Abstract:
A power conversion system includes a power transformer for receiving AC power at a first voltage from an input side and for delivering AC power at a second voltage to an output side. A power converter is also included in the power conversion system wherein the power converter includes an input side converter on the input side and an output side converter on the output side coupled through a plurality of DC links. A converter controller in the power converter provides control signals to the input side converter and the output side converter for regulating an active power and a reactive power flow through the power converter. Each of the input side converters and the output side converters includes at least two power converter transformers coupled between respective power converter bridges coupled to the plurality of DC links and the input side or to the plurality of DC links and the output side.

Description:
BACKGROUND 
       [0001]    Embodiments of the invention relate generally to an electric power grid and more specifically to a system and method for transmitting electric power. 
         [0002]    The basic structure of an electric power system comprises various hardware elements such as generators, transformers, power lines, and real-time monitoring equipment, as well as software such as power flow analysis software, fault detection software, and restoration software for generation, transmission, and distribution of electricity. 
         [0003]    A frequently occurring situation in an electric power system is the need to transmit more power over the system than it was originally designed for. In cases where there is a need to transmit more power, and building new transmission lines is prohibitive due to cost, right-of-way, or environmental constraints, increased utilization of existing transmission lines and equipment is desirable. 
         [0004]    Furthermore, with increased distributed generation, the integration of distributed generators into existing power systems presents technical challenges such as voltage regulation, stability, power quality problems. Power quality is an essential customer-focused measure and is greatly affected by the operation of a distribution and transmission network. 
         [0005]    Flexible alternating current transmission system (FACTS) devices may be one of the solutions to the above problems. FACTS devices are power electronic-based devices and are able to provide active and reactive power compensations to power systems. However, FACTS devices are costly and in present configurations, a fault on the FACTS device may result in a power outage to a significant number of customers. 
         [0006]    For these and other reasons, there is a need for an improved power conversion system and method. 
       BRIEF DESCRIPTION 
       [0007]    In accordance with an embodiment of the present invention, a power conversion system including a power transformer for receiving AC power at a first voltage from an input side and for delivering AC power at a second voltage to an output side is provided. The power conversion system also includes a power converter including an input side converter on the input side and an output side converter on the output side coupled through a plurality of DC links. The power converter also includes a converter controller for providing control signals to the input side converter and the output side converter for regulating an active power and a reactive power flow through the power converter. Each of the input side converters and the output side converters includes at least two power converter transformers coupled between respective power converter bridges coupled to the plurality of DC links and the input side or to the plurality of DC links and the output side. 
         [0008]    In accordance with another embodiment of the present invention, a method of transmitting electric power from an input side to an output side is provided. The method includes coupling the input side and the output side through a power transformer and a power converter including an input side converter on the input side and an output side converter on the output side coupled through a plurality of DC links. The method also includes regulating an active power and a reactive power flow through the power converter by controlling the input side converter and the output side converter and disconnecting the power transformer from the input side or the output side or both sides during a fault condition on the power transformer. Each of the input side converters and the output side converters comprises at least two power converter transformers coupled between respective power converter bridges coupled to the plurality of DC links and the input side or to the plurality of DC links and the output side. 
     
    
     
       DRAWINGS 
         [0009]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0010]      FIG. 1  is a diagrammatical representation of an overall electric power system; 
           [0011]      FIG. 2  is a graphical representation of a power-voltage (P-V) curve; 
           [0012]      FIG. 3  is a diagrammatical representation of a power conversion system in accordance with an embodiment of the present invention; 
           [0013]      FIG. 4  is a schematic representation of a power converter of  FIG. 3  in accordance with an embodiment of the present invention; and 
           [0014]      FIG. 5  is a block diagram of a controller in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Furthermore, the terms “connected” and “coupled” are used interchangeably and could mean direct or indirect connections unless noted. 
