Hallo Mahmood,

electrolytic capacitors of 450V rating can be connected in series to achieve higher voltage rating. Of course this also results in a decrease of equivalent capacitance. Assume you connect 4 capacitors of each 10µF/450V rating in series, then you get an equivalent of 2.5µF/1800V.
But this simple series circuit can be very dangerous, when leakage current performance of these electroytics are very different. Then, it's very possible, that a high percentage of voltage drops across only ONE capacitor, resulting in a loud explosion and spraying out of dangerous electrolytic material right into your face...
When using such series connections of electrolytic capacitors do ALWAYS connect a high ohmic resistor of about 100k...1MOhm in parallel to EACH capacitor. The lower impedant the better!!
Keep in mind, that heat dissipation of resistor can be enormous, when high voltages are applied!

WHEN EXPERIMENTING WITH ELECTROLYTIC CAPACITORS IN SUCH HIGH VOLTAGE ENVIRONMENTS, DO ALWAYS TAKE CARE, THAT THESE CAPACITORS CAN EXPLODE AND MAKE YOU BLIND!!!!!!

If have seen exploding such capacitor one time, and I can tell you it's a real shock: First the loud explosion, then the white smoke steaming out sounding and looking like a rocket...Happily this failing capacitor was sitting in an enclosure (monitor).

Keep also in mind, that above series and parallel configuration does only work in DC applications like supply voltage blocking and decoupling. When working with high currents, these electrolytic capacitors can extremely heat up and also result in explosion!

I would choose another capacitor technology, which does not use this electrolytic material.

A word to semiconductors: Keep in mind that bipolar semiconductors can show 'secondary breakdown effect'. This effect highly reduces current handling capability at high voltages dropping across semiconductor. This can result in a violation of SOAR, especially when a complex load is switched. (Don't forget to take the snubber into account!) One cure is to choose parts with much higher voltage and current rating than might be needed. Also, a careful analysis of load and impedance control can be very helpful.

Take care(!),
Kai