Voltage Level at Trx secondary and Main Bus

What do you mean by somewhat less? In general, transformers have taps at +-2% and +-5% to correct the secondary line voltage. +-5% of nominal line voltage is common and acceptable on a continuous basis. To answer the second question: Yes the load has an effect on the secondary voltage. In general, an inductive full load will reduce it by at least 12%, a resistive load by 6%, and a capacitive load will increase the voltage by a many percent and will bring the danger of resonating at harmonics frequencies. It is all because the transformer and their feeders have a inductive / resistive impedance of about 6-8% with a ratio of about 5:1, XL/R. Add capacitance and you get a second order circuit that can resonate. That is why you should never exaggerate on the power factor correction capacitors.

"In general, transformers have taps at +-2% and +-5% to correct the secondary line voltage" - true but my question is why voltage level is less at main bus which is not integral part of Trx but separate switch panel.

Read the rest of my answer. Check the voltage at the primary. Make sure that the transformer has the correct turn ratio.

Sorry if i have not understand rest of your answer, let me elaborate my doubt....

what ever may be the turn ratio and tap setting, if we check the voltage level at secondary say V1 and voltage level at LT panel Bus say V2, now my observation is

V2 < V1 on load. Connected load have some effect but it should be at load end not at main bus which is the purpose of bus. what could be reason ? is it only the load which is affecting the voltage level at bus or some other reason?

The Normal specification of transformer in India is 11kv/433V. The 433 V is the open circuit voltage of transformer.

you never get 433 v at bus bar of LT panel. even though VD=0 or negligible at cable connecting Trx secondary and LT panel.