Sebi tweaks CAS framework, restores 30-minute VWAP for expiry settlements

Sebi has changed the price used to settle derivatives on expiry days. Index derivatives will no longer rely on the closing auction session. Instead, their settlement value will use the index calculated from the volume-weighted average prices of its underlying stocks during the final 30 minutes of normal trading. Stock derivatives will use the underlying stock's VWAP over the same period.

For example, if a stock derivative expires, its settlement price will be based on that stock's trading prices and volumes from the final 30 minutes. For an index derivative, Sebi will calculate the index using the relevant VWAPs of the stocks in that index. This separates settlement from the auction process.

The closing auction will still determine official closing prices for roughly 200 stocks. Normal stock trading will now continue until 3:30 pm, while index derivatives generally trade until 3:45 pm, except those expiring that day, which close at 3:30 pm.

Volume-weighted average price, or VWAP, combines transaction prices with the volume traded at each price. Trades involving more shares have greater influence than smaller trades. It therefore reflects the prices at which substantial trading occurred during a chosen period, rather than giving every transaction equal weight.

Under Sebi's rule, a stock derivative expiring that day uses the underlying stock's VWAP from the final 30 minutes of normal trading. For an index derivative, the index value is calculated from the VWAPs of the stocks in that index. This creates a defined calculation based on underlying securities and their trading activity.

The article does not give a separate technical reason for selecting exactly 30 minutes. It states that Sebi retained this methodology while separating derivatives settlement from CAS, after concerns about low auction participation and uncertain indicative index values. CAS continues to set cash-market closing prices.

The closing auction session covers about 200 stocks. Its role is to determine the official closing prices for those stocks, rather than to calculate derivative settlement prices on expiry days. Sebi's revised framework keeps these two functions separate.

During CAS, the exchange collects eligible buy and sell orders before matching them. The matching process then determines a single closing price. In contrast, expiry settlement for a stock derivative uses the underlying stock's VWAP from the final 30 minutes of normal trading. For an index derivative, the index value is calculated from the VWAPs of its constituent stocks during that period.

This means one stock can have a CAS closing price and a separate derivative settlement price. The revised timings support that distinction. Normal trading continues until 3:30 pm, CAS runs from 3:30 pm to 3:40 pm, and the cash market later reopens from 3:50 pm to 4 pm for closing-price trades.

A closing auction session is a period used to discover an official closing price. Instead of executing orders immediately, the exchange collects buy and sell orders during the session. It then matches them after the order-entry period closes. The aim is to make closing-price discovery more efficient and reduce the effect of orders placed near the session's end.

Under the revised process, the first minute calculates the reference price and transfers orders from normal trading. Between 3:31 pm and 3:36 pm, traders may place both limit and market orders. After that, only limit orders are allowed. Existing market orders cannot be changed or cancelled. Order entry ends randomly between 3:38 pm and 3:40 pm, followed by matching.

CAS continues to determine official closing prices for the covered stocks. However, expiry-day derivatives use final-30-minute VWAPs instead. The cash market reopens from 3:50 pm to 4 pm for trades at the closing price.

Sebi separated derivatives settlement from CAS after market participants raised concerns about how the auction worked. The article highlights low participation in the auction and uncertainty over indicative Nifty and Sensex values. These conditions kept participants on edge and led derivatives traders to complain about sharp losses on expiry days.

The revised approach gives each process a distinct job. CAS will continue discovering official closing prices for roughly 200 stocks. Derivatives settlement will instead use the underlying stocks' VWAPs from the final 30 minutes of normal trading. Stock derivatives use the stock VWAP, while index derivatives use an index calculated from constituent-stock VWAPs.

The change followed protests and Sebi's request for public comments on the auction session, market timings, and settlement methodology. Ravi Varanasi described the separation as pragmatic. Under the new schedule, stock trading ends at 3:30 pm before CAS, while most derivatives trade until 3:45 pm.

Derivatives traders can face sharp losses when the price used to settle their contracts differs significantly from the level they expected. The article links this risk to low participation in CAS and uncertainty over indicative Nifty and Sensex values. These conditions kept market participants on edge, particularly on expiry days.

For example, a trader holding an index derivative might expect settlement to reflect normal trading in the underlying stocks. If an auction-based measure appears uncertain or moves sharply, the final settlement can affect the trader's gain or loss. Under the revised rule, index settlement uses an index calculated from constituent-stock VWAPs during the last 30 minutes of normal trading. Stock derivatives use the underlying stock's VWAP.

The new framework is intended to separate this settlement calculation from CAS. CAS still sets cash-market closing prices for about 200 stocks. The article presents the change as a response to protests and concerns raised by market participants.

A derivative is a financial contract whose value is linked to an underlying asset, such as a stock or a stock index. Its expiry settlement price therefore depends on a specified value for that underlying. The article discusses stock derivatives and index derivatives, but does not provide a fuller explanation of contract types.

Under Sebi's revised rule, a stock derivative expiring that day is settled using the underlying stock's VWAP during the final 30 minutes of normal trading. An index derivative uses the index value calculated from the VWAPs of the stocks included in that index during the same period. Trading volume affects each VWAP, so the calculation reflects both prices and activity.

CAS remains relevant to the cash market. It determines official closing prices for about 200 stocks, but those auction prices are no longer used for expiry-day derivative settlement. Index derivatives expiring that day close at 3:30 pm, while stock derivatives trade until 3:45 pm.