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Power Flow Control on the Italian network by means of phase-shiftingtransformers
Enrico Maria Carlini*, Gabriele Manduzio*, Dietrich Bonmann**
* TERNA Italy** ABB Germany
SUMMARY
This paper presents the Italian experience in the installation of PSTs in the 380 kV Rondissone
substation, with particular regard to the characteristics of these devices, to the single line diagram and
lay-out adopted, to the operational strategy and maintenance considerations.
Design restraints such as maximum switching power of on-load tap changers, short circuit withstand
and transportation are briefly discussed. The special challenges in performing dielectric and thermal
factory testing on such large units are presented. Insulation coordination of the PST with the
surrounding network deserves special attention, since a PST is a series connected network element.
The selection of the station surge arresters protecting the PST is highlighted. The operational strategy
of a PST needs to determine criteria for the control of the phase angle. The automatic tap changer
controller allows different control modes: maintaining a remotely set power flow or remote control of
the tap position. The insertion and de-insertion of the PST requires special switching sequences.
A description of the operational impact of the PSTs is also provided as well as elements on the co-
ordination with other PSTs already installed. The paper finally describes solutions adopted by other
utilities in the world in similar PSTs projects.
KEYWORDS
PST, Phase shifting Transformers, power flow control, NTC, maintenance, operation experience.
21, rue dArtois, F-75008 PARIS A2 206 CIGRE 2006http : //www.cigre.org
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1 General Considerations About Import and Load Flows Between Italy, France and
Switzerland
The exploitation of the deregulated electricity market implies the utilization of the interconnection
transmission capacities wherever available under security conditions and this to happen within a
global European market, in the perspective of a pan-European real political/economic entity.
Considering the present and near future structural situation of power generation facilities in Italy (by
far, expectedly more expensive than in EU countries), the already high import capacity could be
further increased, both by better use of existing interconnections and also by the construction of new
interconnectors.
The northern Italian grid is interconnected to the neighboring countries with 17 circuits (8 circuits at
380 kV, 8 circuits at 220 kV and 1 circuit at 132 kV). These interconnections can be ideally grouped
in 3 corridors connecting Italy to France, Switzerland and Austria-Slovenia. In 2004, the net yearly
import to Italy through the northern interconnection has been equal to about 44 TWh. Each of
mentioned corridors is influenced by the protection scheme and by the dispatching criteria of the TSOs
of the neighbor countries.
Fig. 1: Power flow (GWh) exchange between Italy and other countries. Year 2004.
Compared to a total theoretical gross transfer capacity during winter period, equal to more than 12000
MW, the net transfer capacity (NTC) available today in import direction to Italy is actually equal to
7150 MW in winter peak hours and 6050 MW in summer peak hours. These NTC values are
calculated on the basis of the ETSO (European Transmission System Operators) endorsed
methodologies and are assessed for different typical time frame (peak and off-peak hours) and
different grid scenarios (in terms of statistical distribution of power flows on the interconnection). The
grid constraints are firstly determined by the electrical characteristics of the lines and the n-1 securityoperation of the meshed interconnection operation; loop flows, transient overloads, as well as the
transmission reliability margin (TRM) are also taken into consideration.
During the process of NTC assessment, the availability of operative countermeasures to solve grid
congestions, occurring in case of normal operating condition or consequently to the loss of grid
elements, are checked as well.
These strategies usually include:
automatic or manual tripping for the breakers of parallels of the bus bars, where possible;
automatic or manual connecting of power flow control devices as Phase shifting Transformers
(PSTs);
automatic or manual opening of internal lines a 132-220 kV for local congestions resolutions;
automatic or manual disconnection by using short time acting devices for generators placed on key-locations for congestions, introduced by occurring disturbances or system contingencies;
modulation of the generation reserve for every national transmission grid.
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In this framework, therefore, PSTs actually represent one of the available tools to relieve congestion
and, in general, to reduce the effects of the parallel flows and optimize the flows distribution on the
interconnection. This can significantly increase the level of operational security, within long and
medium-term system planning as well as in day-ahead operational planning and real-time operation
level.
2 Technical Characteristics of PST2.1 Functional SpecificationThe load flow and contingency calculations and feasibility studies regarding manufacture and
transportation of the PSTs led to the following nominal characteristics.
