  The development of energy conversion techniques enhances the coupling between
the gas network and power system. However, challenges remain in the joint
optimal dispatch of electricity-gas systems. The dynamic model of the gas
network, described by partial differential equations, is complex and
computationally demanding for power system operators. Furthermore, information
privacy concerns and limited accessibility to detailed gas network models by
power system operators necessitate quantifying the equivalent energy storage
capacity of gas networks. This paper proposes a multi-port energy storage model
with time-varying capacity to represent the dynamic gas state transformation
and operational constraints in a compact and intuitive form. The model can be
easily integrated into the optimal dispatch problem of the power system. Test
cases demonstrate that the proposed model ensures feasible control strategies
and significantly reduces the computational burden while maintaining high
accuracy in the joint optimal dispatch of electricity-gas systems. In contrast,
the existing static equivalent model fails to capture the full flexibility of
the gas network and may yield infeasible results.
