Patent Publication Number: US-11043803-B2

Title: Reference signal generating method for distance and directional protection elements

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to power system monitoring and protection and, more particularly, to generating a reference signal that may be used as a polarizing quantity in protection elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the disclosure are described herein, including various embodiments of the disclosure with reference to the figures listed below. 
         FIG. 1  is a block diagram of an electric power system with an intelligent electronic device (IED) that monitors a power line for events, in accordance with an embodiment. 
         FIG. 2  is a functional block diagram of a process performed by the IED of  FIG. 1  to generate a reference signal that is used as a polarizing quantity in monitoring operations of the power line, in accordance with an embodiment. 
         FIG. 3  is a plot of q components (output from an adaptive notch filter) used in the process of  FIG. 2 , in accordance with an embodiment. 
         FIG. 4  is a plot of difference angles from the process of  FIG. 2 , in accordance with an embodiment. 
         FIG. 5  is plot of d and q components before and after a low pass filters (LPFs) used in adding inertia to the reference signal, in accordance with an embodiment. 
         FIG. 6  is a plot of estimated (output from a delay after an integrator) and delayed (with added delay from a saturation function) angular frequency tracking a set change to angular frequency, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     Electrical power generation and delivery systems are designed to generate, transmit, and distribute electrical energy to loads. These power systems may include equipment, such as electrical generators, electrical motors, power transformers, power transmission and distribution lines, circuit breakers, switches, buses, transmission lines, voltage regulators, capacitor banks, and the like. Such equipment may be monitored, controlled, automated, and/or protected using intelligent electronic devices (IEDs) that receive electric power system information from the equipment, make decisions based on the information, and provide monitoring, control, protection, and/or automation outputs to the equipment. 
     IEDs may use distance and directional elements in power system protection. For example, IEDs may use a polarizing quantity, such as a positive sequence voltage, in estimating direction and distance to a fault. Memory filters may be added to ensure proper operation of protection elements during faults located close to the relay, when values of voltage amplitudes may be close to zero. Such memory filters may be based on an assumption that phase angles of the positive sequence voltages (V 1 ) change very slowly or will not change due to inertia of synchronous generators. 
     However, modern power systems may use low-inertia sources that are connected to the power system indirectly, through inverters. In power systems with low-inertia sources, parameters of generated voltages may change by inverter control algorithms without any inertia. Accordingly, there is a need to accurately track changes of the polarizing quantities while adding inertia to prevent step changes of V 1  parameters that may cause misoperations of IEDs, such as protection relays. 
     As explained below, IEDs may accurately estimate angular frequency ω by tracking power system angular frequency cos and add inertia to changes of estimated V 1  parameters. For example, an IED may receive A-phase, B-phase, and C-phase electrical measurements of a power system. The IED may transform the A-phase, B-phase, and C-phase measurements to d-component, q-component, and 0-component signals. The IED may include an adaptive notch filter that reduces or eliminates a double frequency component that may be present when step changes of frequency and amplitude occur and/or when the A-phase, B-phase, and C-phase measurements have different amplitudes. By reducing or eliminating the double frequency component, the IED may generate a more accurate estimated ω which may allow for more accurately tracking changes to the polarizing source. Further, the IED may separately add inertia to the estimated angular frequency used in generating a reference signal. 
       FIG. 1  illustrates a simplified one-line diagram of an electric power system  20  having a synchronous generator  22  and a low-inertia inverter-based power source  24 , such as a photovoltaic array  26  and an inverter  28 . The photovoltaic array  26  and the synchronous generator  22  may provide power to loads at buses  30 ,  32 , and  34  via the power line  40 . While a simplified one-line diagram is shown, this is meant to be illustrative and the systems and methods described herein may be used in any suitable power system. Further, note that while a single phase of the power line  40  is shown, the transmission system may be, for example, a three-phase transmission system with an A-phase, B-phase, and C-phase. 
