elimination of bottlenecks in transmission – … · elimination of bottlenecks in transmission...

"JORNADAS TECNICAS SOBRE "SESION PLENARIA CIGRÉ 2004" SESION MONOGRAFICA SOBRE FACTS - HVDC 30 de noviembre y 1 de diciembre de 2004

Elimination of Bottlenecks in Transmission – Benefits of FACTS and HVDC

Juan Miguel Pérez de Andrés, Miguel Mühlenkamp, Dietmar Retzma

Siemens AG 1. Introduction The growth and extension of AC systems and consequently the introduction of hhave been driven by a fast growth of power demand over decades. Power systems by applying interconnections to the neighboring systems to achieve technicadvantages. Regional systems have been built-up towards national grids and latesystems with the neighboring countries. Large systems came into existence, coverwhole continents, to gain the well known advantages, e.g. an ability to use larger anpower plants, reduction of reserve capacity in the systems, utilization of the mresources, and to achieve an increase in system reliability. Global studies consumption in the world follows closely the increase of population. In the neconsumption in developing and emerging countries is expected to increase for 220countries, however, only for 37%.

Priorities in future developments will be given to low costs at still adequate tereliability. Environmental constraints will also play an important role. Embedding osources, such as wind farms and dispersed generation will be a key-issue in many c 2. Development of Power Systems The development of electric power supply began more than one hundred years agand neighboring establishments were supplied first by DC via short lines. At tcentury, however, AC transmission has been introduced utilizing higher voltagesfrom remote power stations to the consumers. Power systems have been extinterconnections to the neighboring systems to achieve technical and economical adsystems have been built-up towards national grids and later to interconnectedneighboring countries.

In Europe, 400 kV became the highest voltage level, in Far-East countries mosAmerica 550 kV and 765 kV. The 1150 kV voltage level has been anticipated countries and also some test lines have already been built. However, it is not e

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nn

igher voltage levels have been extended al and economical r to interconnected ing parts of or even d more economical

ost efficient energy show that power

xt 20 years, power %, in industrialized

chnical quality and f renewable energy

ountries.

o. Residential areas he end of the 19th to transmit power ended by applying vantages. Regional systems with the

tly 550 kV, and in in the past in some xpected in the near

future that voltage levels above 800 kV will be utilized to a larger extent. The developments in AC transmission voltages are depicted in Fig 1.

1600

1200

1000

1400

1920 1940 1960 1980 2000

800

400

0

kV

1900Year

12

34

5

6

1910 1930 1950 1970 1990

200

600

Source: Siemens PTD SE NC - 2000

1 110 kV Lauchhammer – Riesa / Germany (1911)

2 220 kV Brauweiler – Hoheneck / Germany (1929)

3 287 kV Boulder Dam – Los Angeles / USA (1932)

4 380 kV Harspranget – Halsberg / Sweden (1952)

5 735 kV Montreal – Manicouagan / Canada (1965)

6 1200 kV Ekibastuz – Kokchetav / USSR (1985)

EHV: 800 kV = “Realistic”

1600

1200

1000

1400

1920 1940 1960 1980 2000

800

400

0

kV

1900Year

12

34

5

6

1910 1930 1950 1970 1990

200

600

Source: Siemens PTD SE NC - 2000

1 110 kV Lauchhammer – Riesa / Germany (1911)

2 220 kV Brauweiler – Hoheneck / Germany (1929)

3 287 kV Boulder Dam – Los Angeles / USA (1932)

4 380 kV Harspranget – Halsberg / Sweden (1952)

5 735 kV Montreal – Manicouagan / Canada (1965)

6 1200 kV Ekibastuz – Kokchetav / USSR (1985)

EHV: 800 kV = “Realistic”

Fig. 1: Development of AC Transmission

The increase in AC voltage levels has been followed by the development of new technologies in the field of high voltage substation technologies, and by innovations in design and manufacturing of the equipment, such as GIS (Gas-Insulated Switchgear) for both in-door and out-dour substations and GIL (Gas-Insulated Lines). GIL are an economically viable alternative to conventional AC power cables. The high safety standards permit trouble-free and EMC friendly operation even in tunnels along traffic routes, including railways and streets.

