2014 Paris Session
http : //www.cigre.org
A1-112
Increased grid performance using synchronous condensers in multi in-feed
multi-terminal HVDC System
A. DI GIULIO, G.M. GIANNUZZI, V. IULIANI, F. PALONE, M. REBOLINI, R.
ZAOTTINI, S. ZUNINO,
Terna Rete Italia
Italy
massimo.rebolini@terna.it
INTRODUCTION
The EHV/HV network of Sardinia (Fig.1) is part of the Italian national grid, owned and
operated by Terna. In order to increase the transmission network flexibility, Terna has
recently awarded a tender for a 500 Mvar synchronous condensers system, to be installed in
Codrongianos substation in 2014.
Off-the-shelf 2-pole generators, normally used for combined cycle power plants, have been
identified as the most proven, cheap and reliable technology for implementing the
synchronous condenser solution.
The paper deals with the characteristics of the synchronous condensers, their auxiliaries and
their control system.
1. DESCRIPTION OF THE SARDINIAN NETWORK
The island of Sardinia is connected to mainland EHV grid by the Sa.Co.I. (Sardinia – Corsica
– Italy) and by the SaPeI HVDC-LCC links. The HV Corsica network is also synchronously
connected to the Sardinia one by the SarCo cable; main data of these links are reported in
Table I.
Absence of natural gas pipelines makes electrical grid the only energy network in the island;
this also limits the development of new, efficient, combined-cycle power plant.
Conventional oil and coal fired power plants are now being rapidly replaced by distributed
generation by renewable sources, mainly windfarms. As visible in Table II, the installed wind
power plants account for 66% of peak (about 1500 MW) load and the PV power plant account
for another 45%[1]; the pumped hydro power plant in Taloro allows for a significant daily
energy storage (240 MW, i.e. 16 % of the peak load); an innovative 20 MW battery energy
storage system will be also put in service in 2014 by Terna Storage.
From these points of view (presence of high renewable penetration, multi-terminal [2], multiinfeed HVDC links and significant energy storage) the Sardinian network already shows the
characteristics of the network of the future.
Due to the huge amount of installed power in some environmental conditions, the renewable
power plants could, in principle, supply the entire Sardinian load; however the replacement of
conventional generation jeopardizes voltage regulation capability of the Sardinian system, its
inertia and short circuit currents.
1
Terna examined the possible solutions to overcome these issues and enhancing renewable
power plant development and operation without reducing grid security and flexibility. This
analysis has been carried out considering international practice and past experience of
synchronous condensers in the Italian grid; Table III summarizes the conclusions of the
survey. As evident from table III [2] the synchronous condenser is the only device able to
increase both short circuit power and grid inertia and its inherent overload capability is not
achievable with static systems; for these reasons it has been chosen for providing dynamic
reactive power support to the Sardinian network.
TABLE I – LINKS DATA
Rated Voltage
Commissioning Year
Link name
Rated Power
SaPeI
2 x 500 MW
±500 kV
2011
HVDC-LCC
Technology
SaCoI
300 MW
200 kV
1965/1992
3-terminal HVDC – LCC
SarCo
135 MVA
150 kV
2006
50 Hz 3-core cable
TABLE II – POWER PLANTS IN SARDINIA (2013)
Power Plants
Installed power
% of peak load
Thermoelectric
2640 MW
176%
Pumped Hydro (PH)
240 MW
16%
Hydro (no PH)
220 MW
15%
Wind
988 MW
66%
Photovoltaic
670 MW
45%
TABLE III – REACTIVE POWER SUPPORT DEVICES
SC
SVC
STATCOM
Dynamic reactive power support
Yes
Yes
Yes
Short circuit power
Yes
No
No
Inertia
Yes
No
No
Overload capability
Yes
Limited
Limited
Normalized cost [3]
100%
140%
150%
2. SYNCHRONOUS CONDENSER TECHNOLOGY
Synchronous condensers (SCs) have been used in HV network since the beginning of the past
century[4], for voltage regulation and reactive power support purposes; their use slowly
declined in the last decades due to introduction of static compensation devices such as SVCs
and STATCOMs. SCs are still widely used where dynamic reactive power support is needed
(e.g. in conjunction with HVDC-LCC converters)[5], also due to their better behavior during
nearby short circuits [6][7]. SCs are also often claimed to need more frequent and expensive
maintenance with respect to static systems; this can be an issue because, in general, electrical
substation are nowadays unmanned. On the basis of its SCs operational experience and after a
technological survey, Terna concluded that the present state of art of synchronous machines
technology overcome most of their reliability and safety issues; more in detail the following
improvements have been introduced, since the last SC had been specified in the Italian grid:
•
Increase of the maximum size of air-cooled machines up to 400 MVA and 21 kV.
