About zones and bid price ladder

The flexibility of the Dutch transmission network varies, depending on the transport load, and is divided into four zones. Based on the estimated transport load (in view of the programmes delivered by the shippers), GTS publishes the sizes of the zones per hour at least two hours before the start of the gas day. These hourly values do not change following publication.

The four zones are: 
1. Dark green zone
As long as the SBS remains within the dark green zone, GTS will not take balancing actions. 
2. Light green zone
If the predicted SBS enters the light green zone, GTS will buy (if the system is short) or sell (if the system is long) gas on the BPL. GTS will accept bids for gas that can be delivered in the three hours (lead time of 30, 90 or 150 minutes) following the call of the BPL by GTS. The volume of gas GTS will buy or sell on the BPL is (in principle) the volume the SBS exceeds the border between the dark green and light green zone, minus bids that have been called but not yet delivered. 
3. Orange zone
If the predicted SBS enters the orange zone, GTS will buy or sell gas on the BPL. GTS will only accept bids for gas that can be delivered in the hour following the call on the BPL by GTS (lead time of 30 minutes). The volume of gas that GTS will buy or sell on the BPL is (in principle) the volume the SBS exceeds the border between the dark green and light green zone, minus bids that have been called but not yet delivered.
4. Red zone
If the predicted SBS enters the red zone, GTS will buy or sell gas on the BPL, if GTS does not expect this to be sufficient to maintain system integrity, GTS will declare an emergency situation. In the event of an emergency situation, GTS can instruct shippers to change the flows at entry points (including entry points with connected gas storage facilities) and – as a last resort - at exit points.