*3.2. Natural Gas System*

The supply subsystem and demand subsystem constitute the natural gas market, with the variable of the natural gas satisfaction rate being the indicator of market efficiency. Pipeline defects, import shortages and extreme weather are the risk factors affecting the market, and a decline in the satisfaction rate induced by these risks may trigger an increase in response strategies such as the natural gas compression rate and the emergency supply ratio.

In the supply subsystem, the different sources of natural gas were divided into domestic natural gas supply and international natural gas imports. According to the annual data derived from the available reports, the daily amount of domestic natural gas supply was set as 5.687 hundred million cubic meters per day (hMm3/d) and the normal import rate as 4.603 hMm3/d. However, import shortages may influence the actual import rate of natural gas. As defined in Equation (1), if the supply is cut or deliberate attacks occur at time 20, an excessively large decline emerges, and these events can barely be resolved within months. The supply of natural gas is transported to the end-users, during which, pipeline defects may occur, creating continual disturbances on the pipeline's efficiency, as denoted by Equation (2). If the natural gas supply declines, the emergency natural gas supply can supplement the supply shortage, which is constrained by the natural gas reserve capacity, the emergency supply capacity and the emergency supply ratio, as shown in Equation (3). According to real data and emergency policies, the natural gas reserve capacity was set as 261 hMm3/d and the maximum daily emergency supply capacity as 2.058 hMm3/d. The emergency supply ratio is highly dependent on the natural gas market's satisfaction rate; hence, a lookup function was used in Equation (4). When the satisfaction rate is lower than 95%, a proportion of emergency gas is supplied, and if the gap increases, the level of urgency rises. Full capacity is used if the market satisfaction rate is below 80%.

Import shortages = IF THEN ELSE (Time plus > 20, 0.9, 0) (1)

Pipeline defects = RANDOM NORMAL (0.05, 0.25, 0.15, 0.05, 0.15) (2)

Emergency natural gas supply rate = IF THEN ELSE (Natural gas reserve capacity > 0, Emergency supply capacity <sup>×</sup> Emergency supply ratio, 0), (3)

Emergency supply ratio = lookup (Natural gas market satisfaction rate). (4)

In the demand subsystem, demands from the residential sector, the electricity sector and other sectors such as industry and transportation were considered. The daily residential gas demand rate was 3.6 hMm3/d, with the other sectors' gas demand rate being equal to 5.16 hMm3/d. The demand for electricity generation is dependent on the proportion of natural gas in electricity generation and the electricity demand generated by natural gas, as denoted in Equation (5). According to the data for 2021, natural gas accounts for 3% of electricity generation in China. Extreme weather in the winter may cause a natural gas demand peak. Normally, for every degree below 0 ◦C the temperature drops, the natural gas demand rises by 2%, as shown in Equation (6). The rise in demand may also cause dissatisfaction in the natural gas market, and a strategy of demand compression can be applied to mitigate the gap. As the residential sector always has the highest supply priority, we can assume that this compression occurs in other sectors such as in the industry. Equation (7) demonstrates that a high-level emergency triggers demand compression, and these actions experience a delay from the time when the market disruption occurred.

Electricity generation gas demand rate = Consumption of natural gas per kwh × Electricity demand generated by natural gas, (5)

Natural gas demand peak = IF THEN ELSE (Extreme weather ≥ 0, 0, 0.02 × (−Extreme weather)), (6)

Natural gas demand compression rate = DELAY1 (IF THEN ELSE (Natural gas market satisfaction rate <sup>≥</sup> 0.8, 0, 0.2), 10), (7)
