Offshore Wind Farms – VSC-based HVDC Connection

 

ABSTRACT:

Due to significantly higher and more constant wind speeds and the shortage of suitable sites for wind turbines on the land, offshore wind farms are becoming very attractive. The connection of the large offshore wind farms is possible with HVAC, classical HVDC and Voltage Source Converter (VSC based) HVDC technology. In this paper their main features will be given. From the economical and technical viewpoint, the type of connection depends on the size of the wind farm and on the distance to the connection point of the system.

As very promising technology, especially from the technical viewpoint, the focus of this paper will be put on the VSC-based HVDC technology. Its main technical features as well as its model will be detailed. At the end, obtained simulation results for different faults and disturbances for one offshore wind farm connected with VSC-based HVDC technology will be presented.

KEYWORDS:

  1. HVDC
  2. IGBT
  3. Offshore wind farm connection
  4. PWM
  5. Requirements
  6. Stability
  7. VSC

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

 

Fig. 1. Principal scheme of VCS-based HVDC connection

EXPECTED SIMULATION RESULTS:

 

 Fig. 2. Active and reactive power at the connection point during reactive power control

Fig. 3. Active and reactive power at the wind farm side during reactive power control

Fig. 4. Active power, reactive power and voltage at system and wind farm side in case of single phase short circuit near to the connection point – 100ms

Fig. 5. Active power, reactive power and voltage at system and wind farm side in case of single phase short circuit at the wind farm side – 100ms

 CONCLUSION:

The connection of an offshore wind farm depends primarily on the amount of power that has to be transmitted and the distance to the connection point.

Primarily due to comparatively small size and short distance to the connection point as well as due to its lower costs and experience, all actual offshore wind farms and those planned to be installed are still using/plan to use HVAC connection.

The advantages of using a HVDC solution are more significant with increase of the distance and power.

The VSC-based HVDC technology is due to its technical advantages like: active and, especially, reactive power control (voltage control), isolated operation, no need for an active commutation voltage etc. very good solution for an offshore wind farm connection. Performed simulation and their results of simulated faults and disturbances show that the technical requirements can be fulfilled.

REFERENCES:

[1] European Wind Energy Association. (2004). Wind Energy – The Facts. [Online]. Available: http://www.ewea.org

[2] Global Wind Energy Council. (2004). [Online]. Available: http://www.gwec.net

[3] F.W. Koch, I. Erlich, F. Shewarega, and U. Bachmann, “Dynamic interaction of large offshore wind farms with the electric power system”, in Proc. 2003 IEEE Power Tech Conf., Bologna, Italy, vol. 3, pp. 632-638.

[4] J.G. Slootweg and W.L. Kling, “Is the Answer Blowing in the Wind?”, IEEE Power and Energy Magazine, vol. 1, pp. 26-33, Nov./Dec. 2003.

[5] Wind Energy Study 2004. [Online]. Available: http://www.ewea.org

Electric Springs—A New Smart Grid Technology

 

ABSTRACT:

The scientific principle of “mechanical springs” was described by the British physicist Robert Hooke in the 1660’s. Since then, there has not been any further development of the Hooke’s law in the electric regime. In this paper, this technological gap is filled by the development of “electric springs.” The scientific principle, the operating modes, the limitations, and the practical realization of the electric springs are reported. It is discovered that such novel concept has huge potential in stabilizing future power systems with substantial penetration of intermittent renewable energy sources. This concept has been successfully demonstrated in a practical power system setup fed by an ac power source with a fluctuating wind energy source. The electric spring is found to be effective in regulating the mains voltage despite the fluctuation caused by the intermittent nature of wind power. Electric appliances with the electric springs embedded can be turned into a new generation of smart loads, which have their power demand following the power generation profile. It is envisaged that electric springs, when distributed over the power grid, will offer a new form of power system stability solution that is independent of information and communication technology.

KEYWORDS:

  1. Distributed power systems
  2. Smart loads
  3. Stability

 SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1. The experimental setup for the electric spring (with control block diagram).

