We report a frequency-output fiber-optic voltage sensor for high-voltage lines. The sensor is based on the linear piezoelectric effect of a PZT ceramic tube. A length of single-mode fiber was wound onto the tube and was inserted in one arm of a Mach-Zehnder interferometer. When an ac voltage is applied to the piezoelectric, the interferometer generates a frequency-modulated signal, which frequency features depend on the applied voltage. The frequency-output operation of the interferometer overcomes the interferometer phase drift and requires minimum polarization control. Moreover, the output signal contains information about the waveform of the applied voltage that can be reconstructed. We also demonstrate vector addition and subtraction of voltages by using a second piezoelectric.