Abstract:
A power regulation system for an electrical grid has a store of electrical energy connected through a first switch to a source of electrical energy. The response time of the store is faster than that of the source. A second switch is connected to the store at one side with the opposite side for connection to the grid. A first controller monitors energy stored in the store and energy available from source and selectively controls the first switch to close to transfer energy from the source to the store. A second controller monitors energy stored in said store and, on receiving an indication that additional energy is needed in the grid, if the energy stored in the store exceeds a supply threshold, controls the second switch to close to transfer energy from the store to the grid.

Description:
BACKGROUND 
       [0001]    The present invention relates to methods and systems for regulating power in an electricity grid system. 
         [0002]    As is well known in the art an AC electricity grid system is organized as a grid or network that interconnects multiple power generating facilities with multiple loads. The grid is typically geographically diverse and is subdivided into smaller geographic areas for regulation of the power. The generating capability and the loads may vary with time and the operation of the grid must be controlled to keep the power distributed—especially the voltage and frequency—within defined characteristics. 
         [0003]    If supply is less than demand, the frequency decreases and if supply is greater than demand, frequency increases. As a result, frequency regulation is an important part of the normal operation of modern AC electricity grids and has historically been accomplished by adjusting classic generation sources (such as hydro, gas or coal power generators) up or down to match the total demand. Independent System Operators (ISOs) are responsible for monitoring their geographic area and ensuring that the mismatch between supply and demand remains small. To do this, the ISOs monitor the total inflows and outflows from their area, the total being generated, and the total demand (or load). The ISOs use this information to determine if more or less power needs to be generated and sends a control signal to the generators under their control. As a result, the AC frequency of the entire system is managed during both regular operation (customers increasing/decreasing their demand) and emergency conditions (when a generator trips out due to a fault). 
         [0004]    The IEEE publication “Generation Scheduling and Control”, Thomas M Athay, Proceedings of the IEEE, Vol. 75, No. 12, December 1987, provides a summary of how power generation control and frequency regulation are accomplished on most conventional AC electricity grids. The overall objective is to control the electrical power generator units so as to supply the continuously changing customer power demand in an economical manner. This task is undertaken by an Automatic Generation Control (AGC) program operating within the ISOs. Much of the associated functionality is provided by an AGC program operating within a control center computer. In very general terms, the AGC monitors the current state of the AC electricity grid (typically voltage and frequency) and outputs regulation signals for controlling each generation unit to keep the voltage and frequency within acceptable limits. An essential aspect of an interconnected system is that all available generators in the system respond to changes in frequency, via their governor speed control mechanisms and their AGC regulation signals, to bring the frequency to a new steady state, thereby balancing total system generation to total system load. 
         [0005]    A limitation of conventional AGC regulation methods is that generation units (e.g. hydro-electric, steam-turbine, diesel etc.) frequently have a response time of several seconds, whereas total system load can vary (due to customers connecting and disconnecting loads to and from the grid) much more rapidly. Consequently, it can be very difficult for conventional AC electricity grids to track short-period variations in total system load. The result is that the frequency and voltage in the AC electricity grid will typically fluctuate in time, and the AGC system will operate to constrain these fluctuations to a predetermined range. 
         [0006]    The publication “Energy Storage: The Missing Link in the Electricity Value Chain” by the Energy Storage Council (ESC) (2002), proposes the use of grid-connected energy storage systems which operate to supply and absorb power to and from the grid to enable improved frequency regulation in the distribution grid. As noted in the ESC paper, energy storage systems such flywheels and batteries are capable of responding to system load changes (or, equivalently, AGC regulation signals) more rapidly than conventional power generation units, which enables more accurate frequency regulation in the grid. 
         [0007]    While the use of grid-connected energy storage systems enables improved frequency regulation relative to conventional AGC methods, further improvements are desirable. 
       SUMMARY 
       [0008]    A power regulation system for an electrical grid has a store of electrical energy connected through a first switch to a source of electrical energy. The response time of the store is faster than that of the source. A second switch is connected to the store at one side with the opposite side for connection to the grid. A first controller monitors energy stored in the store and energy available from said source and selectively controls the first switch to close to transfer energy from the source to the store. A second controller monitors energy stored in said store and, on receiving an indication that additional energy is needed in the grid, if the energy stored in the store exceeds a supply threshold, controls the second switch to close to transfer energy from the store to the grid. 