Nominal frequency: 50 Hz
Nominal PST voltages: 400 kV / 400 kV
Nominal line current: 2350 A
Nominal throughput power: 1630 MVA
Voltage shift type: Asymmetrical (quad-booster)
Off-load regulation angle: 18 (Advance)
On-load regulation angle12, with line current = 70% of In
Max commutation time over full regulating range: 180 s
Busbar short circuit current: 40 kA
Maximum sound pressure level at nominal voltageand frequency
78 dB(A)
Table I: Main items of the functional specification for the phase-shifting transformers
Limited space available in the substation led to the requirement of one three-phase unit for each of the
two transmission lines. The units had to meet the dimensional limits of the Italian railway since road
transportation had to be limited to an absolute minimum.
The regulation angle was specified in a functional manner as 12, under a line current of 70% of
rated current. This allowed the potential suppliers to find the best design compromise betweenminimum short circuit impedance needed for short circuit withstand of windings and tap changer and a
minimum no-load regulation angle for smallest possible physical size.
The N-1 criterion was applied to the operation of the coolers, and in addition an emergency operation
at 120% rated load with N-1 coolers as well as operation at rated load without coolers for 20 minutes
each were specified. The asymmetric quad booster concept for the PST was selected because it
allows maximizing the utilization of the on-load tap changers switching capacity. The moderate
variation of output voltage with angle regulation in the asymmetric quad booster concept could be
tolerated by the system in this case.
Fig. 1a: Exciter and booster transformers, winding connection. Voltage Phasor diagram.
Since the asymmetric quad booster concept does not need a 400 kV centre tap in the serieswindings, the connections between series and shunt transformer are greatly simplified.
Series unit
Main (shunt,
exciting) unit
V1S
V2S
V3S
U1reg
V
u1inj
V1SV1LV
u1inj
U1reg
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As pointed out in the general considerations, the PSTs have to be manoeuvred from a stand-by
operation to maximum advance phase shift in the shortest possible time in order to relieve overloads
on transmission lines in case of certain contingencies.
3 Realization of the Phase Shifting Transformer
The optimisation of the electrical design led to the following results:
Each PST consists of a three-phase series transformer with six 400 kV oil/air bushings and a three-phase shunt transformer with three 400 kV oil/air bushings. The connections between the regulation
windings of the shunt transformer and the excited windings of the series transformer are made via
oil/oil bushings in an oil filled duct. This solution allows separate handling and factory testing of the
transformers with well-defined interfaces and minimum exposure of the windings to the atmosphere
during transportation and on-site assembly. Electromagnetic forces due to external fault currents were
dimensioning for the minimum achievable short circuit impedance of the series transformer. The short
circuit impedance of the shunt transformer was designed as low as possible in order to minimize the
no-load phase angle. Short circuit forces in the shunt transformer were not a challenge.
Transient oscillations could be well controlled by proper winding design, such that no internal surge
arresters were necessary. The on-load tap changer consists of three single-phase units with a common
motor drive with coarse/fine-regulation and 33 positions. It fulfils the requirements regarding speed of
operation, short circuit withstand and overload ability.
Series transformer
Rated voltage induced in series winding 74875 V
Rated current through series windings 2353 A
Two-winding rated power 528.5 MVA
Shunt transformer
Rated voltage 400000 V
Rated current through series windings 725.8 A
Two-winding rated power 502.8 MVA
Complete PST
Short circuit impedance rel. to 1630 MVA at 0 angle regulation 11%Short circuit impedance rel. to 1630 MVA at 18 angle regulation 13.5%
Load losses 2596 kW
No-load losses 269 kW
Total weight 734000 kg
Dimensions L x W x H 18.2 x 12.6 x 10.25 m
Table II: Main technical data of phase-shifting transformers in Rondissone substation
Fig. 2: First of the two PSTs during assembly on site.
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4 Factory Testing
Where applicable the type and special tests according to IEC 60076 were applied.