EXPECTED SIMULATION RESULTS:

 Fig. 2. Measured steady-state electric spring waveforms under “neutral” mode. . Va=4.5vac QES=17.5 var, .[Electric spring voltage is near zero.]

Fig. 3. Measured steady-state electric spring waveforms under “capacitive” mode. Va=97.9vac QES=-349.9 var

Fig. 4. Measured steady-state electric spring waveforms under “inductive” mode. Va=94.3vac QES=348.4 var.

                               

Fig. 5. Measured root-mean-square values of the mains voltage vs, noncritical load voltage vo and electric spring voltage va before and after the electric spring is activated. [Electric spring is programmed for voltage boosting function only.]

Fig. 6. Measured power of the critical load and noncritical loads [Electric spring is programmed for voltage boosting function only.]

Fig. 7. Measured root-mean-square values of the critical load (mains) voltage  vs, noncritical load load voltage vo and electric spring voltage va before and after the electric spring is activated. [Electric spring is programmed for both voltage boosting and suppression functions.]

Fig. 8. Measured power of the critical load and smart load. [Electric spring is programmed for both voltage boosting and suppression functions.].

 CONCLUSION:

The Hooke’s law on mechanical springs has been developed into an electric spring concept with new scientific applications for modern society. The scientific principles, operating modes and limits of the electric spring are explained. An electric spring has been practically tested for both voltage support and suppression, and for shaping load demand (of about 2.5 kW) to follow the fluctuating wind power profile in a 10 kVA power system fed by an ac power source and a wind power simulator. The electric springs can be incorporated into many existing noncritical electric loads such as water heaters and road lighting systems [26] to form a new generation of smart loads that are adaptive to the power grid. If many noncritical loads are equipped with such electric springs and distributed over the power grid, these electric springs (similar to the spring array in Fig. 1) will provide a highly reliable and effective solution for distributed energy storage, voltage regulation and damping functions for future power systems. Such stability measures are also independent of information and communication technology (ICT). This discovery based on the three-century-old Hooke’s law offers a practical solution to the new control paradigm that the load demand should follow the power generation in future power grid with substantial renewable energy sources. Unlike traditional reactive power compensation methods, electric springs offer both reactive power compensation and real power variation in the noncritical loads. With many countries determined to de-carbonize electric power generation for reducing global warming by increasing renewable energy up to 20% of the total electrical power output by 2020 [22]–[25], electric spring is a novel concept that enables human society to use renewable energy as nature provides. The Hooke’s law developed in the 17th century has laid down the foundation for stability control of renewable power systems in the 21st century.

 REFERENCES:

[1] Hooke’s law—Britannica Encyclopedia [Online]. Available:

http://www.britannica.com/EBchecked/topic/271336/Hookes-law

[2] A. M. Wahl, Mechanical Springs, 2nd ed. New York: McGraw-Hill, 1963.

[3] W. S. Slaughter, The Linearized Theory of Elasticity. Boston, MA: Birkhauser, 2002.

[4] K. Symon, Mechanics. ISBN 0-201-07392-7. Reading, MA: Addison- Wesley, Reading,1971.

[5] R. Hooke, De Potentia Restitutiva, or of Spring Explaining the Power of Springing Bodies. London, U.K.: John Martyn, vol. 1678, p. 23.

Electric Springs—A New Smart Grid Technology

ABSTRACT:

 The scientific principle of “mechanical springs” was described by the British physicist Robert Hooke in the 1660’s. Since then, there has not been any further development of the Hooke’s law in the electric regime. In this paper, this technological gap is filled by the development of “electric springs.” The scientific principle, the operating modes, the limitations, and the practical realization of the electric springs are reported. It is discovered that such novel concept has huge potential in stabilizing future power systems with substantial penetration of intermittent renewable energy sources. This concept has been successfully demonstrated in a practical power system setup fed by an ac power source with a fluctuating wind energy source. The electric spring is found to be effective in regulating the mains voltage despite the fluctuation caused by the intermittent nature of wind power. Electric appliances with the electric springs embedded can be turned into a new generation of smart loads, which have their power demand following the power generation profile. It is envisaged that electric springs, when distributed over the power grid, will offer a new form of power system stability solution that is independent of information and communication technology.

