Harmonic balance method is an approximate method for analyzing the existence of periodic solutions and it is widely used in analysis of global stability of nonlinear control systems. For the relay systems the harmonic balance method has an elegant form admitting various further refinement. In this work we consider its extensions known as the Tsypkin method and the locus of a perturbed relay system (LPRS) method. Advantages and limitations of these approaches are illustrated on the example of Keldysh model.