         [0009]    In an embodiment, a system for frequency regulation in an AC electricity grid has a set of one or more power regulation units for connection to the grid. Each power regulation unit has a variable frequency AC source/sink, a capacitor array acting as a DC energy store, a variable frequency AC/DC power converter coupled between the AC source/sink and the capacitor array, and a variable frequency AC/DC power converter connected between the capacitor array and the AC electricity grid. A first converter controller is configured to control the power converter between the grid and capacitor array to control bi-directional flows of electrical power between the capacitor array and the AC electricity grid. A second converter controller is configured to control the power converter between the source/sink and the capacitor array to control energy flow between the AC source/sink and the capacitor array. 
         [0010]    In an aspect, there is provided a power regulation system for an electrical grid comprising: a store of electrical energy; a source of electrical energy, wherein an energy supply response time of the store is faster than that of the supply; a first switch connecting said store to said source; a second switch connected to the store at one side and having an opposite side for connection to the grid; a first controller for monitoring energy stored in the store and energy available from said source and for selectively controlling the first switch to close to transfer energy from the source to the store; a second controller for monitoring energy stored in said store and, on receiving an indication that additional energy is needed in the grid, if the energy stored in the store exceeds a supply threshold, controlling the second switch to close to transfer energy from the store to the grid. 
         [0011]    In another aspect, there is provided a power regulation system for an AC grid, comprising: a capacitive store able to supply energy on command within a first response time; a first switch connected at one side to said capacitive store and for connection, at an opposite side, to a kinetic battery able, at least intermittently, to supply energy on command within a second response time, said first response time being faster than said second response time; a second switch comprising a power inverter, said second switch connected at one side to said capacitive store and for connection at an opposite side to said AC grid; a first controller connected to monitor voltage of said capacitive store and energy available from said kinetic battery and for, where said voltage of said capacitive store falls below a lower threshold and said kinetic battery has available energy, controlling said first switch to close to transfer energy from said kinetic battery to said capacitive store; a second controller connected to monitor voltage of said capacitive store and intermittently receiving a request to supply additional energy to said grid, said second controller for, on receiving a request to supply additional energy to said grid, where voltage of said capacitive store exceeds a supply threshold, controlling said second switch to close to transfer energy from said capacitive store through said power inverter to said AC grid. 
         [0012]    In a further aspect, there is provided a power regulation system for an electrical grid, comprising: a store of electrical energy able to supply energy on command within a first response time; a first switch connected at one side to said store of electrical energy and for connection, at an opposite side, to a source of electrical energy able, at least intermittently, to supply energy on command within a second response time, said first response time being faster than said second response time; a second switch connected at one side to said store and for connection at an opposite side to said electrical grid; a first controller for monitoring energy stored in said store and energy available from said source and for, where said indication of energy stored in said store falls below a lower threshold and said source has available energy, controlling said first switch to close to transfer energy from said source to said store; a second controller for monitoring energy stored in said store and for receiving an indication of at least one characteristic of power in said grid and for, where said at least one characteristic of power in said grid indicates a need for additional energy in said grid and energy stored in said store exceeds a supply threshold, controlling said second switch to close to transfer energy from said store to said grid. 
         [0013]    In a yet further aspect, there is provided a method for regulating power on a grid comprising: (a) in a first process: (i) monitoring energy stored in a store and energy available from a source; (ii) where said indication of energy stored in said store falls below a lower threshold and said source has available energy, transferring energy from said source to said store; (b) in a second process independent of said first process: (i) monitoring energy stored in said store; and (ii) on receiving an indication of a need for additional energy in said grid, if energy stored in said store exceeds a supply threshold, transferring energy from said store to said grid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Further features and advantages of the present invention will become apparent from the following detailed description, by way of example only, taken in combination with the appended drawings, in which: 
           [0015]      FIG. 1  is a block diagram schematically illustrating an energy storage system known in the art connected to an AC electricity grid; 
           [0016]      FIG. 2  is a block diagram schematically illustrating an energy storage system in accordance with a representative embodiment connected to an AC electricity grid; 
           [0017]      FIG. 3  is a schematic view illustrating aspects of the system of  FIG. 2 ; and 
           [0018]      FIG. 4  is a schematic view illustrating an energy storage system in accordance with another representative embodiment. 
       