KEYWORDS:

  1. Distributed power systems
  2. Smart loads
  3. Stability

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. The experimental setup for the electric spring (with control block diagram).

EXPECTED SIMULATION RESULTS

Fig. 2. Measured steady-state electric spring waveforms under “neutral” mode. . Va=4.5vac QES=17.5 var, . [Electric spring voltage is near zero.]

 

Fig. 3. Measured steady-state electric spring waveforms under “capacitive” mode. Va=97.9vac QES=-349.9 var

 

Fig. 4. Measured steady-state electric spring waveforms under “inductive” mode. Va=94.3vac QES=348.4 var.

 CONCLUSION:

 The Hooke’s law on mechanical springs has been developed into an electric spring concept with new scientific applications for modern society. The scientific principles, operating modes and limits of the electric spring are explained. An electric spring has been practically tested for both voltage support and suppression, and for shaping load demand (of about 2.5 kW) to follow the fluctuating wind power profile in a 10 kVA power system fed by an ac power source and a wind power simulator. The electric springs can be incorporated into many existing noncritical electric loads such as water heaters and road lighting systems [26] to form a new generation of smart loads that are adaptive to the power grid. If many noncritical loads are equipped with such electric springs and distributed over the power grid, these electric springs (similar to the spring array in Fig. 1) will provide a highly reliable and effective solution for distributed energy  storage, voltage regulation and damping functions for future power systems. Such stability measures are also independent of information and communication technology (ICT). This discovery based on the three-century-old Hooke’s law offers a practical solution to the new control paradigm that the load demand should follow the power generation in future power grid with substantial renewable energy sources. Unlike traditional reactive power compensation methods, electric springs offer both reactive power compensation and real power variation in the noncritical loads. With many countries determined to de-carbonize electric power generation for reducing global warming by increasing renewable energy up to 20% of the total electrical power output by 2020 [22]–[25], electric spring is a novel concept that enables human society to use renewable energy as nature provides. The Hooke’s law developed in the 17th century has laid down the foundation for stability control of renewable power systems in the 21st century.

REFERENCES:

 [1] Hooke’s law—Britannica Encyclopedia [Online]. Available: http://www.britannica.com/EBchecked/topic/271336/Hookes-law

[2] A. M. Wahl, Mechanical Springs, 2nd ed. New York: McGraw-Hill, 1963.

[3] W. S. Slaughter, The Linearized Theory of Elasticity. Boston, MA: Birkhauser, 2002.

[4] K. Symon, Mechanics. ISBN 0-201-07392-7. Reading, MA: Addison- Wesley, Reading,1971. [5] R. Hooke, De Potentia Restitutiva, or of Spring Explaining the Power of Springing Bodies. London, U.K.: John Martyn, vol. 1678, p. 23.

Novel Development of A Fuzzy Control Scheme with UPFC’s For Damping of Oscillations in Multi-Machine Power Systems

ABSTRACT:

This paper presents a novel development of a fuzzy logic controlled power system using UPFCs to damp the oscillations in a FACTS based integrated multi-machine power system consisting of 3 generators, 3 transformers, 9 buses, 4 loads & 2 UPFCs. Oscillations in power systems have to be taken a serious note of when the fault takes place in any part of the system, else this might lead to the instability mode & shutting down of the power system. UPFC based POD controllers can be used to suppress the oscillations upon the occurrence of a fault at the generator side or near the bus side. In order to improve the dynamic performance of the multi-machine power system, the behavior of the UPFC based POD controller should be coordinated, otherwise the power system performance might be deteriorated. In order to keep the advantages of the existing POD controller and to improve the UPFC-POD performance, a hybrid fuzzy coordination based controller can be used ahead of a UPFC based POD controller to increase the system dynamical performance & to coordinate the UPFC-POD combination. This paper depicts about this hybrid combination of a fuzzy with a UPFC & POD control strategy to damp the electro-mechanical oscillations. The amplification part of the conventional controller is modified by the fuzzy coordination controller. Simulink models are developed with & without the hybrid controller. The 3 phase to ground symmetrical fault is made to occur near the first generator for 200 ms. Simulations are performed with & without the controller. The digital simulation results show the effectiveness of the method presented in this paper.

