Patent Publication Number: US-2010109616-A1

Title: System and method for reactive power compensation and flicker management

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/198,278 filed on Nov. 4, 2008, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to power quality and more particularly to stabilization of power quality using dynamic reactive power compensation and flicker management in non-linear power generation (e.g., Wind and Solar) and distribution systems. 
     Electricity distribution utilities supply power to consumers and commercial users through an electricity distribution grid. The power quality supplied via utility grid is quantified by parameters such as stability, symmetry and current waveform characteristics. In addition to existing electricity generation sources, e.g., hydroelectric and thermal, in recent times renewable energy generation sources are being joined to the grid, e.g., solar and wind power. 
     Renewable electricity generation sources, e.g., solar and wind power, generate non-linear loads because they are dependent on variable natural sources like sunlight and wind. Non-linear sources in electricity generation need to be stabilized before they can be connected to the electricity distribution grids. Stabilization can also introduce distortions in power quality. For example, mass application of various power electronic devices and frequent fluctuation of loads negatively impact the grid power quality. 
     The non-linearity and distortion in grid power quality can lead to multiple problems; some such problems are described next. For example, power quality in the grid deteriorates due to low power factors, high power losses, high capital and maintenance costs and low efficiency. Deterioration in power quality can lead to reactive power impact, voltage drops and voltage flicker of the grid resulting in driving and protection equipment&#39;s malfunction or shutdown. Some illustrations of problems caused by harmonic currents are: grid voltage distortion, faulty protective equipment, and the amplification of resonance and harmonic currents of the capacitors, which can lead to capacitor overload or over-voltage failures. Further examples of problems are: increased loss of transformer leading to overheating, electrical equipment overheating, motor instability, accelerated insulation deterioration, reduced efficiency in electric arc furnaces and higher losses and disturbance in communication signals. Due to power quality problems, the three-phase imbalance with negative sequence currents can lead to vibrations of electric machines. 
     Power quality and stability can be improved in non-linear power generation systems by including a fast responding power stabilizing system in the systems as described next. For example, a Static Synchronous Compensator (STATCOM) can provide quick power stabilization, for a relatively high cost, in wind-farm like power generating systems. A Thyristor Switched Capacitor (TSC) can be combined with the STATCOM to improve power quality in non-linear power generation systems. 
     A combination of TSC and STATCOM needs to resolve transient disturbances resulting from the switching action of the TSC. During the switching action of the TSC, a long transient duration could result in grid power quality problems. In another approach, components can be increased, with a significant cost penalty, in the system to avoid high-frequency inrush current and a corresponding voltage transient when TSC is connected to the grid. 
     SUMMARY 
     A system and method for controlling transient disturbances in an electrical system, particularly electrical systems with non-linear power sources, is described. The system includes a Thyristors Switched Capacitor (TSC) in combination with a STATCOM connected to electrical phases. The TSC configuration includes a diode and a thyristor. 
     The switching sequences of the TSC achieve a switching time of T/3 of the line frequency cycle. The system can be implemented in a delta or a Wye-type configuration. The switching sequences can be in any combination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a prior-art Thyristor Switched Capacitor (TSC); 
         FIG. 1B  illustrates a TSC for power quality regulation; 
         FIG. 2  shows a prior-art three phase TSC configuration; 
         FIGS. 3A-3C  show a prior-art TSC configuration&#39;s transient performance graphs for the FIG.  2 &#39;s TSC configuration; 
         FIG. 4  shows a multi phase TSC configuration with a thyristor-diode combination; 
         FIGS. 5A-5C  show the transient performance graphs for the FIG.  4 &#39;s TSC configuration; 
         FIGS. 6A-6B  show switching sequences for TSC configurations; 
         FIG. 7  shows a delta type configuration of the TSC; and 
         FIG. 8  shows a wind power system with a power quality management system 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, which are not intended to limit the invention,  FIG. 1A  illustrates a prior-art Thyristor Switched Capacitor (TSC).  FIG. 1B  illustrates a TSC for power quality regulation. A TSC includes a thyristor switch and a capacitor bank. TSCs have switching capabilities to support the supply voltage of distribution systems and correct the power factor of the connected loads. TSCs generally are resistant to mechanical wear, noise-free and capable of transient-free switching. 
     The prior-art TSC  10  includes a bi-directional thyristor valve that includes thyristors  12  and  14  connected in parallel. The exemplary TSC  10  is connected to a single phase electrical distribution system. The TSC  10  is expected to be switched on with minimum transient disturbance. The TSC  10  also includes a capacitor  16  and an inductor  18  to control a surge current inflow. Before being switched on, the voltage level of capacitor  16  is zero. To achieve minimum transient when the TSC  10  is switched on, the capacitor is kept pre-charged to a certain voltage level before the switching action happens. 
     With reference to  FIG. 1B , a TSC  20  for a single phase system includes a diode  22  in an anti-parallel arrangement with a thyristor  24 . The function of TSC  20  is to reduce the transient disturbance that occurs at switching time to a minimum. A TSC  20  also includes a capacitor  26  and a surge limiting reactor, e.g., a series inductor. The TSC  20  is switched off at the current zero crossing with no transient disturbance (similar to TSC  10 ). When connected to a grid, the voltage level of capacitor  26  is kept the same as the maximum voltage level of the grid due to the presence of the diode  22  in the circuit. The TSC  20 , unlike TSC  10  which needs capacitor  16  to be pre-charged, can be switched on when the grid voltage reaches the peak value. 
     The TSC  10  requires an additional circuit (not shown) to pre-charge the capacitor  16 , but TSC  20  requires no such circuit because of presence of diode that keeps the capacitor  26  charged to the maximum grid voltage level. 
     The working of TSC  20  is described next. If the supply voltage for the TSC  20  is given by ν=V sin(ω o t+α), time is measured when the thyristor is gated, corresponding to the angle α on the voltage wave. The voltage equation in terms of the Laplace transforms will be: 
     
