rbowers@nyiso.com writes to the NYISO_TECH_EXCHANGE Discussion List:

On April 24, 2001, the NYISO deployed code changes affecting Class 1 PURPA
transactions in the billing system and modified bid status flagging in the 
MIS.
These changes impacted settlements for 4/10, 4/11, 4/13, and 4/18 - forward.

There was a code design problem with the PURPA code change that resulted in an
error in the computation of balancing energy for some class 1 units that
resulted in their purchasing their entire day-ahead schedules in the balancing
market.  There was also an oversight in the implementation of the modified bid
status flagging in the MIS such that those bids were not transferred into the
billing system, impacting bid production cost guarantee settlements for the
units with a modified bid status.

The NYISO will be issuing a supplemental invoice for April 2001 settlements on
Tuesday, May 15th to adjust for the aforementioned issues.  Funds due the 
NYISO
for this supplemental invoice should be transferred into the NYISO Clearing
Account by close of business Wednesday, May 23, 2001.  Funds due Market
Participants from the NYISO for this supplemental invoice will be transferred
into Market Participant accounts by close of business Friday, May 25, 2001.

PLEASE NOTE THAT: Funds due the NYISO for the April 2001 settlements which 
were
invoiced on May 7, 2001 should be transferred into the NYISO Clearing Account 
by
close of business Wednesday, May 16, 2001, as scheduled.  Funds due Market
Participants from the NYISO for the April 2001 settlements invoiced on May 7,
2001 will be transferred into Market Participant accounts by close of business
Monday, May 21, 2001, as scheduled.