    
    
       [0019]    It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
       DETAILED DESCRIPTION 
       [0020]      FIG. 1  is a block diagram schematically illustrating a conventional flywheel energy storage. In the system of  FIG. 1 , the AC electricity grid (or a connection to it) is represented by a 3-phase AC bus  2 . A power conditioning unit  4  is connected to the 3-phase AC bus  2 , and enables bi-directional energy flow between the AC electricity grid and a high-voltage DC bus  6 . A bank of energy storage units  10  is connected to the high-voltage DC bus  6 . Each energy storage unit  10  includes a flywheel  14 , and a flywheel controller/converter  12  to regulate the supply of power between the high-voltage DC bus  6  and the flywheel  14 . In operation, a supervisory controller  8  interacts with each of the flywheel controller/converter  12  to control the flow of electrical power into and out of the energy storage system in accordance with a regulation signal  16  received from an AGC process of the AC electricity grid, for example. By this means, the energy storage system can selectively absorb or supply electrical power from or to the AC electricity grid so as to facilitate frequency regulation. In the power absorbing mode, the flywheel controller/converter  12  operates to absorb electrical power from the high-voltage DC bus  6 , invert the absorbed power to AC, and feed the AC power to the flywheel  14 . On the other hand, in the supplying mode, the flywheel controller/converter  12  absorbs electrical power from the flywheel  14  and converts the 3-phase AC power to DC on the high-voltage DC bus  6 . 
         [0021]    Referring to  FIG. 2 , there is shown a system usable for power regulation in an AC electricity grid in accordance with one embodiment. In the embodiment of  FIG. 2 , the system comprises a set of one or more power regulation units  20  connected to a 3-phase AC bus  2  and controlled by a common supervisory controller  8 . 
         [0022]    In the illustrated embodiment, the 3-phase AC bus  2  may represent either a connection to the AC electricity grid or the AC electricity grid itself. Each power regulation unit  20  comprises a variable frequency AC source/sink  22  providing an AC energy store; a capacitor array  24  providing a DC energy store; a variable frequency AC/DC power converter  26  coupled between the AC source/sink  22  and the capacitor array  24 , and a variable frequency AC/DC power converter  28  connected between the capacitor array  24  and the 3-phase AC bus  2 . 
         [0023]    The variable frequency AC source/sink  22  may be provided as any of a variety of devices capable of producing and/or absorbing variable frequency AC power. In some cases, the variable frequency AC source/sink  22  may be provided as a kinetic battery such as a flywheel energy storage device or a pumped water storage system (which uses water to rotate a generator to generate power). As will be appreciated, kinetic batteries are capable of sourcing (i.e. outputting) AC power in accordance with the total energy stored in the device, and sinking (i.e. absorbing) AC power within the available storage capacity of the device. In some embodiments, all of the power regulation units  20  are configured with the same type of variable frequency AC source/sink  22 . In other embodiments, different types of variable frequency AC source/sink  22  may be employed in different ones of the power regulation units  20 . Applicant&#39;s US Patent publication No. US 2011/0298293, published Dec. 8, 2011, discloses flywheel energy storage systems of a type that may be used for this purpose. The entire content of US Patent publication No. US 2011/0298293 is incorporated herein by reference. 
         [0024]    The capacitor array  24  preferably comprises one or more capacitors configured to provide a highly responsive DC energy store. As is known in the art, capacitors are capable of sourcing and sinking electrical energy with very high transient power levels and very short response times, although their energy storage capability is limited relative to both the variable frequency AC source/sink  22  and conventional power generation units. 
         [0025]    In combination, the capacitor array  24  and the variable frequency AC source/sink  22  enable a staged response to load changes in the AC electricity grid. In particular, the capacitor array  24  enables a very rapid initial response to a control signal, and then the slower responding variable frequency AC source/sink  22  sustains the required power sourcing or sinking operation for a longer duration of time. 
         [0026]    Interaction between the capacitor array  24  and the variable frequency AC source/sink  22  is mediated by the variable frequency AC/DC power converter  26 , under control of a converter controller  30 , which may comprise an AC control PID loop. The converter controller  30  monitors the DC voltage across the capacitor array  24  and controls the variable frequency AC/DC power converter  26  and the variable frequency AC source/sink  22  to supply or absorb energy accordingly. The variable frequency AC/DC power converter  26  comprises both a rectifier and an inverter, so that the variable frequency AC source/sink  22  can operate in both energy sourcing and energy sinking modes of operation. 
         [0027]    The converter controller  30  therefore operates to control the variable frequency AC/DC power converter  26  to manage the flow of energy between the variable frequency AC source/sink  22  and the capacitor array  24  in an effort to maintain the voltage across the capacitor array  24  within a predetermined range. 
         [0028]    The converter controller  30  may operate in a power sourcing mode of operation, in which electrical energy from the variable frequency AC source/sink  22  is rectified and supplied to the capacitor array  24 . For example, the converter controller  30  may control the variable frequency AC/DC power converter  26  to supply energy from the variable frequency AC source/sink  22  in order to maintain the DC voltage of the capacitor array  24  at or above a predetermined minimum level. 