•
Increase of mean time between maintenances (MTBM) and failures (MTBF).
•
Static excitation systems.
•
Static starting systems.
It must be also pointed out that, according to Terna experience, most of the outages are related
to auxiliaries and cooling systems and not to the machine itself, as visible in Fig. 2. The need
2
for hydrogen cooling in particular had a severe impact, in the past, due to safety regulation
and longer maintenance outages.
By contrast, off-the-shelf 2-pole generators (normally used for combined cycle power plants)
have thus been identified as the most proven, cheap and reliable technology for synchronous
condenser solution. The cost of air cooled units is also lower if compared to H2/H2O cooled
ones.
The state of the art in synchronous machines does not allow for the construction of a single, 2pole, 500 MVA air cooled unit: as a consequence two separate 250 Mvar units (directly
derived by 300 MVA generators), will be installed in the SC system.
3. SYNCHRONOUS CONDENSER DATA
The main purposes of the SC system purpose are to provide:
• Short circuit power
• Inertia
• Reactive power support
With reference to this purposes, the rating data have been chosen in order to maximize costeffectiveness of the system considering the industrial practice and avoiding prototypal
machines. As regards machine type, non-salient rotor design has been preferred, because the
first presents a better cost/MVA ratio and because under-excitation behavior of a non-salient
machine have been considered sufficient.
The most significant input data for the specification of SC system has been the target short
circuit power contribution (ΔScc), which network studies showed to be 2500 MVA. ΔScc, in
turn, depends on the ohmic values of subtransient reactance Xd’’ of the SC and on the short
circuit impedance Zcc of the step-up transformer. The use of a standard synchronous
machines (turbo-alternators ) results in xd’’ ≈ 10%; furthermore the minimum value of zcc ,
suggested in [8][8] in order to withstand short circuits, is 12.5%.
As a consequence, given the size of the synchronous machines of 250 Mvar, the requested
ΔScc has been attained by specifying step-up transformers with a rated power 330 MVA and
zcc =12.5%.
This leads to the desired value of incremental short circuit power ΔScc = 2550 MVA at
Codrongianos node. Synchronous condenser and step up transformer data are summarized in
table IV
4. COOLING SYSTEM
Several cooling methods have been used for large synchronous condensers:
 Direct Air Cooling (DAC)
 Totally Enclosed Air to Water (TEWAC)
 Hydrogen-Water cooling (H2/H2O)
 Fully water cooling [9]
The older condensers previously installed in the Italian Network used H 2/H2O cooling; this
solution, although still diffused, especially for very large generators, has some drawbacks,
like increased maintenance outages and fire/explosion risks. Fully Water cooled synchronous
condensers have been firstly realized during the 70’s [9], but at present this solution is not
used for power plants.
3
TABLE IV – SYNCHRONOUS CONDENSER AND STEP-UP TRANSFORMER DATA
Synchronous Condensers Data
Reference standard
Installation
Rated power (Mvar)
Rated voltage (kV)
Maximum continuous voltage (kV)
Rated Frequency (Hz)
Cooling method
Overload
Synchronous reactance (d / q axis)
Sub-transient reactance (unsaturated / saturated)
Inertia constant (s)
Total losses at full power
Insulation class (design / rated)
IEC 60034
Outdoor
+250 / -125
19 kV
20 kV
50
Totally Enclosed Air-to-Water
150 % for 30 s / 200 % for 10 s
180 % / 170 %
14.3 % / 10.1 %
≥1,75
< 3000 kW
B/F
Step-up Transformer Data
Reference standard
Rated power (MVA ONAN / ONAF)
Rated voltages (kV)
Short circuit impedance
IEC 60076
230 / 330
400/19 kV
12.5%
400 kV line
230 kV line
150 kV line
132 kV line
500 kV dc line
200 kV dc line
Fig. 1. HV/EHV Sardinian network.
