Because the voltage isolation is high and the Third harmonic frequency is close to the cut off frequency, the filter will be costly. By using the zero sequence harmonic, the costly filter can be replaced by a cable that connects the neutral point of the Star-Delta transformer on the right side in Figure 2 in the Link,

http://www.researchinventy.com/papers/v2i1/F021025032.pdf

with the ground. Because the Delta winding appears open circuit to the 3rd harmonic current, all harmonic current will flow through the Y winding and concentrate to the grounding cable There fore, the large high pass filter is eliminated.

1.       International Advanced Research Journal in Science, Engineering and                National Conference on Renewable Energy and Environment (NCREE -2015)    IMS Engineering College, Ghaziabad

             Vol. 2,Special Issue 1, May 2015 Copyright to IARJSET DOI 10.17148/IARJSET 5

 Introduction and Control Approach of Distributed Power Flow Controller (DPFC)

Shivanand,Rituraj Jalan,V K Maurya

 In a three phase system the 3rdharmonic each phase is identical, which means they are

zero sequence components. Because the zero sequence harmonic can be naturally blocked by star delta transformers and these are widely incorporated in power systems, there is no extra filter required to prevent harmonic leakage

    DPFC CONTROL SCHEMES:

A.   Central Control

 The central control generates the reference signals for both the shunt and series converters of the DPFC. It is focused on the DPFC tasks at the power-system level, such as power-flow control, low-frequency power oscillation damping, and balancing of as symmetrical components.

According to the system requirement, the central control gives corresponding voltage reference signals for the series converters and reactive current signal for the shunt converter. All the reference signals generated by the central control are at the fundamental frequency.

 B. Series control

Each series converter has its own series control. The controller is used to maintain the capacitor DC voltage of its own converter, by using 3rd harmonic frequency components, in addition to generating series voltage at the fundamental frequency as required by the central control.

 C. Shunt control

The objective of the shunt control is to inject a constant 3rd harmonic current into the line to supply active power for the series converters. At the same time, it maintains the capacitor DC voltage of the shunt converter at a constant value by absorbing active power from the grid at the fundamental frequency and injecting the required reactive current at the fundamental frequency into the grid.

 Fig. 8 in the Link Fundamental + 3 rd Harmonic Current (output voltage of Shunt converter)

 Fig. 8 Contains two Frequency components i.e.fundamental and 3 rd Harmonic frequency components. The constant 3 rd Harmonic current injected by shunt converter.

 Now it is necessary to remove third harmonics components. The output of shunt converter is now fed to series converter which blocks the third harmonics components of input current.

 Fig. 9 of the Link Shows the Output Current from Series converter, which also has some harmonics. Extra high pass filters and Star-Delta transformers are used to remove harmonics completely from the input current waveform

 Link: 

http://www.iarjset.com/upload/2015/si/ncree-15/IARJSET%202%20P103.pdf

2.    RESEARCH INVENTY: International Journal of Engineering and Science ISBN: 2319 -6483,ISSN: 2278-4721, Vol.2, Issue 1

    (January2013), PP 25-32

     www.researchinventy.com

       Simulation of Distributed Power Flow Controller (Dpfc)

      Sarimalla,Pedakotaiah andSantosh A

Because the voltage isolation is high and the harmonic frequency is close to the cut

off frequency, the filter will be costly. By using the zero sequence harmonic, the costly filter can be replaced by a cable that connects the neutral point of the Star-Delta transformer on the right side in Figure 2 in the Link, with the ground. Because the Delta winding appears open circuit to the 3rd harmonic current, all harmonic current will flow through the Y winding and concentrate to the grounding cable as shown in Figure 3

There fore, the large high pass filter is eliminated.

Figure 3 shows grounded transformer to filter zero sequence harmonic

Another advantage of using the 3rd harmonic to exchange active power is that the grounding of the Star-Delta transformers can be used to route the harmonic current in a meshed network. If the network requires the harmonic current to flow through a specific branch, the neutral point of the Star-Delta transformer in that branch, at the side opposite to the shunt converter, will be grounded and vice versa.

 Figure 4 shows a simple example of routing the harmonic current by using the grounding of the Star-Delta transformer. Because the floating neutral point is located on the transformer of the line without the series converter, it is an open circuit for 3rd harmonic components and therefore no 3rd harmonic current will flow through this line

  http://www.researchinventy.com/papers/v2i1/F021025032.pdf

the following Article on Elimination of Harmonics in General may be of Interest.l

3. Hammond Power Solutions Inc.

Transformers, Harmonic Currents and Phase Shifting

 Phase Shifting and Harmonics: The best way to eliminate harmonics is to use a technique known as “phase shifting.” The concept of phase shifting involves separating the electrical supply into several outputs; each output being phase shifted with the

other outputs with an appropriate angle for the harmonics to be eliminated. The idea is to displace the harmonic currents in order to bring them to a 180° phase

shift so that they cancel each other out.

 Hence, an angular displacement of  −60° is required between two three-phase out

puts to cancel the 3rd harmonic currents

 −30° is required between two three-phase outputs to cancel the 5th and 7th harmonic currents

 −15° is required between two three-phase outputs to cancel the 11th and 13th harmonic currents

 Phase-Shifting Transformers Designed for Non-Linear Loads  :

 The level of harmonic currents may be reduced using phase-shifting transformers. Low impedance plays a crucial role in reducing voltage distortion. New low-impedance phase-shifting transformers allow the treatment of harmonic currents while providing a path of low impedance. Moreover, these transformers have been designed to withstand the additional overheating caused by harmonic currents. The quality and reliability of

the electrical system can be improved considerably through the use of a single piece of equipment

 Harmonic Mitigating Transformer :

 (Using 0° or -30° primary-secondary angular displacement)

The primary of the transformer has a delta connection and the secondary has a special double winding connection. Although there is only one secondary three-phase output, the electromagnetic effect of its secondary winding with a zigzag connection ends up cancelling the 3rd, 9th and 15th harmonic currents.

Features of the Harmonic Mitigating Transformers include:

• A capacity for handling nonlinear loads.

• Low-impedance cancellation of the 3rd, 9th and 15th harmonic currents (zigzag-connected secondary).

• A reduction in voltage distortion (3rd harmonic voltage reduced by low zero-sequence impedance).

 When two transformers with a delta-zigzag connection (-30° and 0°) are used for phase-shifting, the 3rd harmonic currents are cancelled due to the secondary. The 5th and 7th harmonic currents are cancelled in the electrical supply common to both transformers and the voltage distortion is reduced.

 If a single delta-zigzag transformer (0°) is used in a system made up of Delta Wye transformers (-30°), the 5thand 7th harmonic currents originating from the delta-zigzag

transformer (0°) will attempt to cancel the 5th and 7thharmonic currents originating from the Delta Wye transformer (-30°) already present. This reduces the 5th and 7th harmonics in the system.

 Link:

http://www.hammondpowersolutions.com/files/HPS_article_Harmonics_PhaseShifting.pdf

P.S.

Thank you Subramanyam Sir.

Please refer to these articles:

https://www.researchgate.net/publication/313576646_From_PID_to_Nonlinear_State_Error_Feedback_Controller

https://www.researchgate.net/publication/312046377_Improved_Sliding_Mode_Nonlinear_Extended_State_Observer_based_Active_Disturbance_Rejection_Control_for_Uncertain_Systems_with_Unknown_Total_Disturbance

https://www.researchgate.net/publication/309655805_On_the_Improved_Nonlinear_Tracking_Differentiator_based_Nonlinear_PID_Controller_Design

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