     The electric power system  20  may be monitored, controlled, automated, and/or protected using one or more IEDs  60 . In general, IEDs in an electric power generation and delivery system may be used for protection, control, automation, and/or monitoring of equipment in the system. For example, IEDs may be used to monitor equipment of many types, including electric transmission lines, electric distribution lines, current transformers, busses, switches, circuit breakers, reclosers, transformers, autotransformers, tap changers, voltage regulators, capacitor banks, generators, motors, pumps, compressors, valves, and a variety of other types of monitored equipment. 
     As used herein, an IED (such as IED  60 ) may refer to any microprocessor-based device that monitors, controls, automates, and/or protects monitored equipment within the electric power system  20 . Such devices may include, for example, remote terminal units, differential relays, distance relays, directional relays, feeder relays, overcurrent relays, voltage regulator controls, voltage relays, breaker failure relays, generator relays, motor relays, automation controllers, bay controllers, meters, recloser controls, communications processors, computing platforms, programmable logic controllers (PLCs), programmable automation controllers, input and output modules, and the like. The term IED may be used to describe an individual IED or a system comprising multiple IEDs. 
     The IED  60  may monitor electrical parameters of the power line  40 . For example, the IED  60  may include a current transformer (CT)  62  that monitors the current of the power line  40  and/or a potential transformer (PT) that monitors the voltage of the power line  40 , which may be used in protection operations of the electric power system  20 . 
     The IED  60  may include a processor  80 , memory  82 , a communication interface  84 , a display screen  86 , input circuitry  88 , and output circuitry  90 , which may be communicatively coupled to each other via one or more communication buses  92 . The memory  82  may be a non-transitory computer readable medium for storing data and executable instructions. Such programs or instructions executed by the processor  80  may be stored in any suitable article of manufacture that includes one or more tangible, computer-readable media at least collectively storing the instructions or routines described below. In some embodiments, the input circuitry  88 , the memory  82  and/or the processor  80  may be implemented as hardware components, such as via discrete electrical components, via a field programmable gate array (FPGA), and/or via one or more application specific integrated circuits (ASICs) and may be referred to generally as processing circuitry  94 . Further, the instructions or routines may be provided to the processor  80  to produce a machine, such that the instructions, when executed by the processor  80 , implement the operations/acts specified in the flowchart described below with respect to  FIG. 2 . 
     The IED  60  may include a display screen  86  that allows a user to view information on the IED  60 , such as fault information (e.g., polarizing quantities, fault voltages, fault currents, etc.). In some embodiments, the display screen  86  may be a touchscreen display. 
     The input circuitry  88  may include a one or more inputs  100 , such as input ports and/or pins operably connected to sensors, such as the current transformer(s)  62  and/or the potential transformer(s)  64  to communicate electrical properties of the power line  40  to the IED  60 , as described above. For example, the current transformer  62  may provide an analog signal representing (e.g., proportional to) current of the power line  40  via the inputs  100 . Further, the potential transformer  64  may provide an analog signal representing (e.g., proportional to) the voltage of the power line  40  to the IED  60  via the inputs  100 . The input circuitry  88  may include analog-to-digital converter(s) (ADC)  96  that may provide digitized samples of the current and voltage to the processor  80  to allow for monitoring and protection operations by the processor  80 . The input circuitry  88  may include potential transformers  98  that may reduce the voltages received to voltages that may be sampled by the ADC  96 . In other embodiments, merging units may provide digital voltage measurements or current measurements. While these are given as examples, note that any suitable inputs may be used in monitoring the power line  40 . While the inputs are shown with sensors of a single line for simplicity, in embodiments described below, each phase of a multi-phase system, such as a three-phase system, may be monitored via respective CTs and PTs on each of the phases. 