         [0029]    In addition to the energy sourcing mode of operation, the converter controller  30  may operate in an energy sinking mode of operation, in which DC energy from the capacitor array  24  is converted to a variable frequency AC power and supplied to the variable frequency AC source/sink  22 . For example, the converter controller  30  may control the variable frequency AC/DC power converter  26  to supply energy to the variable frequency AC source/sink  22  in order to maintain the DC voltage of the capacitor array  24  at or below a predetermined maximum level. 
         [0030]    As will be appreciated, both of the energy sourcing and energy sinking modes of operation are dependent on the available capacity of the variable frequency AC source/sink  22  to source or sink electrical energy. Accordingly, the converter controller  30  is preferably responsive to one or more signals indicative of the state of the variable frequency AC source/sink  22 , and may enter a stand-by mode of operation based on the indicated state. For example, consider an embodiment in which the variable frequency AC source/sink  22  is provided as a kinetic battery. Kinetic batteries typically are capable of storing energy up to a predetermined maximum amount of energy. As such, the ability of the kinetic battery to source energy to the capacitor array  24  at any given time is dependent on the actual amount of energy stored in the battery at that time. Conversely, the ability of the kinetic battery to sink energy from the capacitor array  24  at any given time is dependent on the available storage capacity of the battery, that is, the difference between the actual amount of energy stored in the battery and the maximum amount of energy that the battery is capable of storing. Accordingly, when the actual amount of energy stored in the battery is below a predetermined minimum level, the converter controller  30  may enter a first stand-by mode, in which the variable frequency AC/DC power converter  26  is prevented from supplying any further energy to the capacitor array  24  from the variable frequency AC source/sink  22 . Conversely, when the actual amount of energy stored in the battery is above a predetermined maximum level, the converter controller  30  may enter a second stand-by mode, in which the variable frequency AC/DC power converter  26  is prevented from supplying any further energy to the variable frequency AC source/sink  22  from the capacitor array  24 . 
         [0031]    Interaction between the capacitor array  24  and the 3-phase AC bus  2  is mediated by the variable frequency power converter  28  under control of a converter controller  32 , which may comprise an AC control PID loop. The converter controller  32  is responsive to a control signal  34  from the supervisory controller  8 , to select one of a power absorbing mode and a power supplying mode. In the power absorbing mode of operation, the power converter  28  is controlled to absorb electrical power from the 3-phase AC bus  2 , rectify the absorbed power to DC, and feed the DC power to the capacitor array  24 . On the other hand, in the supplying mode, the power converter  28  is controlled to absorb DC power from the capacitor array  24 , convert the DC power to 3-phase AC, and supply to the 3-phase AC power to the 3-phase AC bus  2 . 
         [0032]    As may be appreciated, both of the power absorbing and power supplying modes of operation are dependent on the energy available from the capacitor array  24 . This energy availability is approximately proportional to the DC voltage of the capacitor array  24 . Accordingly, the converter controller  32  is preferably responsive to the capacitor array voltage in addition to the control signal  34 . 
         [0033]    For example, consider a case in which the control signal  34  requests the supply of power to the 3-phase AC bus  2 . In this case, if the capacitor array voltage is above a predetermined lower limit, then the converter controller  32  selects the power supplying mode and controls the power inverter  28  to supply power to the 3-phase AC bus  2  from the capacitor array  24 . If, on the other hand, the capacitor array voltage is (or drops) below the lower limit, then the converter controller  32  may select a stand-by node, and prevent (or stop) the supply of power to the 3-phase AC bus  2 . 
         [0034]    Conversely, consider a case in which the control signal  34  requests the absorption of power from the 3-phase AC bus  2 . In this case, if the capacitor array voltage is below a predetermined upper limit, then the converter controller  32  selects the power absorbing mode and controls the power inverter  28  to supply power to the capacitor array  24  from the 3-phase AC bus  2 . If, on the other hand, the capacitor array voltage is (or rises) above the upper limit, then the converter controller  32  may select a stand-by node, and prevent (or stop) the supply of power to the capacitor array  24 . 
         [0035]    Following the above description, it will be seen that the converter controller  30  and the converter controller  32  operate independently of each other, while using the capacitor array voltage to control the flow of power into (and through) the system. As a result centralized control of both the converter controller  30  and the converter controller  32  from the supervisory controller  8  is not required. 
         [0036]    Possible control algorithms for converter controllers  30  and  32  may be based on the following thresholds schematically illustrated in  FIG. 3 : 
         [0000]    Ec_min_supply—a minimum energy in the capacitor array for energy to be supplied from the array;
 
