Abstract:
A device and method is disclosed for providing electrical grid-tied power converter anti-islanding protection, which is autonomous to the operation of the power converter. The device can protect any number of paralleled power converters and can independently detect an islanding condition. The method involves actively perturbing the electrical grid at a common point of coupling between converter and grid, observing the effect of the perturbation, comparing the perturbed and non-perturbed grid characteristics and disconnecting the converter from the grid if an island is indicated. Perturbation is accomplished by switching a reactive load or loads onto the grid or by changing a transformer tap. 
     The invention is targeted for solar photovoltaic applications where a number of distributed power converters are used. The invention eliminates the interaction of anti-islanding algorithms between multiple power converters, determines islanding conditions with a greater degree of certainty and reduces current distortion over prior-art anti-islanding methods.

Description:
BACKGROUND OF THE INVENTION 
     All regulatory-code-compliant, utility-grid-interactive power converters must operate according to grid-connect standards for a given jurisdiction. In the United States, the governing standard is IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, commonly known as IEEE 1547. In short, this standard specifies that distributed resources, such a solar photovoltaic power converters, must provide a minimum power quality, must not operate outside of set utility voltage and frequency limits and must detect an islanding condition and automatically cease power production. 
     Islanding or run-on is an abnormal operating condition where a grid-interactive power converter continues to supply power to grid-connected loads following the loss of the utility grid source. In simple terms, all grid-interactive power converters are current sources, whereas the grid source, typically the electric utility grid, is a voltage source. The power converters, the grid source and grid-connected loads are connected at a point commonly referred to as the point of common coupling (PCC). All anti-islanding detection algorithms use passive and active methods. The passive method involves monitoring the voltage and frequency at the PCC and checking for voltages or frequencies outside of specified limits. The passive method alone will not detect an island under balanced conditions where available power out of the converter matches the power into grid-connected loads. All active methods assume a worst-case balanced condition and periodically and purposefully attempt to disturb a balanced condition to force the voltage or frequency at the PCC past passive detection trip points. Almost all prior art disturbance methods are integrated with the power converter control systems and work by periodically jogging the amplitude or phase of the power converter sinusoidal current references to force a detectable change at the PCC. If the grid voltage source is present, purposefully jogging or distorting the power converter output current will not affect the voltage or frequency at the PCC. If however, the grid voltage source is lost, and therefore is not controlling the voltage at the PCC, then sufficient changes in power converter current can result in detectable changes in voltage or frequency across the grid-connected loads. IEEE 1547 requires that an island condition be detected and that the power converter either be shutdown or disconnected from the PCC within two seconds after formation of an island. As such, under all conditions where the power converter is sourcing power at the PCC, the power converter current is typically shifted in phase or amplitude for at least one cycle every second to “test” for an island. 
     Prior art approaches to anti-islanding protection have a number of basic limitations. (i) Distortion—Ideally, the power converter would source perfect, low-harmonic-distortion sinewave currents into the grid voltage source at the PCC. Prior art methods rely on purposefully distorting the converter output current, typically for at least one cycle every second, as part of the island detection algorithm. (ii) Interaction—When multiple power converters are connected at the PCC, interactions between individual power converter anti-islanding algorithms may cause nuisance tripping or in the other extreme may cause an island to go undetected. (iii) Non Autonomous Operation—Almost all prior art methods use the power converter control as an integral part of the anti-islanding algorithm. (iv) Absolute Trip Levels—The magnitude of grid disturbance required with prior art methods must be great enough force the grid past absolute under/over, voltage/frequency trip points. (v) Cost—In prior art systems with a large number of distributed DC-to-AC power converters, each inverter is required to have an integrated anti-islanding protection system. 