     In the illustrated embodiment, the circuit breaker  42  is communicatively coupled to the IED  60  via the output  90 . For example, the processor  80  may send a trip signal via the output  90  to open the circuit breaker  42 , thereby disconnecting the low-inertia power source  24  and the bus  30  from the remaining electric power system  20 . 
     As mentioned above, the IED  60  may have distance and directional elements in protection of the power line  40  of the electric power system  20 . For example, the IED  60  may use a distance that a fault occurred to determine a zone associated with the fault (e.g., zone  1  or zone  2 ), which may be used in coordinating tripping of the circuit breaker  42 . In this example, the IED  60  may respond to a fault (e.g., trip circuit breaker  42 ) located in zone  1  faster than a fault located in zone  2  to allow for more localized protection in zone  2  to respond. Directional overcurrent protection may use directional elements to supervise the operation of overcurrent elements. 
     The IED  60  may use a polarizing quantity, such as a polarizing voltage or polarizing current, in the distance and directional elements. As a polarizing voltage, measurements of voltages from unfaulted phases or properly rotated and scaled vectors of positive-sequence voltages may be used. However, a potential misoperation caused by self- and cross-polarizations may occur when a fault is close to a relay and the measured voltage is low (e.g., almost zero). As such, there is a need for accurate and fast tracking of voltage parameters and providing a smooth polarizing signal with proper inertia to reduce or eliminate misoperations. 
       FIG. 2  is a functional block diagram of a process  120  that may be performed by the IED  60 , in accordance with an embodiment. For example, the process  120  may be performed by the processor  80  of the IED  60  executing instructions (e.g., code) stored in memory  82  of the IED  60 . As mentioned above, in some embodiments, some or all of the functions may be performed using discrete hardware (e.g., logic circuits, integrated circuits, etc.) of the input circuitry  88  to perform the functional blocks illustrated. 
     The IED  60  may receive input signals indicating the real parts of voltage measurements V A , V B , and V C  of the A-phase, B-phase, and C-phase of the power system via respective potential transformers  64  on each phase of the power line  40 . The process  120  includes a modified synchronous reference frame phase-locked loop (SRF-PLL)  122 , part adding additional inertia  124 , and block providing reference signals  126 . The SRF-PLL  122  includes an abc/dq0 transform  130 , an adaptive notch filter  134 , a PI controller  136 , and an integrator  138 . A delay  140 , after the integrator  138 , is included to avoid an algebraic loop. 
     At dq0 transform block  130 , the A-phase, B-phase, and C-phase voltage measurements may be transformed into dq0 components signals for the voltage measurements of the power line  40 . In the SRF-PLL  122 , the q-component signal results from Park&#39;s transform and is described by equation (1): 
                     q   =         -     2   3       ⁢   sin   ⁢           ⁢       (     ω   ⁢           ⁢   t     )     ·     V     A   ⁢   a         ⁢   cos   ⁢           ⁢     (         ω   S     ⁢   t     +     ϕ   A       )       -       2   3     ⁢       sin   ⁡     (       ω   ⁢           ⁢   t     -       2   ⁢   π     3       )       ·     V     B   ⁢   a         ⁢           ⁢   cos   ⁢           ⁢     (         ω   S     ⁢   t     -       2   ⁢   π     3     +     ϕ   B       )       -     2   3         ⁢     
     ⁢     sin   ⁢           ⁢       (       ω   ⁢           ⁢   t     +       2   ⁢   π     3       )     ·     V     C   ⁢   a         ⁢           ⁢     cos   ⁡     (         ω   S     ⁢   t     +       2   ⁢   π     3     +     ϕ   C       )                 Eq   .           ⁢   1               
where V xa  and ϕ x  are the amplitude of voltage and phase angle between measured voltages and Park&#39;s transform of A, B, and C phases, ω s  and ω are the system and estimated angular frequency respectively, and t is the time at which the measurements were taken. By controlling the estimated angular frequency ω, the control loop attempts to find a stable point where the error signal is zero. At this point, ω and ω s  are equal and phase differences ϕ are zero.
 