Lightning and switching impulse tests were applied to the individual units. The switching impulse on
the series transformer could only be performed as a potential test, i.e. with source and load side
bushings connected together, as described in IEEE C57.135, since the transformation ratio of the
series transformer would have led to very high voltages in the excited windings. In normal operationthe voltage that can occur across the excited windings is limited by the connection to the regulating
windings of the shunt transformer.
Power frequency voltage withstand tests and sound level tests were performed on the individual units
as well as on the complete PST. The test winding in the exciter transformer has permanent bushings
suitable for induced voltage and partial discharge testing of the complete PST as well. Under normal
operation a metallic cover protects these bushings.
Load loss and temperature rise tests up to 120% of rated load could be performed on the individual
transformers. Due to the very high currents through the HV bushings of the series transformer this test
is often a challenge for transformer test rooms.
Measurements of phase angle regulation and zero sequence impedance voltages were performed on the
complete PST.
5 Verification of Insulation Coordination
At the end of the electrical design stage a transient model of the PST for implementation in ATP was
created. Various single-phase and three-phase switching events as well as lightning strokes at various
locations of the system were simulated in order to verify the dimensioning of the station surge
arresters located on both sides of the PSTs. Special consideration was given to the fact while a
lightning hits one end of a series winding the opposite end of the series winding may be at the peak of
opposite polarity of the operating AC voltage.
6 Single Line Diagram Adopted
The scheme adopted for the phase shifter, installed on the 380 kV double line Rondissone
Albertville, is shown in the figure below.
Fig. 3: Single line diagram adopted for the PST installation on the 380 KV double circuit line
Rondissone Albertville
This type of configuration allows to have the following advantages.
1. When the PST is out of order, every 380 kV connection with Albertville is on service.
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2. The normal operation of connection with the open by - pass and PST in stand - by is assured.3. It is possible:
to insertthe PST when the line is in service, without interruptions of grid connection;
the return of PST in stand by, when the line is in service and without interruptions ofgrid connection;
to put the PST out of order, when the line is in service.
With regard to the PST and by pass control system, the following conditions can be obtained:
PST in stand by connected on the busbar A and by pass closed on busbarA;
PST in stand by connected on the busbar B and by pass closed on busbar B;
PST closed on busbar A and open by pass;
PST closed on busbar B and open by pass;
open PST;
open by pass.
7. Testing, Operation and Maintenance Aspects
As can be seen in table II the PST has a short circuit impedance voltage at rated load of 11 to 13.5%. If
the PST is inserted into the line the PSTs impedance tends to reduce the power flow, which is the
opposite of what is necessary in a post-contingency situation when the power flow in the line shall be
increased as fast as possible in order to avoid overloading of parallel lines. This can be avoided by
bringing the PST into an equivalent zero tap position before opening the bypass. Equivalent zero
position means that the voltage drop caused by the load current is approximately balanced by the
voltage injected by the series windings of the PST. The switching sequence would be:
PST in stand-by, connected to the busbar; by-pass closed, carrying load current.
Close PST branch, tap changer in 0 or in another parking position. Load current remains
in the by-pass.
Move tap changer to equivalent zero position. The voltage injected by the series windings
of the PST superposes a circulating current through by-pass and PST over the load current.As a result the current in the by-pass becomes almost zero, the load current commutates into
the PST branch.
Open by-pass, load current flows through PST.
Normal power flow control mode of the automatic power flow controller can be enabled, if desired.
This switching sequence can be performed manually or as an automated command sequence. The
equivalent zero tap positions as a function of load current are given to the SCADA system in the
form of a table. The parking position of the tap changer can be set to a value that allows to minimize
the time for reaching the equivalent zero position.
The automatic power flow controller allows running the power lines at a constant power flow or at a
fixed tap position. The set values for power flow (in MW) or tap position can be set locally or via the
SCADA system.
With regard to maintenance each PST corresponds to two large 400 kV transformers in a strategically
important location and is treated like that according to TERNA standard procedures. In order to
support the maintenance planning, and in order to provide more early indicators of incipient faults all
four transformers are equipped with TEC (Transformer electronic control) system and a bushing
monitoring system. The data from the four TECs are transmitted via optical fiber to a common server
PC located in a substation building. The data from the four TECs and the four bushing monitoring
systems can be accessed from remote locations via Intranet using standard web browsers.