     BRIEF SUMMARY OF THE INVENTION 
     A device and method is disclosed for providing anti-islanding protection for an electrical grid-tied power converter, which is autonomous to the operation of the power converter. The device can protect any number of paralleled power converters and can independently determine if an islanding condition exists. The method involves actively perturbing the electrical grid at a common point of coupling between converter and grid, observing the effect of the perturbation, comparing the perturbed and non-perturbed grid characteristics and disconnecting the converter from the grid if a run-on or islanding condition is indicated. Perturbation is accomplished switching a reactive load or loads onto the grid or by changing a transformer tap connection at the point of common coupling. 
     The invention is targeted for use in solar photovoltaic power plants which have a number of distributed DC-to-AC power converters. The invention eliminates the interaction of anti-islanding algorithms between multiple power converters, determines islanding conditions with a greater degree of certainty, reduces current distortion over prior art anti-islanding methods and provides a least-cost anti-islanding solution for systems with distributed power converters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an application of the invention as a protective anti-islanding device in a grid-interactive power system. 
         FIG. 2  illustrates the preferred embodiment of the invention where, in order to perturb the system, reactive loads are periodically switched onto the point of common coupling between a power converter, an electric utility grid source and grid-connected loads. 
         FIG. 3  illustrates a second embodiment of the invention where, in order to perturb the system, a transformer tap is periodically switched at the point of common coupling between a power converter, an electric utility grid source and grid-connected loads. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows how the invention is applied in a system. The system comprises a number of distributed photovoltaic power sources each with dedicated DC-to-AC power converters,  1  through N, protective anti-islanding device (the invention)  2 , distribution transformer  3 , grid-connected loads  4 , AC grid source  5  and switch  6 . The invention further comprises isolation contactor  7  and perturb and observe circuit block  8 . Under normal operating conditions, isolation contactor  7  and switch  6  are closed and the net power transfer into grid source  5  is the combined output of DC to AC power converters,  1  through N, less the power consumed by grid-connected loads  4 . An islanding fault condition is established when switch  6  is opened (functionally representing the loss grid source  5 ) and the power available from photovoltaic-to-AC power converters,  1  through N, matches the power consumed by grid-connected loads  4 . An island or run-on condition may exist indefinitely as long as this source and load balance is maintained. The perturb and observe circuit block  8  periodically attempts to disturb the voltage, phase, frequency or power transfer at the point of common coupling  9 , observes the effect of the perturbation, compares perturbed and non-perturbed grid characteristics and disconnects the converter, by opening contactor  7  if a run-on or island condition is indicated. Perturbation is accomplished by switching a reactive load or loads onto the grid, by changing a transformer tap connection or by shorting a number of transformer winding turns at the point of common coupling  9 . 
       FIG. 2  illustrates the preferred embodiment of the invention where, in order to perturb the system, reactive loads  41  and  42  are periodically switched onto the point of common coupling (with respect to power transfer) between power converter  30 , electric utility grid source  90  and grid-connected loads  80 . In this embodiment, the invention is protective device  40  and comprises all the elements circumscribed by the illustrated boundary line. The input connections of device  40  and the output connections of power converter  30  are designated as terminals  31 ,  32  and  33 . Power converter  30  may be one power converter or many power converters connected in parallel, all regulating 3-phase current. When isolation contactor  55  is closed, this 3-phase current is sourced into the low voltage terminals  71 ,  72  and  73  of distribution transformer  70  and in turn is sourced from high voltage terminals  91 ,  92  and  93  of distribution transformer  70  into utility grid voltage source  90 . The aggregate power converter 3-phase output current is synchronized with the 3-phase utility source voltage to enable power transfer into utility source  90 . Grid-connected loads  80  are shown across terminals  91 ,  92  and  93 . An island is formed when utility source  90  is lost and power is only supplied from power converter  30  to loads  80 . 
