Increased grid performance using synchronous ...

Study Committee A1 Rotating Electrical Machines

Paper A1_112_2014

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

Motivation The EHV/HV network of Sardinia is part of the Italian national grid, owned and operated by Terna. In order to increase the transmission network flexibility, Terna installed a 2x250 Mvar synchronous condenser system in Codrongianos sub station. SaCoI

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.

SaPeI SarCo

The Sardinian Network The Sardinian network is connected to Corsica by SarCo 150 kV cable and by the 3-terminal SaCoI HVDC. The new SaCoI HVDC, sharing also the same anode of the SaCoI, allow direct connection with continental Italy. The Sardinian network is thus an example of multi-infeed, multi terminal HVDC network. Link name Rated Power SaPeI 2 x 500 MW SaCoI 300 MW SarCo 135 MVA

Rated Voltage ±500 kV 200 kV 150 kV

Technology HVDC-LCC 3-terminal HVDC – LCC 50 Hz 3-core cable

Due to the large number of windfarm (988 MW) and PV (670 MW) in some cases the generation from renewable power sources exceed the local load. Proper reactive power support and short circuit capacity are thus of paramount importance for the safe operation of the grid. Power Plants Thermoelectric Pumped Hydro (PH) Hydro (no PH) Wind Photovoltaic

Installed power 2640 MW 240 MW 220 MW 988 MW 670 MW

% of peak load 176% 16% 15% 66% 45%

Synchronous condensers (SC) are the most effective solution for increasing grid performances in multi-infeed scenarios with substantial generation from renewable energy sources, increasing both short circuit power and inertia. Solution

SC

SVC

STATCOM

Dynamic reactive power support

Yes

Yes

Yes

Short circuit power Inertia

Yes Yes

No No

No No

Overload capability

Yes

Limited

Limited

Normalized cost [3]

100%

140%

150%

Codrongianos Station

HV/EHV transmission network in Sardinia

Main SC data and characteristics The new 2 x 250 Mvar SCs have some innovative differences with respect to traditional ones: • Round rotor design, because of its better cost/Mvar ratio • 2 poles design, in order to reduce weight and cost • Air to water cooling (TEWAC) in order to simplify maintenance • Fast static starting system (< 15 min) • Adiabatic cooling system for the primary water circuit • 200% overload capability (10 s) • Completely unmanned operation from remote control centre. Synchronous Condensers Data Manufacturer Ansaldo Energia Installation Outdoor Rated power (Mvar) +250 / -125 Rated voltage (kV) 19 kV Max.cont.voltage (kV) 20 kV Rated Freq. (Hz) 50 Cooling TEWAC Overload 200 % for 10 s Xd/Xq 180 % / 170 % Xd” (unsat – sat) 14.3 % / 10.1 % Inertia constant (s) ≥1,75 Total losses 2900 kW Insulation class (design / rated) B/F Step-up Transformer Data Manufacturer ABB Rated power (MVA ONAN / ONAF) 230 / 330 Rated voltages (kV)

400/19

Short circuit impedance

12.5%

http://www.cigre.org

Study Committee A1 Rotating Electrical Machines

Paper A1_112_2014

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

Static Excitation/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 standsill to synchronous operation.

Cooling system TEWAC cooling allow for a reliable operation, with low losses and noise. Internal heat exchangers, water pumps and cooling towers are redundant in order to increase reliability and maintainability.

250 Mvar Synchronous condenser during installation

degraded network conditions

SCR SaPeI

4,04

2,26

3,13

without new SCs 1,32

SCR SaCoI

13,03

7,54

10,08

4,40

MIESCR SaPeI

2,76

1,31

2,02

0,58

MIESCR SaCoI

3,22

1,73

2,38

0,92

with new SCs

Adiabatic coolers in Codrongianos substation

Effect of SC on the Sardinian network

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.

without new SCs

with new SCs

600% 500%

Presence of bulk HVDC transmission with LCC technology arise the need for a “Strong” network. Under this regards, the most widely used parameters for assessing the robustness of a multi-infeed 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 can reach troublesome values for stable LCC-HVDC operation.

degraded network conditions and high renewables production

400%

Technical literature Codrongianos SCs

Xd Xd' Xd" H (s) Pcc*

300% 200% 100% 0% Xd

Xd'

Xd"

H (s)

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%

Pcc*

Conclusion • SCs are the most effective technology for increasing short circuit power and inertia and are expected to be an useful component in the EHV network of the future. • Air cooled, non salient 2-poles design, can be a costeffective solution for SC. • The two 250 Mvars SCs in Codrongianos will improve grid performances and enable a smooth operation of the Sardinian multi-infeed, HVDC system

http://www.cigre.org