         [0016]      FIG. 1  illustrates a single line diagram of an overall electric power system  10  from generation to utilization. Electric power system  10  includes a generating station  12 , a transmission substation  14 , local substations or distribution substations  16  and loads  18 . Generating station  12  may comprise a hydropower generating station, a thermal power generating station, a wind power generating station, or a solar power generating station, for example. Generator  20  in generating station  12  generates electricity at a generating station voltage which in certain embodiments may range from 4 kV to 13 kV. The generating station voltage is stepped up to a higher transmission level voltage such as 110 kV in an embodiment by a generating station transformer  22  for more efficient transfer of the electricity. 
         [0017]    The electricity is transmitted at the transmission level voltage to transmission substation  14  by primary transmission lines  24  that are configured to carry electricity over long distances. At transmission substation  14 , a reduction in voltage occurs for distribution to other points in the system through distribution lines  26 . Further voltage reductions for commercial and industrial or residential loads  18  may occur at distribution substation  16 . Distribution substation  16  may supply electricity at voltages in the range of 4 kV to 69 kV, for example. The voltages may further by reduced by one or two more levels at other local substations (not shown) receiving power from distribution substation  16  to supply the electricity to residential loads at lower voltages such as 120 V or 240 V. 
         [0018]    Current and voltage ratings of transmission lines  24  determine a transmission capacity of transmission lines  24  which is generally measured in terms of MVA loading (S). The MVA loading is a vector sum of an active power or a real power (P) and a reactive power (Q) and is given as P+jQ. Thus, the reactive power Q which does not produce any work or energy puts a limit on the amount of active power P that can be transmitted though the transmission line  24 . However, if the reactive power Q is supplied locally (e.g., at distribution station  16 ), the amount of active power transferred can be increased. 
         [0019]      FIG. 2  shows a graphical representation  30  of a power-voltage (P-V) curve. A horizontal axis  32  represents the active power P in terms of per unit (pu) and a vertical axis  34  represents a line voltage in pu. Three plots  36 ,  38 ,  40  represent P-V curves for three different power factors (i.e., 0.97 lagging, unity, and 0.97 leading respectively). A nose point  42  on each of the curves represents a voltage stability limit at the respective power factor. As will be appreciated by those skilled in the art, a nose point refers to a point after which voltage collapse occurs. That is, although with increased load or the active power, the line voltage also varies slightly but beyond nose point  42 , the voltage decreases sharply to 0 pu. This condition results from reactive power losses significantly exceeding the reactive resources available to supply them. As can be seen from the three curves  36 ,  38 ,  40 , the line voltage variation depends on the power factor and with higher or leading power factors, the nose point occurs at higher voltages. In other words with higher or leading power factors, the system becomes more stable. 
         [0020]    A power conversion system in accordance with an embodiment of the present invention provides reactive power to the power line to improve the power factor and consequently the voltage stability as discussed above. Other applications of the power conversion system include harmonic current compensation, power system oscillations damping, low voltage ride through capability and voltage regulation, for example. 
         [0021]      FIG. 3  shows a power conversion system  50  in accordance with an embodiment of the present invention. Although the example provided in  FIG. 3  depicts a three-phase power conversion system, this is not limiting of the teachings herein. Power conversion system  50  includes a power converter  60  and a main transformer  62  which provide coupling between a lower voltage side  52  and a higher voltage side  56 . In the embodiment shown, lower voltage side  52  is on a generator side  54 , whereas higher voltage side  56  is on transmission line  58 . However, in other embodiments, the lower voltage side may be on a load side and higher voltage side may be on the transmission line side. 
         [0022]    In the embodiment shown, power converter  60  and main transformer  62  are connected in parallel. However, other configurations, such as inputs of power converter  60  and main transformer  62  being connected in series and outputs being connected in parallel or vice versa, are also within scope of this invention. In general, during normal operating conditions, some part of the power from low voltage side  52  is processed through main transformer  62  and remaining part of the power is processed through power converter  60 . The ratio of the power that can be to be processed through power converter  60  and main transformer  62  depends on the rating of main transformer  62  and power converter  60  as well as the control aspects of the application. During an abnormal condition, i.e., when there is a fault on a main transformer, main transformer  62  is disconnected from the low voltage side or the high voltage side or both, and a maximum possible power is transmitted through power converter  60 . The maximum possible power again depends on the voltage or current or power rating of power converter  60 . In one embodiment, the power rating of power converter  60  is small compared to main transformer  62 . For example, if the power rating of main transformer  62  is 500 MVA, then the rating of power converter  60  may be 100 MVA. 
