Venture capital firm A91 Partners is set to lead a Rs 500-crore pre-IPO round in health insurance company Niva Bupa.The investment will happen at a valuation of around Rs 15,000 crore.A91 Partners is investing around Rs 350 crore to Rs 400 crore in the pre-IPO round.The investment will be a full secondary-deal, providing an exit for some existing shareholders.