Figures 4a 4c show some of the data that was downloaded after approximately one year of operation.
However, this huge amount of data may be of interest in case of detailed
investigations after a fault has occurred, but for the operators at the dispatch center it is of little value
during normal operation. Therefore the TEC system continuously supervises whether the oil, winding
and tap changer temperatures are consistent with the actual load, actual ambient temperature andnumber of coolers in operation. In case of discrepancies the TECs provide alarm signals to the
SCADA system for:
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- Hot Spot temperatures- Moisture in oil- Thermal balance tank- Thermal balance for the on-load tap-changers (OLTC)- Top oil temperature in the OLTC- OLTC needs maintenance- OLTC contacts need to be replaced
The bushing monitoring system measures voltages, leakage currents and their phase at the nine 400 kV
bushings of each PST. The system calculates capacitances and loss factors of the bushings. Based on
an internal expert system software the bushing monitoring system issues alarm signals of three levels
of urgency for the SCADA system. Error codes at the local display of the monitoring system give
further indications of the type of bushing problem.
Further alarms are generated by the gas-in-oil sensor (Hydran): Gas-in-oil concentration high and Gas-
in-oil concentration high-high; In case of a condition monitoring alarm signal the remote operator has
the option to view detailed information and recommendations for action via intranet access to the TEC
and bushing monitoring systems.
Fig. 4a: Main parameters recorded by TEC for the PST 2 series unit
Fig. 4b: Main parameters recorded by TEC for the PST 2 exciter unit
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Fig. 4c: Main parameters recorded by TEC for the on-load tap changer. The tap changer consists ofthree single phase units.
In order to support diagnostic measurements in case of need the PSTs design allows access to the
connections between exciting and regulating windings. After draining just very few 1000 liters of oil
from the short oil duct between series and exciter transformers the connections between exciting and
regulating windings can be opened. The terminals are then accessible for low voltage tests of the
individual transformers: ratio, impedance, and resistance.
The 15 kV test winding in the exciter transformer has permanent bushings suitable for induced voltage
and partial discharge testing of the complete PST. In normal operation a metallic cover protects these
bushings.
8 Experience in PSTs utilization, operational impact on the system and co-ordinationTwo main modalities of Rondissone PSTs utilization are currently applied by the National Control
Centre (NCC) of the Italian TSO:
Preventive action on unbalanced distribution of power flowsPSTs act to redistribute power flows on the northern interconnection under fully meshed grid
conditions, aiming at keeping secure operating conditions.
This modality is presently the most frequently adopted and allows increasing the firmness of
the cross-border exchange programs while guaranteeing adequate standard of security.
Corrective action on post-fault situationsPSTs are activated in case of tripping of one of the 380 kV interconnectors between Italy and
Switzerland or in case of tripping of the 380 kV Venaus-Villarodin interconnector between
Italy and France. In the first case, the contribution of PST operation is the increase of the
power flows from France to Italy on the 380 kV double-circuit Rondissone Albertville to
restore in due time security conditions on the rest of the northern interconnection. In the
second case, Rondissone PSTs operation contributes to timely relieve security violations on
the northwestern Italian 220 kV grid.
This modality of utilization allows an increase of the levels of operational security at real-time
level. It has been actually less used in the recent past, in particular due to the introduction in
January 2005 of the new CH-IT double circuit interconnector named S.Fiorano-Robbia.
Other modalities of PSTs utilization are however allowed in operation. It is worthwhile noting thepossibility to adjust flows on the interconnection under conditions of major tie-lines/internal lines
unavailability, for scheduled or unscheduled maintenance activities. In this case, also, PSTs provide a
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further degree of flexibility in achieving the interconnection security, without recurring to reduction of
exchange programs or curative redispatching actions.
Fig. 5: Re-distribution of the power flows in import direction to the Italian system, as determined by
the operation at extreme taps of the two Rondissone PSTs (PSTs in stand-by position, PSTs tap 33)
The experience of the possible impact of Rondissone PSTs operation on neighboring systems have
suggested the need of an appropriate and deep co-ordination of PSTs operation between the TSOs
involved. Co-ordination covers a crucial role to optimize the efficiency of PSTs utilization and
guarantee the best standards of operational security.
