We report the results of spectroscopic observations, obtained with the Gemini Multi-Object Spectrograph, of eight planetary nebulae (PNe) in the dwarf spheroidal (dSph) galaxy NGC 147, a companion of M31. The physico-chemical properties of the six brightest PNe were derived using both the empirical ICF method and photoionization modelling with CLOUDY. Different aspects of the evolution of low- and intermediate-mass stars in a low-metallicity environment are analysed using relationships between chemical abundances. In addition, certain features of the chemical evolution of NGC 147 were examined. In particular, the mean metallicity of PNe, O/H = 8.06+0.09-0.12 (corresponding to [Fe/H]PNe ~ -0.97), is close to the metallicity of the old stellar population, [Fe/H] = -1.0, suggesting a negligible chemical enrichment during a substantial amount of time. Finally, the luminosity-metallicity relationship for the dwarf galaxies of the Local Group is discussed. The location in the luminosity-metallicity diagram of dSphs does not exclude their formation from old dwarf irregular (dIr) galaxies, but it does exclude their formation from the present time dIrs, since the differences between their metallicities are already present in their older populations. The offset in the luminosity-metallicity relationship indicates a faster enrichment of dSphs and, together with the different average abundance ratio [O/Fe ]demonstrates the different star formation histories for these two types of galaxies.