busbar and having an associated prime mover that produces exhaust emissions; and an electrical load connected to the first busbar by means of a first power converter and to the second busbar by means of a second power converter; the method characterised by the steps of: using a power allocation algorithm to minimise exhaust emissions from the prime movers (D1, D3) by finding the minimum value of a cost function Q, where Q = ∑ i = 1 N λf i P i + μ g i P i whilst observing the following constraint, ∑ i = 1 N P i = P tot where: f i ( P i) is a first set of functions estimating the nitrogen oxides emissions rate for the prime movers (D1, D3), g i ( P i) is a second set of functions estimating the carbon dioxide emissions rate for the prime movers (D1, D3), N is the number of generators (G1, G3) and associated prime movers (D1, D3), P i is the power delivered by generator i, where i = 1,..., N, λ and μ are weighting gains, one of which can be zero, and P tot is the total power requirement of the dual fed system; loading or unloading the generators (G1, G3) according to the power allocation algorithm; and supplying power to the electrical load from the first and second busbars (2a, 2b) at a ratio that minimises exhaust emissions from the prime movers (D1, D3). using a power allocation algorithm to minimise exhaust emissions from the prime movers (D1, D3) by finding the minimum value of a cost function Q, where Q = ∑ i = 1 N λf i P i + μ g i P i whilst observing the following constraint, ∑ i = 1 N P i = P tot where: f i ( P i) is a first set of functions estimating the nitrogen oxides emissions rate for the prime movers (D1, D3), g i ( P i) is a second set of functions estimating the carbon dioxide emissions rate for the prime movers (D1, D3), N is the number of generators (G1, G3) and associated prime movers (D1, D3), P i is the power delivered by generator i, where i = 1,..., N, λ and μ are weighting gains, one of which can be zero, and P tot is the total power requirement of the dual fed system; f i ( P i) is a first set of functions estimating the nitrogen oxides emissions rate for the prime movers (D1, D3), g i ( P i) is a second set of functions estimating the carbon dioxide emissions rate for the prime movers (D1, D3), N is the number of generators (G1, G3) and associated prime movers (D1, D3), P i is the power delivered by generator i, where i = 1,..., N, λ and μ are weighting gains, one of which can be zero, and P tot is the total power requirement of the dual fed system; loading or unloading the generators (G1, G3) according to the power allocation algorithm; and supplying power to the electrical load from the first and second busbars (2a, 2b) at a ratio that minimises exhaust emissions from the prime movers (D1, D3).