The performance of power systems decrease with the size and complexity of the networks. This is related to problems with load flow, power oscillations and voltage quality. If power should be transmitted through the interconnected system over longer distances, transmission needs to be supported. This is for example the case in the West-European UCTE system (Fig. 2a), where the 400 kV voltage level is relatively low for large cross-border and inter-area power exchange. Bottlenecks are already identified (NTC - Net Transfer Capacity, Fig 2b), and for an increase of power transfer, advanced solutions need to be applied. Such problems are even deepened by the deregulation of the electrical power markets, where contractual power flows do not follow the design criteria of the existing network configuration.

Large blackouts in America and Europe confirmed clearly, that the synchronous coupling of neighboring grids systems might also include risk of uncontrollable cascading effects in large and heavily loaded systems, see Fig. 3. The figure shows, that the synchronous systems Canada/Ontario-USA have been fully affected by the cascading blackouts, whereas Québec, due to its DC interconnections, “survived” the outage, and in addition, the DC links assisted for power supply and system restoration.

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NORDEL

IPS/UPS

UCTE - 1

AL MAGHREB

UCTE - 2Turkey

NORDEL

IPS/UPS

UCTE - 1

AL MAGHREB

UCTE - 2Turkey

Source: UCTE - 5 / 2003

NTC Values for East-West Power Transfer

Bottlenecks in the UCTE System

Source: UCTE - 5 / 2003





b)a)

Fig. 2: European Power Systems (2003) a) Overview of the Systems b) Bottlenecks in UCTE

Additiintegra

Before the Blackout Source: EPRI 2003

Some of the Reasons were:Overloads and Loop Flows

Leading to Voltage Collapse

Blackout: a large Area is out of Supply

However, some Islands have still local Supply

Québec´s HVDC assist for Power Supply & System Restoration

Before the Blackout Before the Blackout Source: EPRI 2003







Blackout: a large Area is out of SupplyBlackout: a large Area is out of Supply

However, some Islands have still local SupplyHowever, some Islands have still local Supply



Fig. 3: Blackouts 2003 - Example United States

onal problems are expected when renewable energies, such as large wind farms, have to be ted into the system, especially when the connecting AC links are weak and when there is no

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sufficient reserve capacity in the neighboring system available. Fig. 4 summarizes the perspectives of power system developments.

Increasing part of the installed capacity will, however, in the future be connected to the distribution levels (dispersed generation), which poses additional challenges on planning and safe operation of the systems, see Fig. 5.

In such cases, power electronics can clearly strengthen the grid and improve the performance of transmission, sub-transmission and distribution systems.

Fig. 4: Trends in High Voltage Transmission Systems

PrivatisationGlobalisation/Liberalisation

Deregulation - Privatization: Opening of the markets, Independent Transmission Companies ITCs, Regional Transmission Organisations RTOs

PrivatisationBottlenecks inTransmission

Problem of uncontrolled Loop-FlowsOverloading & Excess of SCC LevelsSystem Instabilities & Outages

PrivatisationInvestments inPower Systems

System Enhancement & Interconnections:Higher Voltage LevelsNew Transmission TechnologiesRenewable Energies













Fig. 5: Perspectives of Distribution Network Developments

Today: Tomorrow:

G

G

G

G

G

G

G

G

G

G

G

G

Today: Tomorrow:

G

G

G

G

G

G

G

G

G

G

G

G

3. Perspectives of HVDC and FACTS

Since the 60s, HVDC (High Voltage DC transmission) and FACTS (Flexible AC Transmission Systems) has developed to a viable technique with high power ratings. From the first small DC and AC "mini networks", at the end of the 19th century, there are now systems available transmitting 3 - 4 GW over large distances with only one bipolar DC transmission: 1.000 - 2.000 km or more are feasible with overhead lines. With submarine cables, transmission levels of up to 600 – 800 MW over distances of nearly 300 km have already been attained, and cable transmission lengths of up to 1.300 km are in the planning stage. As a multiterminal system, HVDC can also be connected at several points with the surrounding three-phase AC network.