     In some SRF-PLLs, the q-component signal may be provided directly to the PI controller, which may be sufficient for symmetrical and balanced input signals. Such may be the case for inverters, motor control, etc. However, protection elements on IEDs  60 , such as relays, may receive asymmetrical and/or unbalanced input signals during abnormal conditions, such as during fault conditions. If the SRF-PLL is synchronized with the system such that ω s =ω, equation (1) may be transformed to equation (2) using trigonometric properties. The signal q may be divided into a static DC component (q DC ) and an oscillating AC component (q AC ) that oscillates with a double system angular frequency 2ω. 
     
       
         
           
             
               
                 
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     In a stable state, synchronization with balanced symmetrical voltages V A , V B , and V C , the value of q may be zero. During transient states, caused by changes of parameters of phase voltages, the value of q may vary. Influence of changes of voltage parameters on q may be estimated using equation (2). Rapid change of phase ϕ, by the same value in three phases, cause transient increases in q DC  and q AC . Increased q DC  starts operation of the control loop and ensures re-synchronization (q DC =q AC =0). Change of the amplitude in any of the three-phases, asymmetry, causes an oscillating component q AC  appearance. 
     As illustrated in  FIG. 2 , a notch filter, such as the adaptive notch filter  134 , may be included to compensate for the q AC  component. Notch filters may provide steeper characteristics, smaller delays, and larger attenuation of selected frequency over low pass filters, which may be desirable in power system protection. Further, in power systems with inverter-based resources, the frequency may change rapidly and have values different from nominal. The adaptive notch filter  134  may receive an adaptation signal  142  from the PI controller  136  that may be used to provide an adjusted q-component signal to the PI controller  136  corresponding to the frequency. The adaptive notch filter  134  may be embodied as an infinite impulse response (IIR) IIR Butterworth 2 nd  order low pass filter (LPF) using transformation equation (3). 
                     s   →     B   ⁢         z   2     -   1         z   2     -     2   ⁢   A   ⁢   z     +   1       ⁢           ⁢   where       ⁢     
     ⁢       A   =       cos   ⁢           ⁢     (     2   ⁢     π   ·   2   ·     f   i     ·     t   s         )         cos   ⁢           ⁢     (     π   ·   bnd   ·     t   s       )           ;     B   =     tan   ⁢           ⁢     (     π   ·   bnd   ·     t   s       )                   Eq   .           ⁢   3               
where f i  is the input frequency from the integral component of the PI controller  136  (e.g., adaptation signal  142 ), t s  is the sampling period, bnd is the width of the stopband. That is, the adaptive notch filters  132  and  134  may adjust filter coefficients of the d-component and q-component signals based on the adaptation signal  142 , output form the integral component of PI controller  136 , corresponding to the system frequency to allow the adaptive notch filters  132  and  134  to reduce the AC components in the d and q signals. Discrete transfer function of the adaptive notch filter  134  for f i =60 Hz and bnd=40 Hz is described according to equation (4):
 
     
       
         
           
             
               
                 
                   
