HVDC technology

HVDC technology is utilised where non-synchronous ties between different uti-lities or parts of the same transmission system are required. The most obvious cases

STATCOM power circuit

Figure 1.23 Arc furnace and flicker compensation by STATCOM

of this are where the two systems have different frequencies or voltage levels, or are separated by long sea crossings. HVDC can also be used within an AC trans-mission system to deliver power between two regions directly or in parallel operation with AC transmission. Furthermore, AC overhead lines can be converted to DC operation enabling additional flow of power without the need for recon-ductoring or installation of new transmission circuits.

Advances in HVDC technologies such as ‘voltage-sourced converters (VSCs)’

also offer many interesting applications such as current limiters, solid-state circuit-breakers, transfer switches, dynamic voltage restorers for addressing distribution system problems [18].

Further advances in HVDC technologies such as ‘capacitor commutated con-verters (CCC)’ [19] and ‘VSC transmission’ [20] have been successfully applied to HVDC interconnections and point the way to further future developments.

1.6.3 Control and communication developments

Increasing use of reactive compensation devices and solid-state component-based technologies and the need to control voltage and frequency to much higher quality standards are the driving forces behind the control system developments. At a component level increasing reliability of digital processors has led to almost complete dominance of such devices over the conventional analogue-type control systems.

Increasingly the digital control systems are being installed for protection, indications, measurements, monitoring and control purposes. Furthermore, these devices are being integrated to perform these functions in a single control unit utilising the same system quantities. While this has the promise of savings in terms of installed equipment it requires application of special measures to achieve the same level of dependability obtainable from the existing separate functional arrangements. Lack of formalised standards for digital information exchange between different manufacturers equipment is likely to hamper progress.

Increasing dependence on reactive compensation devices to achieve better system utilisation creates its own control challenges. In many transmission systems adequate voltage control is no longer possible if the control is based purely on local power system quantities. In addition to the immediate fast primary voltage control response provided by generator automatic voltage regulators in the 1–5 s timescale, an automatic secondary voltage control response based upon voltage measurements at a number of points within a specific region is required to ensure stable system behaviour in the 5 s to 10 min timescale. This then needs to be augmented by a tertiary voltage control system ensuring readjustment of target voltages in critical parts of the system at around 10 min intervals.

Use of fast-response power electronic devices requires co-ordinated control of a portfolio of shunt and series devices in a manner not only to make most efficient use of full device capabilities but also to ensure non-conflicting control action between these devices, which may lead to unstable system operation. This parti-cular subject area is at present in its infancy with potential for rapid developments.

Electric power transmission and distribution systems 53

Developments in the application of fibre optic-based communication technol-ogies have facilitated increasing use of digital control systems as well as increased functional integration of protection, indications, measurements, monitoring and control. There is no doubt that while many control concepts hitherto considered are too complex and difficult to achieve, such as the remote control of Substations, they are now becoming a reality thanks to the application of advanced technologies.

The new challenge is in management of these systems to achieve the ever increasing performance expectations of the society from the transmission and dis-tribution systems and in the avoidance of high-impact supply interruptions and blackouts.

With the rapid development of computing and communication systems it was inevitable that so-called smartgrid initiatives have recently developed at a great pace.

These initiatives tend to focus on consumer demand control, remote switching and metering aspects but currently suffer from inadequately defined objectives as well as protocols and standards. At transmission level these initiatives are most appropriate for special protection and system control applications. In distribution systems, however, their application will be subject to consumer approval and placement of privacy safeguards.

Increasing use of Internet-based communications also exposes transmission and distribution utilities to malicious attacks and hacking activity. These issues are likely to require increasing efforts to ensure robustness of systems to such threats.

References

1. Sayers D.P., Forrest J.S., Lane F.J. ‘275 kV Developments on the British Grid System’. Proceedings of the IEEE. May 1952;99(Pt. II) (Paper No. 1309S) 2. Booth E.S., Clarke D., Eggington J.L., Forrest J.S. ‘The 400 kV grid system

for England and Wales’. Proceedings of the IEEE. December 1962;109 (Pt. A, no. 48):493

3. Electrical Transmission and Distribution Reference Book. Westinghouse Electric Corporation

4. Electricity Association. Handbook of Electricity Supply Statistics. London:

Electricity Association; 1989

5. Casazza J.A. ‘The development of electric power transmission’. IEEE Case Histories of Achievement in Science and Technology. IEEE; 1993

6. Ledger F., Sallis H. Crisis management in the power industry – an inside story. Routledge; 1995

7. ‘FACTS overview’, Joint document by IEEE Power Engineering Society and CIGRE, issued as IEEE Publication 95TP108, April 1995

8. Hingorani N.G., Gyugyi L. Understanding FACTS – concepts and technology of flexible AC transmission systems. IEEE Press; 2000

9. Auckland Power Supply Failure 1998. The report of the Ministerial inquiry into the Auckland power supply failure, Ministry of Commerce, New Zealand, 1998

10. CIGRE Report no. 316 by Working Task Force TFC 2.02.24 ‘Defence Plans for Extreme Contingencies’, April 2007

11. Gyugyi L. ‘Converter Based FACTS Controllers’, IEE Colloquium on

‘Flexible AC Transmission Systems – The FACTS’, IEE Publication 98/500, November 1998

12. Erinmez I.A. (ed.). Static Var Compensators, CIGRE report no. 025 by Working Group 38.01, Task Force 2, September 1986

13. Erinmez I.A., Foss A.M. (ed.). Static Synchronous Compensator (STATCOM), CIGRE report no. 144 by Working Group 14.19, September 1999

14. Erinmez I.A. (ed.). Static Synchronous Compensator (STATCOM) for Arc Furnace and Flicker Compensation, CIGRE report no. 237 by Working Group B4.19, December 2003

15. Lahtinen M. ‘New method for flicker performance evaluation of arc furnace compensator’, CIGRE Conference Paper 36-205; CIGRE, Paris, August 2002 16. Gru¨nbaum R., Gustafsson T., Hasler J.-P., Larsson T., Lahtinen M. ‘STATCOM, a prerequisite for a melt shop expansion – performance experiences’, IEEE Power Tech 2003 Conference, Bologna, Italy, 23–6 June 2003

17. CIGRE Report no. 242 by Working Group B4.35 ‘Thyristor Controlled Voltage Regulators’, February 2004

18. CIGRE Report no. 280 by Working Group B4.33 ‘HVDC and FACTS for Distribution Systems’, October 2005

19. CIGRE Report no. 352 by Working Group B4.34 ‘Capacitor Commutated Converters (CCC) HVDC Interconnections’, June 2008

20. CIGRE Report no. 269 by Working Group B4.37 ‘VSC Transmission’ April 2005

21. Knight R.C., Young D.J., Trainer D.R. ‘Relocatable GTO-based static var compensator for NGC substations’, CIGRE conference paper 14-106, Paris, 1998

Editor’s comments: The reader will observe that many of the references listed above [7, 10, 12–15, 17–21] involve international CIGRE related activities. The following footnote key indicates the diversity of strategic technical CIGRE activ-ities, which also extend to international CIGRE Conferences and Electra magazine publications. These will be considered further in later chapters.¹

1CIGRE key: SC, Study Committee; WG, Working Group and JWG, Joint Working Group; TF, Task Force; TB, Technical Brochure; TR, Technical Report; SC, Scientific Paper.

Electric power transmission and distribution systems 55

Chapter 2

Insulation co-ordination for AC transmission