Fig. 6 shows a solution, which is under discussion for embedding of very large amounts of wind energy in Germany (2003: 12 GW, in future 30-50 GW) to be delivered from planned off-shore wind farms in North- and East-sea regions, were the neighboring grids are too weak for such large additional power infeed. By means of HVDC, the energy can be “bundled” and then transported to the load centers in the middle of the country. The benefits of this solution are indicated in the figure.

Share in installed wind energy of 12,223 MW

E. ON Netz: 48 %Vattenfall Europe Transmission: 37 %RWE Net: 14 %EnBW Transportnet

FACTS, based on power electronics, have been developed to improve the performance of weak AC Systems and for long distance AC transmission. FACTS can, however, contribute to solve also technical problems in the interconnected power systems. Excellent operating experiences are available worldwide and the technology became mature and reliable. FACTS is applicable in parallel connection or in series or in a combination of both. The rating of shunt connected FACTS controllers is up to 800 Mvar, series FACTS devices are implemented on 550 and 735 kV level to increase the line transmission capacity up to several GW. Fig. 7 shows the site view of a large SVC (Static Var Compensator) installation in South-America. The SVC has been installed to support the stability of the large transmission system, ref. to the figure. The containerized solution offers many benefits, such as fast installation and commissioning times plus space and cost-savings.

Fig. 6: Embedding of large amounts of Wind Energy by means of HVDC Long Distance Transmission

ze: 1 %

E. ON Netz: 48 %

Share in installed Wind Energy of

Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %

E. ON Netz: 48 %E. ON Netz: 48 %

12,223 MWShare in installed Wind Energy of

Vattenfall Europe Transmission: 37 %Vattenfall Europe Transmission: 37 %

RWE Transportnetz Strom: 14 %RWE Transportnetz Strom: 14 %EnBW Transportnetze: 1 %EnBW Transportnetze: 1 %

12,223 MW

Benefits of such a Solution:o Load Sharingo Generation Reserve Sharing

VattenfallEurope Transmission

Source: E.ON - 2003

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By the use of new, high power direct light-triggered thyristors (LTT), significant benefits can be achieved, as shown in the Fig. 8. Siemens uses this innovative technology for both HVDC and FACTS controllers. Highlights are less electronic components, leading to an increased reliability, in combination with a unique wafer-integrated thyristor over-voltage protection.

In Fig. 9, the stepwise assembly of the thyristors in modules and valve group is shown. An additional, important feature of these high power electronic components is a flame-retardant design of the elements. Recent developments in the FACTS technology are the TPSC (Thyristor Protected Series

V a lves & C o n tro l B enefits:

o Im provem ent o f V o ltage Q ualityo Increased S tab ilityo A vo idance o f O utages

V o ltage C ontro l R eactive P o w er C ontro lP ow er O scilla tion D am pingU nbalance C ontro l (O ption )

V a lves & C o n tro lV a lves & C o n tro l B enefits:

o Im provem ent o f V o ltage Q ualityo Increased S tab ilityo A vo idance o f O utages



Fig. 7: SVC Bom Jesus da Lapa 500 kV, +/-250 MVar, Enelpower, Brazil - Containerized Solution

Fig. 8: Benefits of LTT for HVDC and FACTS

LTT: Technical & Economical Advantages

80 % less Electronic Components

Less Electric Wiring & Fiber Optic CablesReduced Spare Parts Requirements

Wafer-integrated Over-voltage Protection

Maximum Reliability & Availability - Benefits of LTT

Thyristor Valve with Direct-Light Triggering 100 mm Thyristors with integrated Break-over Protection

The safest Valve Technology

LTT: Technical & Economical Advantages

80 % less Electronic Components

Less Electric Wiring & Fiber Optic CablesReduced Spare Parts Requirements

Wafer-integrated Over-voltage Protection

Maximum Reliability & Availability - Benefits of LTT

Thyristor Valve with Direct-Light Triggering 100 mm Thyristors with integrated Break-over Protection

The safest Valve Technology

The active portion of the valve becomes a straightforward assembly of thyristors, heat sinks, and cooling-water piping

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Compensation) and the Short-Circuit Current Limiter (SCCL), both innovative solutions using special, very high power thyristor technology.