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     In the illustrated embodiment, the modified SRF-PLL  122  is shown as being separated from the inclusion of the additional inertia to ensure accurate operation of the SRF-PLL  122  and to allow for modification of the additional inertia without affecting the remaining portions of the algorithm. 
       FIG. 3  illustrates a plot  160  of the q-component signal for step changes in A-phase amplitude for a 62 Hz signal from an SRF-PLL without a notch filter (signal  162 ), with a notch filter (signal  164 ), and with an adaptive notch filter (signal  166 ).  FIG. 4  shows a plot  180  of signals  182 ,  184 , and  186  of the difference angles between the estimated ω and the actual system ω s  for the step change of  FIG. 3 . As shown in  FIG. 3 , a step change in the A-phase amplitude causes the signal  162  without the filter to include a q AC  component oscillating with a double angular frequency 2ω, which causes an oscillation error of ω estimation in signal  182 . 
     As illustrated, systems that use a notch filter prior to the PI controller may have a filtered q-component signal  164  with relatively reduced oscillations compared to the signal  162 , which result in a more accurate estimated ω signal  184 . Further, systems that use an adaptive notch filter  134  may further reduce q-component signal oscillations by using the estimation of frequency f i  in equation (3). As the integral component I corresponds to frequency estimation, the PI controller  136  may provide I as feedback for an indication of f i  in equation (3) via the adaptation signal  142 . The plot  160  includes the signal  166  of the q-component signal output by the adaptive notch filter  134 , which may result in a difference angle with smaller differences between the estimated ω and the actual system ω s , as indicated by the signal  186 . The integral and proportional signals may be integrated  138  and delayed, at delay  140 , to obtain a more reliable estimate of ωt. The estimated ωt may then be used as a feedback signal, from the delay  140 , as an input to the abc/dq0 transform  130  to allow for close the control loop and more accurate subsequent estimations of ωt. Further, the estimated wt may be separately provided from delay  140  to be used in part adding additional inertia. 
     Additional inertia may be added separately to the d and q components and the ω estimation. When the SRF-PLL  122  is synchronized with the power system angular frequency ω s , and voltages of the three phases are balanced, the d and q components may have DC components without an AC component, according to equations (2) and (5). Equation (5) was obtained using Park&#39;s transform and similar simplifications as equation (2). 
                   d   =         d     D   ⁢   C       +     d     A   ⁢   C         =         1   3     ⁢     (         V     A   ⁢   a       ⁢           ⁢     cos   ⁡     (     ϕ   A     )         +       V     B   ⁢   a       ⁢           ⁢   cos   ⁢           ⁢     (     ϕ   B     )       +       V     C   ⁢   a       ⁢   cos   ⁢           ⁢     (     ϕ   A     )         )       +       1   3     ⁢     (         V     A   ⁢   a       ⁢   cos   ⁢           ⁢     (       2   ⁢   ω   ⁢           ⁢   t     +     ϕ   A       )       +       V     B   ⁢   a       ⁢           ⁢       cos   ⁡     (       2   ⁢   ω   ⁢           ⁢   t     +       2   ⁢   π     3     +     ϕ   B       )       ++     ⁢     V     C   ⁢   a       ⁢           ⁢     cos   ⁡     (       2   ⁢   ω   ⁢           ⁢   t     -       2   ⁢   π     3     +     ϕ   C       )           )                   Eq   .           ⁢   5               
Where d DC  is the DC component of the d signal, d AC  is the AC component of the d signal, and the remaining variables are described above with respect to equation (1). During unbalanced conditions and transient states, d and q may have also AC components.
 
     In the illustrated embodiment, the d AC  component may be reduced or eliminated by the adaptive notch filter  132 . However, transient oscillations of d and q outputs from the adaptive notch filters may be caused by step changes of angle ϕ. 
       FIG. 5  illustrates a plot of d-component and q-component signals before and after the low-pass filters (LPFs)  200  and  202  of  FIG. 2  for a step change of A-phase amplitude from 1 to 0 and a step change of angle ϕ A , ϕ B , and ϕ C  of 90 degrees. Referring back to  FIG. 2 , the adaptive notch filters  132  and  134  may output d and q signals  194  and  196  that include transient oscillations due to step changes in amplitude and/or angle. 
     To reduce or eliminate the transient oscillations and add inertia to d and q estimations, the additional inertia block may include IIR LPFs  200  and  202  that receive the d signal  194  and the q signal  196  and provide a filtered d-component signal (d′)  210  and a filtered q-component signal (q′)  212 . For example, the IIR LPFs  200  and  202  may be described with equation (6): 
                         G     L   ⁢   P   ⁢   F       ⁡     (   z   )       =       1   -   C       z   -   C         ;     C   =     exp   ⁢           ⁢     (       -     t   S       ⁢     /     ⁢     T   del       )                 Eq   .           ⁢   6               
where t s  is the sampling period and T del  is the time delay.
 