As in most of the new Combined Cycle Power Plants, TEWAC cooling has been chosen for
the new synchronous condensers. The modular design of the cooling system allow for N-1
security and for easy maintenance.
4
3%
41%
38%
Oil Circuit
Starting System
Cooling System
Protections
18%
Fig. 2. Outage hours causes for the previous Terna’s synchronous condenser (statistics data from 1998
to 2008).
Adiabatic coolers (Fig. 3) have been adopted in order to reduce water consumption and
maintain full power output also in case of very high ambient temperature, without exceeding
the temperature limits of class B insulation. In fact, adiabatic coolers allow for fluid
temperatures lower than ambient dry bulb when there is sufficient difference between the dry
bulb and wet bulb is temperatures (as in most of the year in Codrongianos Substation) and for
a significant reduction in physical size of plant.
In case of faults on the cooling systems or reduced water availability it is possible to use the
adiabatic coolers as Dry Coolers. In this case the temperature limits of class F insulation are
respected, still allowing for a continuous operation at full power of the synchronous
condenser even at very high (40 °C) ambient temperatures.
5. STATIC EXCITATION AND STARTING SYSTEM
Static excitation system has been adopted in order to allow for faster response time and
negative ceiling. The static excitation system allows for a 200 % positive ceiling voltage and
for a 150 % negative ceiling voltage. The maximum current overload is 150 % of the rated
field current for 10 s; a redundant thyristor bridge design allow for N-1 secure operation.
Stating starting system has been adopted because the pony motor, used in the former
synchronous condenser, had been found to be a weak point, accounting for 18% of outage
hours. The new static starting system allow for a 15 minutes starting time, from zero speed to
synchronous operation.
Auxiliary power is drawn, in normal operation, from two 10 MVA 19 kV/15 kV auxiliary
transformers (one for each synchronous condenser); in case of outage of one auxiliary
transformer the remaining one can feed the auxiliary systems of both units. For the
commissioning phase and in case of severe faults, the 15 kV network can also be fed from a
150 kV /15 kV transformer, connected to the sub-transmission network, or directly from the
15 kV distribution network of ENEL.
Notably, the 15 kV auxiliary system will also be connected to the Codrongianos 20 MW
battery energy storage system.
5
Closed
circuit
cooling water (to
synchronous
condenser)
Closed circuit Air-to-Water
heat exchanger
Open circuit water nozzles
Filler
Air filters
Water tank
Fig. 3. Adiabatic cooler simplified scheme
6. EFFECT OF THE SYNCHRONOUS CONDENSER SYSTEM ON THE SARDINIAN NETWORK
As stated in the introduction, the Sardinian network already has some characteristics of the
future transmission grid. Furthermore Sardinia is at present time the only non-connected grid
in which both multi-terminal and multi-infeed HVDC systems are present.
These very particular conditions, in conjunction with the substantial development of
renewable energy power plants and load reduction to the financial crisis, prompt the need for
innovative solutions for improving the performance of the network.
“Smart grid” solution like energy storage systems are of paramount importance and will be
implemented in 2014. However presence of bulk HVDC transmission with LCC technology
also arise the need for a “Strong” network.
Under this regards, the most widely used parameters for assessing the robustness of a multiinfeed network are the Multi – Infeed SCR and the Multi – Infeed ESCR .
Those parameters, for some degraded operating conditions (i.e. multiple faults) of the
Sardinian network, especially in case of very high energy production with renewable power
plants (i.e. with very few thermo electric power plants in service), can reach troublesome
values for stable LCC-HVDC operation, as reported in table V.
The new SCs allow for a significant increase in MIESCR and SCR for both the HVDC links;
this allow for an higher renewable production (or, on the other hand, for a lower number of
conventional power plants to be in service), without reducing system security.
The effectiveness of non salient 2-pole air-cooled Synchronous Condensers for the dynamic reactive
6
support to HVDC multi in-feed systems, if compared to typical salient pole H2/H2O Condenser is
shown in Fig.4.