       
         
           
             
               
                 
                   
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     ω n  is the natural frequency of the circuit. 
     The last two terms in equation (2) above represent the expected oscillatory components of current having the frequency ω n . In practice, resistance causes these terms to decay. Hence, the voltage gap between capacitor initial voltage and the grid voltage at the switching time affects the inrush current peak amplitude. If the voltage gap is very large, the inrush high-frequency current is very large and the corresponding transient time is very long. 
       FIG. 2  shows a prior-art three phase TSC configuration. A TSC configuration  30  connects to three phases via phase A line  32 , phase B line  34  and phase C line  36 . The three TSCs  38 A-C have two thyristor configuration and operate as described above with reference to  FIG. 1A . The three capacitors  40 A-C may require pre-charging before switching for the TSCs  38 A-C can happen. Inductors  42 A-C provide surge current control. 
       FIGS. 3A-3C  shows a prior-art TSC configuration&#39;s transient performance graphs for the  FIG. 2  TSC configuration.  FIG. 3A  shows grid voltages;  FIG. 3B  shows gate commands to switch on and off the Thyristors (SCRs); and  FIG. 3C  shows the thyristor branch currents. The graph  44  shows three grid voltages Voa  46 A, Vob  46 B and  46 C for three phases. The graph  46  shows the gate commands to switch on and off the thyristors  38 A-C (See  FIG. 2 ). The graph  46  shows that the minimum transient time in the TSC configuration shown in  FIG. 2  is 2T/3 of a line frequency period. The capacitors  40 A,  40 B and  40 C (See  FIG. 2 ) are switched on sequentially, i.e., for phase sequence A-B-C representing switching for phases  32 ,  34  and  36  (See  FIG. 2 ) respectively. The phase switching sequence A-B-C for two thyristor TSC configuration of  FIG. 2  leads to a switching time of 2T/3. Referring to  FIG. 3C , the transient lasts a time period of about 2T/3 for the thyristor currents to reach the steady state values. 
       FIG. 4  shows a multi-phase TSC configuration with a thyristor-diode combination. A TSC configuration  52  includes connections to three phases via phase A line  32 , phase B line  34  and phase C line  36 . The three TSCs  54 A-C have a thyristor-diode pair as described above in context of  FIG. 1B . The capacitors  56 A-C do not need any pre-charging. The inductors  58 A-C control or regulate surge current. The TSC configuration can be in a Wye-type arrangement or a delta type arrangement. The Wye-type arrangement is described next and delta type arrangement is described below later. Those skilled in the art will appreciate that either Wye or delta type arrangement could be used for TSC configuration depending on the application need. 
     The TSC configurations of TSCs  54 A-C reduce the transient time as compared to the transient time required for the TSC configurations shown in  FIG. 2  whose working is explained in  FIGS. 3A-C . The capacitor  56 A-C can be switched in any sequence. The illustrative sequence of switching used here is A-C-B representing switching for phases  32 ,  36  and  34  respectively. The illustrative phase switching sequence A-C-B for the TSC configuration  52  leads to a switching time of T/3. The current from the grid flows first through phase A line  32  and then phase C line  36  followed by phase B line  34  (See  FIG. 4 ). There is no significant inrush current when the thyristors switch on. The transition time of T/3 for the TSC configuration of  FIG. 4  hence is much lower than the 2T/3 transition time of the TSC configuration of  FIG. 2 . Other switching sequences can also be used;  FIGS. 6A-6B  illustrate other sequences. Those skilled in the art will appreciate that a particular sequence of switching will depend on a specific application. 
     A Static Synchronous Compensator (STATCOM) (not shown) is a voltage regulating device used on alternating current (AC) electricity transmission grid networks. A STATCOM can be combined with the TSC configuration  52  to provide improve grid power quality, particularly in grid that are connected to non-linear power generation sources, e.g, solar power, wind power or tidal power. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power to an electricity grid network. If connected to a source of power it can also provide active AC power. It is a part of the Flexible AC transmission system device family. A STATCOM works by rebuilding the incoming voltage waveform by switching back and forth from inductive to capacitive load. If it is inductive, it will supply reactive AC power. If it is capacitive, it will absorb reactive AC power. Thus, the STATCOM acts as a source or sink. 
       FIGS. 5A-5C  show the transient performance graphs for the  FIG. 4  TSC configuration.  FIG. 5A  shows grid voltages;  FIG. 5B  shows gate commands to switch on and off the Thyristors (SCRs); and  FIG. 5C  shows the thyristor branch currents. The graph  60  shows three grid voltages Voa  62 A, Vob  62 B and Voc  62 C for three phases. The graph  64  shows the gate commands to switch on and off the thyristors-diode pairs  54 A-C (See  FIG. 4 ). The graph  64  shows that the minimum transient time in the TSC configuration shown in  FIG. 2  is a third (T/3) of a line frequency period. The capacitors  56 A,  56 B and  56 C (See  FIG. 4 ) can be switched on in any combination of the phase sequences as described below (switching capacitors and TSC are similar concepts). 
     With reference to  FIG. 5B , the procedure for the gate control logic is described as:
         1. Measure the input voltages.   2. As an example, after 0.3 s when phase-A reaches its peak value, the gate command for this phase-A Thyristor (SCR), the TSC  54 A (See  FIG. 4 ) is allowed to turn on.   3. Next, when phase-A and phase-B voltages cross each other, and phase-C voltage reaches its negative peak value, the gate command for phase-C Thyristor (SCR), i.e, the TSC  54 C (See  FIG. 4 ) is allowed to turn on, before phase-B voltage reaches its positive peak value.   4. The following sequence determines the Thyristor (SCR) control of phase-B. When phase-B voltage reaches its positive peak value in time, the gate command for phase-B Thyristor (SCR), i.e., TSC  54 B (See  FIG. 4 ) is allowed to turn on.   5. This completes one cycle of the Thyristor gate control sequence.       