In Fig. 10, the benefits of using power electronics for system enhancement are summarized and a comparison of switching frequency of line-commutated thyristor devices and self-commutated VSC (Voltage Sourced Converters) is depicted.

It can be seen, that the thyristor technology still offers significant benefits with regard to the transmission losses, which is a major issue for large projects. This is the main reason, why the markets for VSC technology applications are still small.

Fig. 9: Advanced Power Electronic Components (Example HVDC)

Fig. 10: Use of Power Electronics for HVDC & FACTS - Transient Performance & Losses

More Dynamics for better Power Quality:

Use of Power Electronic Circuits for Controlling P, V & QParallel and/or Series Connection of ConvertersFast AC/DC and DC/AC Conversion

ThyristorThyristor

50/60 Hz

ThyristorThyristor

50/60 Hz

GTOGTO

< 500 Hz

GTOGTO

< 500 Hz

IGBT / IGCT

Losses

> 1000 Hz

IGBT / IGCT

LossesLosses

> 1000 Hz

Transition from Slow to FastTransition from Slow to Fast

Switching Frequency

On-Off Transition 20 - 80 ms

Transition from Slow to FastTransition from Slow to Fast

Switching Frequency


Transition from Slow to FastTransition from Slow to FastTransition from Slow to FastTransition from Slow to Fast

Switching Frequency


1-2 %1-2 %

4-5 %4-5 %

4. Phase Shifting Transformer versus Power Electronics

Phase shifting transformers have been developed for transmission system enhancement in steady state system conditions. The operation principle is voltage source injection into the line by a series connected transformer, which is fed by a tapped shunt transformer, very similar to the UPFC (Unified Power Flow Controller), which uses VSC-Power Electronics for coupling of shunt and series transformer. So, overloading of lines and loop-flows in meshed systems and in parallel line configurations can be eliminated. However, the speed of phase shifting transformers for changing the phase angle of the injected voltage via the taps is very slow: typically between 5 and 10 s per tap, which sums up for 1 minute or more, depending on the number of taps.

For successful voltage or power-flow restoration under transient system conditions, as a thumb rule, a response time of approx. 100 ms is necessary with regard to voltage collapse phenomena and “First Swing Stability” requirements. Such fast reaction times can easily be achieved by means of FACTS and HVDC controllers. Their response times are fully suitable for fast support of the system recovery. Hence, dynamic voltage and load-flow restoration is clearly reserved to power electronic devices like FACTS and HVDC. In conclusion, phase shifting transformers and similar devices using mechanical taps can only be applied for very limited tasks with slow requirements under steady state system conditions. 5. Elimination of Bottlenecks in Transmission Systems Fig. 11 shows an example of the West-European system: 500 MW should be transported from Hungary to Slovenia. It can be seen, that this power flow is spread widely through the neighboring systems. Only a limited amount of power is flowing directly to the target location. Using a power electronic device for power-flow control, e.g. HVDC Back-to-Back as “GPFC” (Grid Power Flow Controller), the power exchange between the two countries can be improved significantly.

DE CZ

Uncontrolled Load-Flow

DE

Power-Flow Controller

CZControl of Load-Flow

Benefits:Directing of Load-Flow

Basis for Power Purchase Contracts

DE CZDE CZ

Uncontrolled Load-Flow

DE

Power-Flow Controller

CZControl of Load-Flow





Fig. 11: UCTE – Load-Flow Improvement by means of Power Electronics

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If the AC systems are linked at different locations, power loop-flows can occur dependent on the changing conditions in both networks and in case of outages of lines. This was for example a strong issue during the events of the US-Canada Blackouts due to local overloads, leading to line trips and system disconnections, see Fig. 12.

