About POS and SBS
About POS and SBS
The regime is based on near real time information about the accumulated balancing position of each shipper (portfolio imbalance signal or POS) and the system (system balance signal or SBS). The SBS is equal to the sum of all POS’s and reflects the balance position of the network system.
If we define damping as the difference between programmed exit and entry, we derive a definition for imbalance: imbalance is the difference between the damping in the programme and the actual difference between entry and exit. For programmes without damping, therefore, the programmed entry and exit are equal and cancel each other out, leaving the imbalance as the difference between near real time entry and exit allocation.
The programmes, and consequently the damping, are fixed the day before the actual day of transportation.
Depending on the type of entry and exit points in a portfolio, “near real time” in practice means “with a delay of approximately 20 minutes”. Using the near real time allocations, GTS calculates the imbalance for each portfolio every hour (if a portfolio consists of an entry and an exit programme, the net imbalance is calculated). The hourly imbalance is added to the POS of the previous hour, to calculate the POS for this hour. When all the POS’s have been calculated, the SBS is calculated as the sum of all POS’s.
The POS will become final approximately 20 minutes after the hour. During the hour, every 5 minutes GTS will predict the SBS, based on predicted POSes, for the end of that hour. The prediction for the SBS is used to determine whether the Bid Price Ladder (BPL) should be called. If and when the BPL is called, the volume of gas to buy or sell on the BPL is calculated based on the predicted SBS.
If the system is long, portfolios that are long are defined as “causers” and portfolios that are short are defined as “helpers”. Contrastingly if the system is short, portfolios that are short are defined as “causers” and portfolios that are long are defined as “helpers”. Along with the SBS, the combined positions of “causers” and the combined positions of “helpers” is published by GTS. By publishing the total POS volume of the “helpers” and “causers”, a shipper that is causing an imbalance will be able to calculate the volume of gas that will be assigned to its portfolio:
If the predicted SBS leaves the dark green zone in the current hour, the BPL is called and an amount of gas is sold (if the system is long) or bought (if the system is short) as to bring the system back to the border of the dark green zone.
Furthermore the accumulated POS volume of the helpers is bought (if the system is short) or sold (if the system is long) at the marginal price and pro-rata allocated to the POSes of the causers. Therefore all helpers of the system (shippers of which the POS is opposite of the SBS) will get a POS of zero (and thus be in balance). The volume of gas involved in this transaction will be pro-rata allocated to the causers of the system imbalance (of which the POS has the same sign as the SBS). This is called assistance gas, and is meant to give an extra incentive to help maintain the system balance.