With this purpose, the Italian and French System Operators have set-up a joint procedure for the co-
ordinated operation of the PSTs installed in 380 kV Rondissone substation and La Praz (F). In fact,due to the grid topology, one of the circuits of the 380 kV Rondissone-Albertville interconnector is
electrically close to the 380 kV La Praz-Villarodin-Venaus corridor, on which the La Praz PST is
installed. Therefore, the uncontrolled variation of the Rondissone PST taps in question could result in
a direct and opposite effect on the La Praz corridor and vice versa.
The procedure allows the optimized utilization of the available means for congestion while increasing
the level of operational security of the northern interconnection and, in particular, of the F-I border.
The common objectives and the tasks and actions of the NCCs during the possible scenarios of
operation are specified. The co-ordination results also in reducing the risk of tap-change hunting by
TSOs when operating conditions change and in the increase reliability of PSTs operation in case of
unavailability of one of the transformers.
In normal operation, two of the three PSTs are operated to manage flows on F-I interconnection. In
case of very unbalanced flows on the Northern Italian grid and specifically of very low flows on F-Iborder, the simultaneous operation of the three PSTs is undertaken to push flows on that border.
Co-ordination in PSTs handling is implemented at three stages:
| Long and medium-term system planning, to perform joint security analyses on the impact of
all the PSTs operation on the interconnected grid.
| Day-ahead operational planning, to optimise the PSTs utilisation based on the day-ahead load-
flow calaculations and n-1 security analyses
| Real-time operation, to relieve security violations through a co-ordinated congestion
management strategy. This modality can be undertaken under fully meshed grid conditions
(preventive way) or following the loss of one grid element (corrective way).
The co-ordination in PSTs operation is based on the mutual exchange of information regarding thesystem state and the objectives of the TSOs in optimising PSTs taps. The real-time exchange of PSTs
selected taps represents an effective mean to monitor the switching margins available for relieving
ITALIA
Villarodin
Venaus
S. Fiorano
RobbiaLienz
Soverzene
Redipuglia
Divaca
Padriciano
Soazza
Bulciago
Gorduno
Mese
Airolo
Ponte
MusignanoPallanzeno
MoerelRiddes
AviseValpelline
Albertville
Rondissone
CamporossoLe Broc-Carros
Lavorgo
4499 MW4499 MW 4053 MW4053 MW
SWITZERLANDSWITZERLAND
1597 MW1597 MW
2087 MW2087 MW
FRANCEFRANCE
856 MW856 MW
824 MW824 MW
SLOVENIASLOVENIA
198 MW198 MW 189 MW189 MW
AUSTRIAAUSTRIA
ITALIA
Villarodin
Venaus
S. Fiorano
RobbiaLienz
Soverzene
Redipuglia
Divaca
Padriciano
Soazza
Bulciago
Gorduno
Mese
Airolo
Ponte
MusignanoPallanzeno
MoerelRiddes
AviseValpelline
Albertville
Rondissone
CamporossoLe Broc-Carros
Lavorgo
4499 MW4499 MW 4053 MW4053 MW
SWITZERLANDSWITZERLAND
4499 MW4499 MW 4053 MW4053 MW
SWITZERLANDSWITZERLAND
1597 MW1597 MW
2087 MW2087 MW
FRANCEFRANCE
1597 MW1597 MW
2087 MW2087 MW
FRANCEFRANCE
856 MW856 MW
824 MW824 MW
SLOVENIASLOVENIA
856 MW856 MW
824 MW824 MW
SLOVENIASLOVENIA
198 MW198 MW 189 MW189 MW
AUSTRIAAUSTRIA
198 MW198 MW 189 MW189 MW
AUSTRIAAUSTRIA
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congestion and to increase the accuracy of the security analyses as far as the PST simulation is
concerned.
Such level of co-ordination between TSOs represents at the moment a unique example in the UCTE
community.
9 Single line diagrams adopted from other utilities in the world in similar projectPSTs devices can be adopted to fit the need of liberalized electricity market requiring ever more global
interconnected electricity networks. Interesting feasibility studies by introducing PST in strategic
European electric nodes to optimize\increase the power flow among different countries, are going on.