     The SRF-PLL  122  tracks system angular frequency accurately with small inertia due to the PI controller  136 . To add inertia to the ω estimation, settings of the PI controller  136  may be changed, however, this may affect operation of the SRF-PLL  122 . Therefore, inertia to the estimated ω may be added separately using an arcsin function  230 , a low-pass filter LPF  232 , and a saturation function  234  in the additional inertia block  124 . If the double frequency part q AC  in equation (2) is reduced or removed by the adaptive notch filter  134  and the voltages are symmetrical, the angle difference between the system angular frequency ω s  and the estimated angular frequency ω can be described by equation (7). 
                   ϕ   =     arcsin   ⁡     (       3   ·   q         V   A     +     V   B     +     V   C         )               Eq   .           ⁢   7               
where V A , V B , and V C  are the amplitudes of voltages of the A-phase, B-phase, and C-phase, and q is the q-component from the dq0 transform. Due to fast operation of the SRF-PLL  122 , the estimation of ϕ using equation (7) may have an accurate value for a short time and then it may rapidly change, due to change of q-component  196  of  FIG. 5 . Therefore, to keep the accurate q value and remove oscillations, the low-pass filter LPF  232  with a properly set time delay and gain may be used. Further, a saturation block to π/2 may be used to ensure limiting the added delay from the LPF  232  to 90 degrees.
 
       FIG. 6  is a plot of the set system ω  242 , ω  244  estimated by SRF-PLL  122 , and delayed ω′  246  for a step change of angle ϕ by 90 degrees. As illustrated, at time t 0 , the set system ω  242  has a step change of angle ϕ by 90 degrees. The estimated ω without a delay may adjust to the step change as shown. The delayed estimated ω′  246  may be more resistant to change due to the additional inertia from the delay. That is, by adding delay to the estimated ω, the delayed estimated ω′ may respond slower to the step change to represent inertia of generators. 
     By including inertia in the estimated ω separately, changes to inertia may be changed according to the application, for example, via user input of the IED  60 . Further, by adding inertia separately, the inertia added to ω does not deteriorate operation of the SRF-PLL  122  that tracks system ω s . Inertia in delayed ω′ may be present during transients cause by changes of input voltage signal parameters, and may have values reflecting the size and sign of changes. 
     Referring to  FIG. 2 , reference signals, such as V′ A , V′ B , and V′ C  and/or α, β, and 0, may be created using estimation of the d′-component  210  and q′-component signals  212  and the estimated delayed angular frequency ω′  246 . In the illustrated embodiment, the 0-component from the abc/dq0 transform  130  is provided to transforms  250  and  252 . While two transforms used to generate reference signals are illustrated, note that one, two, or more reference signals may be used. Estimations of d′ and q′ are delayed DC components—double frequency parts are eliminated using the adaptive notch filters  132  and  134 . For stable conditions, synchronization of the PLL (e.g., q=0), the d component reflects magnitude of positive sequence symmetrical component vector V 1  of input voltages. In such a case, α and β components from the dq0/αβ0 transform  252  correspond to real and imaginary parts of V 1 . Additionally, using the dq0/abc transform  250 , smooth three phase voltage V′ A , V′ B , and V′ C  reference signals, with amplitude equal to V 1 , can be generated. Using the process  120 , real and imaginary parts of V 1  that corresponds to balanced and symmetrical voltages, for unbalanced and unsymmetrical input voltages, can be created. 
     As mentioned above, the reference signals from the transforms  250  and  252  may be used in distance and direction elements in IEDs, such as protective relays. For example, the IED  60  may determine a positive sequence voltage from the reference signals to be used as a polarizing voltage. By using a polarizing voltage that more accurately tracks the estimated angular frequency and that includes additional inertia, distance and direction elements of the IED may be more reliable during asymmetric or transient conditions on the power system. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure. 
     The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as “means for [perform]ing [a function] . . . ” or “step for [perform]ing [a function] . . . ”, it is intended that such elements are to be interpreted under 35 U.S.C. 112(f). However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under 35 U.S.C. 112(f).