IEEE Power & Energy Magazine - November/December 2014 - 46

consists of more than 40,000 km of electrical lines. REE is
also the electricity system operator, responsible for ensuring the secure supply of electrical energy.
The gas transmission operator is ENAGAS Transporte,
founded in 1972. ENAGAS Transporte operates a pipeline network of 10,000 km, three underground storage facilities, and
four (of a total of six on the mainland) regasification plants.
Total storage capacity is 2,646,500 m3. The technical operation of the system is carried out by ENAGAS GTS, which is

Hydro

Renewables

Coal

Dec.

Nov.

Oct.

Aug.

Sept.

July

May

CCGT

June

Apr.

Feb.

Jan.

Mar.

(GWh)

1,000
900
800
700
600
500
400
300
200
100
0

Nuclear

180
160
140
120
100
80
60
40
20
0

Dec.

Oct.

Nov.

Sept.

July

Aug.

June

Apr.

May

Mar.

Feb.

CCGT

Jan.

(GWh)

figure 11. Electricity production in Spain by source of
primary energy, 2013.

Dec.

Oct.

Nov.

Aug.

Sept.

July

May

June

Apr.

Mar.

Electricity Sector
Conventional Market

Feb.

1,600
1,400
1,200
1,000
800
600
400
200
0

Jan.

(GWh)

figure 12. Variability of CCGT electricity production in
Spain in 2013.

figure 13. Gas transmission system outflows in Spain in
2013: conventional market and electricity sector.
46

ieee power & energy magazine

responsible for the correct operation of all regulated facilities-namely, regasification, storage, and transmission.
The gas system is divided into five transmission zones
(see Figure 10) that can all suffer from transmission bottlenecks, which often complicates the supply to consumers in
certain regions. Avoiding unserved demand requires careful
planning by the system operator and sometimes the modification of storage or regasification plans.
From an operational viewpoint, a diverse electricity technology mix is desirable, as shown in Figure 11. Such a mix
needs to incorporate a technology flexible enough in the
short term to provide backup in the face of variability and
uncertainty while guaranteeing supply in the medium term
by securing adequate amounts of primary energy. The units
compensating for production variability-variability that is
mainly due to renewable facilities and, specifically, to wind
power plants-most of the time are CCGTs. CCGTs compensate for the variability of these technologies at two levels:
by providing up and down backup power in the short run and
by provisioning sufficient gas to operate as needed within a
medium-term horizon.
Figure 12 illustrates the highly variable operation of
CCGTs spanning two typical situations: periods with very low
hydro production (droughts) and long periods with low wind
power production ("low wind" seasons).
The gas system operator needs to ensure that appropriate
gas supply is available for the functioning of the balancing
CCGTs. Gas procurement for electricity production is generally managed jointly with the gas procurement for all other
consumption, which complicates the operation of the natural
gas system. This situation is illustrated in Figure 13.
To ensure an adequate supply of natural gas, the gas system operator manages international interconnections via
pipelines, decides LNG procurement levels via contracts,
and operates the country's underground storage facilities,
as illustrated in Figure 14. Each of these activities involves
complex technical decisions.
While supplying via pipelines dominates the procurement of natural gas, both LNG and underground storage
installations play crucial roles for seasonal adjustment and
operational flexibility. The various gas procurement mechanisms are illustrated in Figures 15 and 16.
The main role of underground natural gas storage consists of covering seasonal demand surges, mostly in the
residential sector, which are particularly important during
the winter. Seasonal demand shifts let underground storage
facility operators arbitrage differences in price among seasons (see Figure 17). In particular, storage can be replenished during low-demand, low-price seasons and used
during high-demand, high-price seasons. Although storage
management is not directly linked to CCGT gas supply,
CCGTs impose significant constraints on the operation of
these storage facilities. This linkage will grow along with
the further integration of renewables into the Spanish electric energy system.
november/december 2014

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2014