KEYWORDS:

  1. UPFC
  2. POD
  3. Fuzzy logic
  4. Coordination
  5. Controller
  6. Oscillations
  7. Damping
  8. Stability
  9. SIMULINK
  10. State space model

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

image001

Fig. 1 : A 3-machine, 9-bus interconnected power system model with 4-loads without the controllers

image002

Fig. 2: A 3-machine, 9-bus interconnected power system model with 4-loads & 2 POD-UPFC & the fuzzy controller

 EXPECTED SIMULATION RESULTS:

 image003

Fig. 3 : Simulation result of power angle v/s time (without Fuzzy-POD-UPFC)

image004

Fig. 4 : Simulation result of power angle v/s time (with UPFC & fuzzy control)

image005

Fig. 5 : Comparison of the simulation results of power angle v/s time (without UPFC & with UPFC & fuzzy control)

 CONCLUSION:

AFACTS based multi-machine power system comprising of 3 generators, 9 buses, 3 loads with and without the 2 Fuzzy-POD-UPFC controllers was considered in this paper. SIMULINK models were developed in MATLAB 7 with & without the Fuzzy- POD-UPFC controllers for the considered multi machine model in order to damp out the oscillations. The control strategy was also developed by writing a set of fuzzy rules. The fuzzy control strategy was designed based on the conventional POD-UPFC controller & put before the POD-UPFC in the modeling.

The main advantage of putting the fuzzy coordination controller before the POD-UPFC in modeling is the amplification part of the conventional controller being modified by the fuzzy coordination unit, thus increasing the power system stability. Simulations were run in Matlab 7 & the results were observed on the scope. Graphs of power angle vs. time were observed with and without the controller. From the simulation results, it was observed that without the Fuzzy-POD-UPFC controller, the nine bus system will be having more disturbances, while we check the power angle on the first generator.

There are lot of ringing oscillations (overshoots / undershoots) & the output takes a lot of time to stabilize, which can be observed from the simulation results. But, from the incorporation of the Fuzzy- POD-UPFC coordination system in loop with the plant gave better results there by reducing the disturbances in the power angle and also the post fault settling time also got reduced a lot. The system stabilizes quickly, thus damping the local mode oscillations and reducing the settling time immediately after the occurrence of the fault.

The developed control strategy is not only simple, reliable, and may be easy to implement in real time applications. The performance of the developed method in this paper thus demonstrates the damping of the power system oscillations using the effectiveness of Fuzzy-POD-UPFC coordination concepts over the damping of power system oscillations without the Fuzzy-POD-UPFC coordination scheme.

REFERENCES:

[1]. L. Gyugi, “Unified Power flow concept for flexible AC transmission systems”, IEE Proc., Vol. 139, No. 4, pp. 323–332, 1992.

[2]. M. Noroozian, L. Angquist, M. Ghandari, and G. Anderson, “Use of UPFC for optimal power flow control”, IEEE Trans. on Power Systems, Vol. 12, No. 4, pp. 1629–1634, 1997.

[3]. Nabavi-Niaki and M.R. Iravani, “Steady-state and dynamic models of unified power flow controller (UPFC) for power system studies”, IEEE’96 Winter Meeting, Paper 96, 1996.

[4]. C.D. Schauder, D.M. Hamai, and A. Edris. “Operation of the Unified Power Flow Controller (UPFC) under Practical constraints”, IEEE Trans. On Power Delivery, Vol. 13, No. 2. pp. 630~639, Apr. 1998.

[5]. Gyugyi, L., “Unified power flow controller concept for flexible AC transmission systems”, IEE Proc. Gener. Transm. Distrib., No.139, pp. 323-331, 1992