Source: National Transmission Grid Study; U.S. DOE 5/2002 – “Preview”

Source: National Transmission Grid Study; U.S. DOE 5/2002 – “Preview”

System Enhancement necessary:

Source: ITC 8/2003 – “Blackout”Source: ITC 8/2003 – “Blackout”Source: ITC 8/2003 – “Blackout”

Giant Loop Flows 2.2 - 4.8 GW

Fig. 12: US-Canada Blackout - Overloads and Loop Flows

Fig. 13 gives an example how FACTS and HVDC (in this case UPFC or HVDC B2B as GPFC) can direct power flow across the interconnections between two systems.

360 km

400 MW

Loads

Loads

3 ~

Power- Flow Controller

3 ~

Restoration of the initial Power Flow

200 MW

… Quite Easy

A B

360 km

400 MW

Loads Loads

Loads Loads

3 ~3 ~



3 ~3 ~

Restoration of the initial Power FlowRestoration of the initial Power Flow

200 MW

… Quite Easy… Quite Easy

A B

Fig. 13: Avoidance of Loop-Flows with Power-Flow Controller (GPFC or UPFC)

In Fig. 14 it is shown, how problems with inter-area oscillations have been solved in the Brazilian System. In the Brazilian grid, the situation is critical because of a very long transmission distance

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between the interconnected systems: a 1000 km 500 kV AC interconnection between North and South systems has been realized. In the interconnection two TCSC devices have been installed at both ends of the line which damp the inherent oscillations that occur between the systems. Additionally, 5 FSC have been necessary to reduce the transmission angle.

The recordings from on-site tests show that the interconnection would become unstable without the damping function of TCSC. If only one TCSC is in operation, the interconnection becomes stable, with both devices acting the inter-area oscillations are quite well damped, and redundancy is provided. From site experience, it has been reported, that under increased load conditions, the TCSC damping function is activated up to several hundred times per day.

Fig. 14: Avoidance of Inter-area Oscillations in the 500 kV AC Transmission in Brazil by means of FACTS (Staged-Fault Tests for TCSCs)

From studies and real system tests during the UCTE-CENTREL synchronous interconnections (former UCPTE, see Fig. 15), it is well-known that the system has a strong tendency for large inter-area oscillations, very similar to the above mentioned Brazilian case. Up to now, with the given load flow from France to Spain, these oscillations have been damped successfully by the installations of PSS in CENTREL. However, if the load flow changes in magnitude and/or polarity, additional measures will be essential, ref to Fig. 16.

In Great Britain, in the course of deregulation, new power stations where installed in the north of the country, remote from the southern load centers and some of the existing power stations in the south were shut down due to environmental constraints and for economic reasons.

In studies, it has been analyzed, that this will lead to significant bottlenecks in the North-South transmission system. As a countermeasure, a total number of 27 SVC have been installed, because there was no right of way for new lines or higher transmission voltage levels. Fig. 17 gives an example

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for two of these SVCs, installed in Harker substation in a parallel configuration. Both Harker SVCs have been designed mainly for power oscillation damping (POD, Fig. 17 c).

UCPTE-CENTREL Synchronous Interconnections:

Inter-Area Oscillations with magnitudes up to

1000 MW

Damping Measures necessary

The step from UCPTE to UCTEThe step from UCPTE to UCTE

Fig. 15: UCTE: it is a very large synchronous System

“Stable” Power Flow: 1500 - 2000 MW. Stability Limit: 0 MW.System unstable in Case of Power Reversal.

“Stable” Power Flow: 1500 - 2000 MW. Stability Limit: 0 MW.System unstable in Case of Power Reversal.

Waiting for a new 400 kV Line ?Waiting for a new 400 kV Line ?

or using FACTS and HVDC?or using FACTS and HVDC?