     The graph  66  shows waveforms that represent three-phase thyristor branch currents as a result of this invention. With reference to Voa  62 A, Vob  62 B and Voc  62 C for three phases, the current flows through phase-A first, then phase-C current starts to flow, followed by phase-B current. The sequence is A-C-B. Hence, here is no significant inrush current at Thyristor turn-on. The total transition time is ⅓ of a line frequency period. As compared to the conventional approach, where the turn-on sequence is A-B-C instead, resulting in a longer transition time of ⅔ of a line frequency period. 
       FIGS. 6A-6B  show switching sequences for TSC configurations. A switching sequence of A-C-B phases was described above (See FIGS.  4  and  5 A-C). The TSC configurations  68  and  70  are same as the TSC configuration  52  (See  FIG. 4 ) except for the switching sequence. The TSC configuration  68  illustrates the switching sequence of B-A-C for phases  32 ,  34  and  36  in that sequence. The TSC configuration  70  illustrates the switching sequence of C-B-A. The T/3, line frequency period is obtained in any switching sequence. Those skilled in the art will appreciate that the any switching sequence can be used depending upon the required application. 
       FIG. 7  shows a delta type configuration of the TSC. A delta type TSC configuration  72  includes thyristor-diode pairs  54 A-C and capacitors  56 A-C that are same as those in  FIG. 4 . The pairs are connected to three electrical phases (not shown). Inductors  58 A-C (See  FIG. 4 ) can be included to limit the current surge. Hence, between any two electrical phases, a thyristor and a diode are connected in parallel. This assembly is then in series with a capacitor bank. The anode and cathode arrangement of the thyristor-diode pairs  54 A-C are reversed as compared to known arrangements (not shown). The response time of switching can be improved from 2T/3 to T/3 as those skilled in the art will appreciate. 
       FIG. 8  shows a wind power system with a power quality management system. An exemplary wind power generating system  76  is connected to a STATCOM  78  via a STATCOM series connected impedance (Zs)  80 . A TSC configuration  82  is included in the system. A transmission line impedance (R L     —   jX L )  84  and a utility voltage source,  86  (V 1 ) are also included. The grid voltage is kept at its normal level with STATCOM  78  except there is a time span when the limited capacity of the STATCOM cannot provide full voltage support. When STATCOM  78  reaches a maximum output level, the TSC configuration  82  is switched on and the voltage is recovered immediately. The transient time period can be achieved in a time duration of T/3 as described above. A similar configuration can be applied in solar energy generation system where non-linear loads are generated due to variation in sun&#39;s radiation and other factors, as those skilled in the art will appreciate. 
     The invention has been described in detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.