Ec_lower—a threshold energy in the capacitor array below which it would advisable to increase the energy in the capacitor array;
 
Ec_upper—a threshold energy in the capacitor array above which it would be advisable to decrease energy in the capacitor array;
 
Ec_max_absorb—a maximum energy in the capacitor array for energy to be absorbed by the array;
 
EB_min—a minimum energy in the source/sink; and
 
EB_max—a maximum energy in the source/sink.
 
         [0037]    Converter controller  30  may store two thresholds for the capacitor array  24 : Ec_lower and Ec_upper. Additionally, converter controller  30  may store the two thresholds for the source/sink  26 : EB_min, and EB_max. Converter controller  30  may then operate under the following algorithm, where Ec is the energy in the capacitor array and EB is the energy in the source/sink: 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 while Ec &lt; Ec_lower 
               
             
          
           
               
                   
                 while EB &gt; EBmin → supply energy to the capacitor array 
               
               
                   
                 else stand-by 
               
             
          
           
               
                   
                 while Ec &gt; Ec_upper 
               
             
          
           
               
                   
                 while EB &lt; EBmax → absorb energy from the capacitor array 
               
               
                   
                   
               
             
          
         
       
     
         [0038]    The converter controller  32  may store the following two thresholds for the capacitor array  24 : Ec_min_supply and EC_max_absorb. Converter controller  32  may then operate according to the following algorithm: 
         [0000]    while receiving energy supply request from the supervisory controller 
         [0000]                                            while Ec &gt; Ec_min_supply → supply energy to grid           else stand-by                        
while receiving energy absorb request from the supervisory controller
 
         [0000]    
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 while Ec &lt; Ec_max_absorb → absorb energy from grid 
               