TABLE V – SYNCHRONOUS CONDENSERS EFFECT ON THE SARDINIAN NETWORK
degraded network conditions
with new
SCs
4,04
13,03
2,76
3,22
SCR SaPeI
SCR SaCoI
MIESCR SaPeI
MIESCR SaCoI
degraded network conditions
and high renewables production
without new SCs
with new SCs
2,26
7,54
1,31
1,73
3,13
10,08
2,02
2,38
without new
SCs
1,32
4,40
0,58
0,92
600%
500%
400%
Technical
literature
Codrongianos
SCs
Xd
Xd'
Xd"
H (s)
Pcc*
300%
200%
100%
Technical
literature
200%
35%
25%
120%
290%
Codrongianos
SCs
180%
17%
10%
170%
514%
Δ
-20.00%
-18.30%
-15.00%
41.70%
223.50%
0%
Xd
Xd'
Xd"
H (s)
Pcc*
Fig 4. Comparison of typical (H2/H2O cooled, salient pole) synchronous condenser data[12] and Codrongianos
SCs
7. CONCLUSIONS
The presence of multi in-feed, multi-terminal HVDC systems in a network with a substantial
production from renewable power plants prompts the need for innovative solution for
strengthening the grid. Under this regard the Sardinian network already exhibits some
characteristics of the future EHV network.
Air cooled, non salient 2-poles synchronous condensers, can be an effective solution for
improving the performance of the grid and are expected to be an useful component in the
EHV network of the future.
The two 250 Mvars SCs to be installed in Codrongianos in 2014 have some innovative
characteristics, which makes them significantly more effective if compared to normal (salient
poles) ones.
7
8. REFERENCES
[1] Terna document “Statistical Data on Electricity in Italy”, available online on:
www.terna.it/default/home_en/electric_system/statistical_data.aspx
[2] Mazzoldi, F. ; Taisne, J.P. ; Martin, C.J.B. ; Rowe, B.A. ,“Adaptation of the control equipment to permit 3-terminal
operation of the HVDC link between Sardinia, Corsica and mainland Italy”, Power Delivery, IEEE Transactions on
Volume: 4 , Issue: 2 Apr. 1989 , pp 1269 - 1274
[3] “Principles for Efficient and Reliable Reactive Power Supply and Consumption” Federal Energy Regulatory
Commission, Feb. 4, 2005 Docket no.AD05-1-000 , available online.
[4] Alger, P. L., “Synchronous Condensers”, American Institute of Electrical Engineers, Transactions of the” Jan 1928.
[5] C.V. Thio, J.B. Davies, “New Synchronous Compensators For The Nelson River HVDC System - Planning
Requirements And Specification” IEEE Trans. on Power Delivery Vol,.6 n°2, pp. 922-928, April 1991
[6] O.B. Nayak, A.M. Gole, D.G. Chapman, J.B. Davies. “Dynamic Performance Of Static And Synchronous Compensators
At An HVDC Inverter Bus In A Very Weak AC System” IEEE Trans. on Power Systems, Vol. 9. No. 3. Pp.1350-1358,
August 1994
[7] S.Teleke, T. Abdulahovic, T. Thiringer, and J. Svensson “Dynamic Performance Comparison of Synchronous
Condenser and SVC” IEEE Trans. on Power Delivery, Vol. 23, n°. 3,pp. 1606-1612, July 2008
[8] IEC standard 60076-5 “Power Transformers - Part 5: Ability to withstand short circuit”, 2006
[9] J.A. Oliver, B.J. Ware, R.C. Carruth, “345 MVA fully water-cooled synchronous condenser for Dumont station - part I.
application considerations”, Power Apparatus and Systems, IEEE Transactions on, Issue 6, Nov. 1971, pp. 2578-2764.
[10] Rassegna Tecnica Tecnomasio Italiano Brown Boveri “Compensatori sincroni in Idrogeno da 55 MVA per la
Ricevitrice Sud dell’AEM di Milano”, Anno XX, Oct.-Dec 1959.
[11] de Toledo, P.F. ; Bergdahl, B. ; Asplund, G. “Multiple Infeed Short Circuit Ratio; Aspects Related to Multiple HVDC
into One AC Network” in Proc IEEE /PES Transmission and Distribution Conference and Exhibition: Asia and Pacific,
Dalian, 18 Aug. 2005.
[12] T.J. Miller, “Reactive Power Control in Electric System” New York, John Wiley & Sons Inc,1982
8