Fig. 16: Spain-France Interconnections: an essentially weak Link

Fig. 17: Strengthening the Transmission System in UK with SVC a) No SVC connected

b) SVCs in Voltage control c) SVCs in POD control

In emerging countries, with a high “density” of HVDC applications, similar benefits can be achieved by use of advanced coordinated control functions with power oscillation damping features (POD). The HVDC can support the dynamic behavior of the AC interconnection during fault contingencies and control the load flow in the system. Fig. 18 shows an example of a large power system in the Chinese grid, in which HVDC has been integrated. Because of long transmission lines, the AC system experiences severe power oscillations after systems faults, close to the stability limits. In the figure, oscillations are depicted, first for the case that HVDC is just transmitting power in constant power mode (curve a). It can be seen, that strong power oscillations occur. If, however, damping control of the HVDC is activated (curve b), the oscillations are damped very effectively. Without HVDC, e.g. with a fully synchronous interconnection, such a large power system would be unstable in case of fault contingencies, thus leading to severe outages (Blackout).

In conclusion, the hybrid solution, using both DC and AC links, offers the best possibility for large power system interconnections. In case of interconnected networks, consisting of a number of smaller systems, a configuration according to Fig. 19 is technically and economically the best solution. An AC or DC interconnection between the neighboring areas of the different systems enables local power exchange among these regions. The performance of the AC links can additionally be improved by means of FACTS. Transmission of larger power blocks over long distances is, however realized by HVDC long distance transmissions directly between the locations of power surplus and power demand. The HVDC can at the same time strengthen synchronous interconnections to avoid possible dynamic problems, which exist in such complex configurations.

There are practically no technical limitations for the size of the interconnected system when using such a configuration, as shown in Fig. 19.

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Power System

Dynamic Results

5 10 150

0

600

900

1200

-600

300

-900

-300

Time (s)

Pow

er f

low

in o

ne li

neH

uish

ui-H

echi

(MV

A)

20

1500

a

b

a

b

Dynamic Results

5 10 150

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Time (s)

Pow

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Pow

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(MV

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a

b

a

b

GuiyangAnshun

Nay

ong

Anshun

TSQBASUO

Anshun Huishui

Hechi

Lubuge

TSQ-I

Luoping

HVDC TSQ

Liuzhou

Yantan

Pingguo

Baise

TSQ-II

Nanning

Yulin

Laibin

Hezhou

Gaomin

Luodong

Zhaoqing

BeijiaoGuangzhou

Wuzhou

TSQDAW_27

YunnanGuangxi

Guizhou

Guangdong

HVDC GuiGuang

GuiyangAnshun

ong

Anshun

TSQBASUO

Anshun Huishui

Hechi

Lubuge

TSQ-I

Luoping

HVDC TSQ

Liuzhou

Yantan

Pingguo

Baise

TSQ-II

Nanning

Yulin

Laibin

Hezhou

Gaomin

Luodong

Zhaoqing

BeijiaoGuangzhou

Wuzhou

Nay

TSQDAW_27

YunnanGuangxi

Guizhou

Guangdong

HVDC GuiGuang

Fig. 18: Examples of Power Oscillations without a) and with b) Power Modulation of HVDC Control in the Chinese Grid

SystemA

SystemB

SystemC

SystemD

SystemE

SystemF

Large Hybrid Interconnections with HVDC and FACTS:Large Hybrid Interconnections with HVDC and FACTS:

HVDC - Long Distance DC Transmission

High Voltage AC Transmission / FACTSDC Interconnection (B2B)

SystemA

SystemB

SystemC

SystemD

SystemE

SystemF

Large Hybrid Interconnections with HVDC and FACTS:Large Hybrid Interconnections with HVDC and FACTS:

HVDC - Long Distance DC Transmission

High Voltage AC Transmission / FACTSDC Interconnection (B2B) Fig. 19: Hybrid Interconnection

for large Power Systems

6. Conclusions In future, the loading of existing power systems will further increase, leading to bottlenecks and reliability problems. As a consequence of “lessons learned” from the large Blackouts in 2003, the needed enhancement of power systems will be done by FACTS and HVDC. In emerging countries, networks will grow fast and introduction of new higher voltage levels is envisaged in some countries. In these countries, because of the need to transmit power over long distances, application of FACTS and HVDC will play an important role, leading to hybrid AC/DC systems.

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elimination of bottlenecks in transmission – … · elimination of bottlenecks in transmission...

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