Due to strong sextupole corrections, the vertical dynamic aperture of low-emittance storage ring is rather small. Therefore, in-vacuum small-period undulators may provide high enough field. In particular, significant electrostatic field value can be obtained near electrodes. To eliminate the field emission, the high-field surfaces has to be at positive electric potential. In this paper we consider the feasibility of undulator using the comb of such electrodes. It worth noting that variable-period design may be rather simple for electrostatic undulators. Results of researches supported by RBRF grant 15-02-07776 were used in this work.