         [0023]    Furthermore during normal operation, main transformer  62  merely transmits the power coming from lower voltage side  52  to higher voltage side  54  by changing the level of the voltage. Whereas, power converter  60  injects a controllable active and reactive power into higher voltage side  54 . The amount of current that power converter  60  injects into higher voltage side  54  depends on the application. The applications as discussed in the preceding paragraph may include reactive power compensation, harmonic current compensation, transient and steady state stability management etc. 
         [0024]      FIG. 4  shows a schematic of power converter  60  of  FIG. 3  in accordance with an embodiment of the present invention. Power converter  60  includes an input side converter  72  and an output side converter  74  coupled through a plurality of DC link capacitors  75 . In one embodiment, a three-phase input signal is received by input side converter  72  from a generator (not shown); while a three-phase output signal is provided by output side converter  74  to a transmission line. The input side and the output side may be the high voltage side and the low voltage side respectively or the low voltage side and the high voltage side respectively. In one embodiment, the input side and the output side may be at the same voltage level if main transformer  62  is not used. A converter controller  70  monitors as well as controls the condition of input side converter  72  and output side converter  74  based on reference signals which may include voltage signals, current signals, or active and reactive power signals, for example. 
         [0025]    Output side converter  74  comprises at least two output side converter bridges  76  and at least two output side converter transformers  88 . In the embodiment shown, six output side converter bridges  76  and six respective output side converter transformers  88  are utilized. Each component is described in further detail below. 
         [0026]    In one embodiment, converter controller  70  is configured to control output side converter bridges  76  to switch at a low frequency and generate a corresponding converter output voltage including a fundamental voltage component and harmonic components. In an embodiment, the low switching frequency ranges from 60 Hz to 180 Hz for a fundamental frequency of 60 Hz. In another embodiment, output side converter bridges may be operating at high frequency. For example, in one embodiment, the high frequency may be 2 kHz. The converter output voltage of power converter bridges  76  is generated on a plurality of AC lines  99 . 
         [0027]    Output side converter transformers  88  are configured to generate an output voltage  100  by changing the level of the voltage of AC line  99  to match it to the voltage of the output side of the power line. Resultant output voltage  100  comprises a sum of the fundamental voltage components of the output voltage of each output side converter bridge  76 . In one embodiment, resultant output voltage  100  is substantially free of any harmonic component that exists in the converter output voltages of output side converter bridges  76 . Substantially free refers to a resultant output voltage that does not include low order harmonic components such as 5 th , 7 th , 11 th  or 13 th . 
         [0028]    Each output side converter bridge  76  is coupled to a primary winding  78  of a respective output side converter transformer  88 . Typically each output side converter transformer comprises a three-phase transformer. In one embodiment, primary winding  78  of each output side converter transformer  88  is connected in a three phase delta mode (i.e., three phase delta winding). 
         [0029]    In another embodiment, a secondary winding  80  of output side converter transformer  88  comprises a single winding per phase with open neutral. Secondary windings  80  of all output side converter transformers  88  are then typically connected in series. In one embodiment, secondary windings  88  may be connected in parallel. In another embodiment, some secondary windings may be connected in series and groups of series connected windings may be connected in parallel. In yet another embodiment, secondary windings  88  may be oriented such that each of the secondary windings is phase-shifted by an angle with respect to a secondary winding of another transformer to cancel low order harmonics in output voltages. In an alternative embodiment, the primary and secondary winding may comprise a zigzag winding. 