               
                   
                 else stand-by 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    It will be apparent from these control algorithms that while the energy of the capacitor array stays between Ec_lower and Ec_upper, the capacitor array will supply energy to, and absorb energy from, the grid without any energy flowing between the capacitor array and the source/sink. If, however, in supplying energy to the grid, the energy in the capacitor array drops below Ec_lower, controller  32  will be triggered to commence supplying available energy from the source/sink to the capacitor array. The difference between Ec_lower and Ec_min_supply may be chosen to be sufficiently large that it may be expected that in many situations, even given its slower response time, the source/sink will begin supplying energy to the capacitor array before the energy in the array drops below Ec_min_supply. Thus, the source/sink will assist in maintaining the energy in the capacitor array above the point where the capacitor array would cut out and cease supplying energy to the grid. Similarly, if, in absorbing energy from the grid, the energy in the capacitor array rises above Ec_upper, controller  32  will be triggered to have the source/sink commence absorbing energy from the capacitor array in order to assist in maintaining the energy in the capacitor array below the point where the capacitor array would cut out and cease absorbing energy from the grid. 
         [0040]    In other embodiments, the variable frequency AC source/sink  22  is replaced with as a variable frequency generator such as a wind-turbine for example. As will be appreciated, variable frequency generators are capable of sourcing AC power at a variable frequency, but cannot sink and store energy for later use. Where source/sink  22  is replaced with a variable frequency generator, the variable frequency AC/DC power converter  26  is replaced by a rectifier, which operates to rectify AC power from the variable frequency AC source/sink  22  to a DC voltage, which is supplied to the capacitor array  24 . 
         [0041]    In other embodiments, the AC source/sink  22  is replaced with a DC source/sink, such as a battery array. In such instance, each variable frequency AC/DC power converter  26  is replaced by a switch which closes whenever energy is to be transferred to or from the capacitor array. 
         [0042]    In other embodiments, the grid is a DC grid rather than an AC grid. In this instance, each variable frequency AC/DC power converter  28  is replaced by a switch which closes whenever energy is to be transferred to or from the grid. 
         [0043]    In other embodiments, each capacitor array  24  is replaced by any other kind of capacitive system. 
         [0044]    In other embodiments, each capacitor array  24  is replaced by a different kind of store of electrical energy, such as a battery array, provided the store has a faster response time than the source/sink. 
         [0045]      FIG. 4  schematically illustrates a specific embodiment. Turning to  FIG. 4 , a system  100  for regulating power of an AC grid has a flywheel system  122  with a flywheel  140  connected to a generator  142 . The AC output of the generator inputs an input side of a three way switch  144  of a power converter  126 . One pole  146  of the switch is unterminated, one  148  connects to the output of the power converter  126  through a power inverter  150 , and the third  152  connects to the output through a rectifier  154 . The output of the power converter  126  inputs capacitive array  124 . A controller  130  is input from a flywheel RPM sensor  158  and a capacitive array voltage sensor  160 . The controller outputs a control signal to a switch controller which is schematically illustrated as a servo motor  162 . 
         [0046]    The AC grid inputs the input side of a three way switch  164  of a power converter  128 . One pole  166  of the switch is unterminated, one  168  connects to the output of the power converter  128  through a power inverter  170 , and the third  172  connects to the output through a rectifier  174 . The output of the power converter  128  inputs capacitive array  124 . A controller  132  is input from a supply/absorb request line  134  and a capacitive array voltage sensor  180 . The controller  132  outputs a control signal to a switch controller which is schematically illustrated as a servo motor  182 . 
         [0047]    Controllers  130  and  132  may operate under the control of the algorithms described in conjunction with  FIG. 3 , where the energy of the capacitive array is represented by the voltages measured by voltage sensors  160 ,  180  and the energy of the flywheel is represented by the rotational speed (RPM) of the flywheel. 
         [0048]    Two voltage sensors are illustrated in  FIG. 4  to emphasize the independent operation of controllers  130  and  132  but both controllers could instead take a voltage signal from the same sensor. The two switches  144 ,  164  are schematically illustrated as mechanical switches but solid state switches may be substituted. 
         [0049]    The embodiments described above are intended to be illustrative only and modifications will be apparent to those of skill in the art. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.