In order to calibrate high voltage dividers in testing laboratories and HVDC converter substations, a new measurement infrastructure for on-site calibration up to 1000 kV was established in 2013. A 1000 kV wide-band modular HVDC divider was built divided into stackable 200 kV HV modules for agile on-site assessment, with protection against damage from transients, and for measuring of ripple voltage. Nine 200 kV modules were built by the five NMIs and stacked to one 1000 kV divider and one 800 kV voltage divider. The scale factors of the two dividers were determined in a comparison campaign in June 2013, resulting in a combined expanded uncertainty better than 20 μV/V at 1000 kV, and better than 15 μV/V for 200 kV. Traceability of five modular systems have since been maintained using three methods; where the first method is calibration of the HV resistance of each module and LV arm, calculating the scale factors. The second method uses the Hamon transfer method, applied by RISE with five 200 kV modules. The third method is by comparison of the scale factor which includes the LV arm against a lower voltage reference divider. This paper presents the stability and robustness of all divider systems from June 2013 to spring 2022. The long-term stability is presented for the resistance of both the high voltage and the low voltage arm as well as the scale factor. The result shows an unprecedented stability of a HV divider system.