         [0030]    In a more specific embodiment, converter controller  70  is configured to control output side converter bridges  76  to switch with a phase shift. The gating signals for output side converter bridges  76  are derived so that the fundamental components of the converter output voltages are shifted in phase with respect to one another. In one embodiment, the phase-shifted gating signals, when combined with phase shifting in secondary winding  80  of output side converter transformers  88 , results in canceling of the low-order harmonic components from the resultant output voltage. The order of harmonics cancelled depends on the number of pairs of converter-transformer units used. The number of pairs and level of phase shifting can be selected such that a high power quality resultant output voltage is derived at a relatively low switching frequency. 
         [0031]    The structure of input side converter  72  is similar to the output side converter  74 . That is input side converter  72  also includes at least two input side converter bridges  102  and two input side converter transformers  104 . Input-side converter bridges  102  are each coupled to the input lines via the corresponding input-side converter transformer  104 . In addition, input-side converter bridges  102  are coupled to secondary windings  106  of input-side converter transformers  104 , whereas primary windings  108  of input-side converter transformers  104  may be connected in series or parallel or in a combination of series-parallel as described with secondary windings  80  of power converter transformer  88 . Further, input-side converter bridges  102  of input-side converter system  72  may be switched in a similar manner as output side converter bridges  76 . 
         [0032]    In a further embodiment, converter controller  70  is further configured to control an active power flow from the input side converter bridges  102  and output side converter bridges  76 . In one embodiment, the active power output is controlled by controlling a phase angle of the fundamental component of the resultant output voltage on the output side whereas the reactive power input is controlled by controlling an amplitude of the voltage of the DC link capacitors  75 . 
         [0033]    In another embodiment, power converter  60  is further configured to control a reactive power flow from the input side and output side converter bridges  76  and  102 . In this embodiment, the reactive power is typically controlled by adjusting a resultant magnitude or amplitude of the fundamental component of the resultant output voltage on the output side or the input side. 
         [0034]    In one embodiment, converter controller  70  may control the input side and output side converter bridges  102  and  76  to generate a reference current or voltage command signals on the output side which may further result in changes of active and reactive power flow. The reference current or voltage command signals may depend on the application for which present power converter  60  may be employed. For example, the applications may include reactive power compensation, harmonic current compensation, power system oscillations damping, low voltage ride through capability and voltage regulation. It should be noted that, in general, irrespective of the application, when voltages and currents of power converter  60  are controlled, the active and reactive power also gets controlled by default. This is so because active and reactive power are finally functions of voltages and currents 
         [0035]    In one embodiment, connections of windings of input and output side converter transformers may be changed from one type to another when main transformer  62  ( FIG. 3 ) is disconnected from the system. For example, in one embodiment, groups of series connected primary windings of input side transformers may be connected in parallel which may further be connected in series with main transformer  62  during normal conditions. However, when main transformer  62  is taken out due to fault or for maintenance purposes, the connection of primary windings of input side transformers may be changed such that the primary windings of all input side transformers are connected in series. The connection changes may be done to reconfigure power converter  60  to handle the system voltages and transfer the maximum possible power from input side to the output side. 
         [0036]      FIG. 5  shows an exemplary block diagram of a controller  120  in accordance with an embodiment of the present invention. Controller  120  includes an output side converter controller  122  and an input side converter controller  124 . It should be noted that controller  120  is only a part of the converter controller  70  of  FIG. 4 . In other words, output side converter controller  122  and input side converter controller  124  shown are only for individual output and input side converter bridges  76  and  102 . In one embodiment, multiple such controllers  120  may be utilized in the converter controller  70  for multiple converter bridges, such as those shown in  FIG. 4 . These multiple controllers may be connected together in one embodiment or may be reduced to a single controller without deviating from the scope of the invention. 
         [0037]    Both controllers  122  and  124  utilize an angle θ generated from a three phase-phase locked loop (PLL) (not shown) in transformation matrices which transform voltage or current signals from one reference frame to another reference frame. Output side controller  122  receives reference signals, output side bridge active power P o * and output side bridge reactive power Q o * as inputs. In one embodiment, the output side bridge active power signal P o * and output side bridge reactive power signal Q o * may be generated by dividing the total output active and reactive power required from output side converter  74  ( FIG. 4 ) by a number of power converter bridges utilized in output side converter  74 . For example, if the total output active and reactive power required from output side converter  74  is 50 MW and 50 MVAR respectively and the number of converter bridges utilized are 5, then each of the converter bridges will need to output 10 MW active power and 10 MVAR reactive power respectively. 
