Gas quality

Natural gas quality
 
General
Natural gas used by the final customers in the Republic of Croatia comes into the transmission system from different sources (onshore and offshore domestic production, import via interconnections with the neighbouring countries (Hungary and Slovenia), underground gas storage). Although the gas from different sources can be of different chemical composition, the rule is that natural gas that is input into the transmission system of the Republic of Croatia has to meet standard quality parameters stipulated by the General conditions of gas supply. In this way it is provided that also the gas at the transmission system exit always meets the stipulated parameters. As the transmission system operator, Plinacro does not perform any technological procedures which may have an impact on chemical composition of gas and other parameters of gas quality.
Pursuant to the provisions of the Transmission System Network Code the transmission system operator monitors gas quality in the transmission system with the aim to prevent occurrence of gas of non-standard quality, to determine calorific value of gas for calculating the energy of gas and to inform on the quality of gas in the transmission system. A transmission system operator monitors gas quality by its own gas quality monitoring system, which enables recording and central collection of natural gas quality data from all sampling locations, regardless of the ownership of the equipment for the gas quality parameters measurement, frequency of sampling and laboratory gas analysis.
 
Determining gas quality
Since 01 October 2019 a new gas quality monitoring system has been applied in Plinacro. The system relies on the use of process chromatographic gas analysers that were allocated and installed at 28 locations on the transmission system. These are automatic devices that continuously, in cycles of at least two analyses in an hour, determine the composition and quality of gas at 48 automatic gas sampling locations. Manual sampling and laboratory gas analyses are carried out only for a small number of transmission system entries of a small capacity, and they deliver gas to a limited part of the transmission system. A list of all automatic and manual gas sampling locations on the transmission system has been published within the document.
 
Calculation of gas energy
The applicable by-laws stipulate the use of measuring unit of energy [kWh] in all business transactions on the natural gas market. The volume of gas energy input or off-taken at the transmission system entries and exits is calculated by multiplying the measured gas volume (standard m³ at reference conditions 15°C and 1.01325 bar) by the determined net calorific value of gas (kWh/m³, at the same reference conditions and combustion temperature of 15°C) in the same period.
 
On the transmission system
For about 80% of annual transported gas volume, for energy calculation purposes, average daily calorific value determined on the same location at which gas flow is measured is used (transmission system entries, UGS, exit MRSs of large capacity). For exit stations of lower capacity, average daily calorific value determined at the nearest sampling location located on the same gas pipeline and in the same gas stream.
The rules for associating gas sampling locations and gas flow metering locations for energy calculation purposes are stipulated by the Transmission System Network Code and explained in detail in the document.

On distribution systems
The activities of distribution system operators are stipulated by the Distribution System Network Code. Upon expiry of half-monthly period the transmission system operator publishes on its web page half-monthly and monthly weighted net calorific value for exits towards the distribution systems used by distribution systems operators, that is, suppliers, on the billing metering point for the calculation of the off-taken gas volume in energy.

Natural gas quality – data publishing