To point out :
Netherlands-Germany Interconnection (Meeden substation)
France-Italy interconnection (La Praz substation)
UK National Grid (Keadby substation)
9.1 PST Meeden substation. [1]
In recent years the liberalization of the European market has pointed out the need to strengthen the
interconnection between Netherlands and Germany. TenneT, the Dutch Transmission System Operator
(TSO), has been faced therefore with the desire of the market to increase the available cross-border
capacity. The installation of PSTs at the Dutch 380 kV substation Meeden in series with the two
interconnections to the grid of E.ON Netz, allows to upgrade the NTC of the cross-border line
Meeden-Diele/Conneforde with about 1100 MVA. The Netherlands has five interconnections,
comprising 10 circuits, with two neighboring countries. Because of the geographical and electro
technical location of the country, main interconnection flows are directed from Germany to the
Netherlands. The loading of the circuits in the South border, is often causing a transfer limitation.
Installing phase shifters in the 380 kV substation Meeden at the German border in the North would be
most beneficial regarding import capacity.
Fig. 6: Single line diagram adopted for the PST installation at the Meeden substation
9.2 PST La Praz substation. [2]
As mentioned beforehand, the 400 kV interconnection between France and Italy consists of a high
capacity two circuit 400 kV line (north Albertville Rondissone line) and a weaker one circuit 400 kV
line (south Venaus Villarodin). The south line has a far lower capacity than the north line, which
raises two problems, It does not allow optimum use of the water resources in the Alps region and does
not enable to increase the European exchanges.
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Fig. 7: Single line diagram adopted for the PST installation at the La Praz substation
The PST has been thought with a rating of 1181 MVA and the maximum phase shift has beenspecified to be 15, so as to enable the power system to remain stable after the loss of the double north
line. An automatic controller will alter the phase shift according to the situation and the kind of event
occurring: in normal operating conditions, the phase angle will be set near to zero so as to avoid
disturbances for the Italian TSO. The phase angle will be shifted to its maximum for the most severe
contingency (loss of the double line). The scheme in figure 7, consisting of a PST with a by pass
disconnector, allows:
use of tap changer in real time;
easy utilizations of the PST in the different conditions;
a reasonable flexibility of operation.
9.3 PST Keadby substation. [2]
The main difference between French and United Kingdom scheme is use of a circuit breaker at the
bottom of circuit.
Fig. 8 Single line diagram adopted for PST installation at the Keadby substation
The largest quadrature boosters in the UK are connected in the 400 kV system and have a throughput
rating of 2750MVA and the ability to shift the angle by up to 11.3 degrees in either direction by meansof 19 voltage taps in each direction. The Quadrature Booster in question is a 2750 MVA, 400/400 kV
+/-j80 kV unit to be installed at the National Grid Transco (NGT) 400 kV substation at Keadby, near
Scunthorpe on Humberside.
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10 Conclusion
In the deregulated European electricity market the trading of electrical power over larger distances as
well as the installation of new generation, especially wind power, create new load flow patterns that
often challenge the capacity or security of existing transmission lines. PSTs can be inserted into a line
with just two extra disconnectors for less demanding applications or with a bypass and two or three
extra circuit breakers for the highest demands of flexibility and protection. Different modes of anglecontrol can be chosen. The design and installation of two PSTs at Rondissone substation near Torino
is an example of an application in a strategically important network node and is described in some
detail. The special considerations given to maintenance and condition monitoring are described. The
modality of PSTs operation is described as well as its impact and sensitivity on the redistribution of
flows across the northern Italian interconnection; elements on the co-ordination with La Praz PST are
also provided. Three further applications of PSTs are briefly highlighted. Phase-shifting transformers
are a demanding yet well-proven means of controlling quasi-stationary load flows.
BIBLIOGRAPHY
[1] W.L. Kling, D.A.M. Klaar, J.H. Schuld, TenneT bv A.J.L.M. Kanters, C.G.A. Koreman, TenneTbv H.F. Reijnders, C.J.G. Spoorenberg, Smith Transformatoren B.V.
Cigr paper C2-207, 2004
[2] M.H. Baker, Reason to prefer a Phase-shifting transformer (PST) and which studies are required.Cigr SC 14, Paris session, 2002
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