         [0038]    A current computation block  126  computes d-q domain output side bridge reference current signals i od * and i oq * from output side bridge active and reactive power signals P o * and Q o * respectively. In one embodiment, the d-q domain signals refer to signals in a synchronous reference frame (i.e., a reference frame rotating at synchronous speed). Two proportional integral (PI) regulators  128 ,  130  then generate d-q domain output side bridge reference voltage signals V od * and V oq * based on error signals between d-q domain output side bridge reference current signals i od * and i oq * and d-q domain output side bridge actual current signals i od  and i oq  respectively. The d-q domain actual current signals i od  and i oq  are generated by an abc-dq transformation matrix  132  from a-b-c domain output side actual bridge currents i oa , i ob , and i oc . In one embodiment, the abc-dq transformation matrix  132  may be given as 
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         [0039]    where θ is given as ωt, ω representing frequency of transmission line voltage in rad/seconds and t representing time in seconds. 
         [0040]    A dq-abc transformation matrix  134  converts d-q domain voltage signals V od * and V oq * into a-b-c domain output side bridge reference voltage command signals V oacmd , V obcmd  and V occmd . The dq-abc transformation matrix may be given as: 
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         [0000]    A modulator  136  then generates gating signals for output side bridge converter  76  ( FIG. 4 ) to generate the voltage command signals V oacmd , V obcmd  and V occmd . In one embodiment, modulator  136  may be a pulse width modulation (PWM) modulator. Further the PWM modulator may a sine-triangle PWM modulator or space vector PWM modulator. 
         [0041]    The structure of the input side controller  124  is more or less similar to output side controller  122 . The objective of the input side controller  124  is to control input side bridge converter  102  ( FIG. 4 ) for maintaining the voltage at the DC link  75  ( FIG. 4 ) and for generating a required reactive power set by an operator. Input side controller  124  receives reference signals, DC link voltage V dc * and input side bridge reactive power Q i * as inputs. As discussed earlier, the input side bridge reactive power Q i * signal may also be generated by dividing the total reactive power required from input side converter  102  by a number of power converter bridges utilized in input side converter  102 . 
         [0042]    A voltage PI regulator  138  generates a d domain input side bridge reference current signal i id * based on an error between the actual DC link voltage V dc  and the reference DC link voltage V dc * whereas a current computation block  140  generates q domain input side bridge reference current signal i iq *. As with output side converter controller  122 , two proportional integral (PI) regulators  142 ,  144  then generate d-q domain input side bridge reference voltage signals V id * and V iq * based on error signals between d-q domain input side bridge reference current signals i id * and i iq * and d-q domain input side bridge actual current signals i id  and i iq  respectively. The actual current signals i id  and i iq  are generated by an abc-dq transformation matrix  146  from a-b-c domain input side actual bridge currents i ia , i ib , and i ic . Finally, a dq-abc transformation matrix  148  converts d-q domain voltage signals V id * and V iq * into a-b-c domain input side bridge reference voltage command signals V iacmd , V ibcmd  and V iccmd , which are then generated by the input side converter bridge  102  ( FIG. 4 ) after receiving gating signals from a modulator  150 . 
         [0043]    It should be noted that controller  120  shown here is merely exemplary and other controller structures which may be used to control embodiments of the power converter of the present invention are very much within scope of this invention. The objective of such controllers may include harmonic current compensation, power system oscillations damping, low voltage ride through capability and voltage regulation. 
         [0044]    One of the advantages of the presented power conversion system is that it utilizes low power and hence cheap modular building blocks comprising converter bridges and transformers compared to high power and costly FACTS devices. Thus, when one of the building blocks fails it can be replaced immediately by a backup building block ensuring continuity of power supply. Further, it can transmit a reduced power even when the main transformer fails. 
         [0045]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.