     In  FIG. 2 , during normal operation of the system, isolation contactor  55  is closed, control circuit  60  periodically issues commands  66  and  67  to close switches  56  and  57  respectively. When switch  56  is closed, capacitive load  41  is connected across terminals  71  and  74 , low voltage line  1 -to-neutral. When switch  57  is closed, inductive load  42  is connected across terminals  72  and  74 , low voltage line  2 -to-neutral. Differential voltage sensors  51 ,  52  and  53  sense low voltage line  1 ,  2  and  3  voltages, respectively, all with respect to neutral and provide analog input signals  61 ,  62  and  63 , respectively, for control circuit  60 . Summing amplifier  54  adds signals  61 ,  62  and  63  and supplies the result as signal  64  for control circuit  60 . With switches  56  and  57  open and with a perfect 3-phase voltage balance on terminals  71 ,  72  and  73 , signal  64  will be zero. In a typical system, signal  64  will indicate some baseline level of imbalance. Control circuit  60  reads signal  64  for a number of line-frequency cycles and stores averaged values, peak values, waveforms, phase shift and/or frequency data which characterize the non-perturbed grid. When either switch  56  or  57  is closed or both are closed, signals  61 ,  62 ,  63  and  64  are read (observed) and this perturbed grid data is compared to the non-perturbed baseline data to determine if an island exists. By way of example, an average baseline RMS value of signal  64  is calculated and stored, switches  56  and  57  are closed and the RMS value signal  64  is again read and compared with the stored baseline value, the addition of capacitive load  41  and inductive load  42 , imbalance the system and therefore cause the RMS value of signal  64  to increase. If this increase is above a predetermined trip point, then an island is indicated. By way of a second example, the time difference between a non-perturbed voltage zero crossing of a given phase-to-phase or phase-to-neutral voltage is compared with that of a perturbed zero-voltage crossing. If a phase relationship is significantly changed, then an island is indicated. In practice, a number different data comparisons will be made to conclusively detect an island. Reactive loads are used as opposed to resistive loads so that power is not dissipated in the perturbation process. Also, if capacitive load  41  and inductive load  42  have equal but opposite reactive impedances, then net system power factor is maintained. Power quality is also maintained by timing the open and close transitions of switch  56  with line  1 -to-neutral voltage zero crossings and in a similar way by synchronizing the transitions of switch  57  with line  2  zero-current crossings. In practice switches  56  and  57  may be bi-directional blocking semiconductor devices, such as back-to-back (emitter-to-emitter or collector-to-collector) insulated gate bipolar transistors. 
       FIG. 3  illustrates a second embodiment of the invention where, in order to perturb the system, a transformer tap is periodically switched at the point of common coupling (with respect to power transfer) between power converter  30 , electric utility grid source  90  and grid-connected loads  80 . In this embodiment, the invention is protective device  40  and comprises all the elements circumscribed by the illustrated boundary line. The input connections of device  40  and the output connections of power converter  30  are designated as terminals  31 ,  32  and  33 . Power converter  30  may be one power converter or many power converters connected in parallel, all regulating 3-phase current. When isolation contactor  55  is closed, this 3-phase current is sourced into the low voltage terminals  71 ,  72  and  73  of distribution transformer  70  and in turn is sourced from high voltage terminals  91 ,  92  and  93  of distribution transformer  70  into utility grid voltage source  90 . The aggregate power converter 3-phase output current is synchronized with the 3-phase utility source voltage to enable power transfer into utility source  90 . Grid-connected loads  80  are shown across terminals  91 ,  92  and  93 . An island is formed when utility source  90  is lost and power is only supplied from power converter  30  to loads  80 . 
     In  FIG. 3 , during normal operation of the system, isolation contactor  55  is closed, control circuit  60  periodically issues command  69  to change a voltage tap from terminal  72 A to  72 B to perturb the system. Differential voltage sensors  51 ,  52  and  53  sense low voltage line  1 ,  2  and  3  voltages, respectively, all with respect to neutral and provide analog signals  61 ,  62  and  63  respectively for control circuit  60 . Current sensor  58  reads line  2  current and provides analog signal  68  for control circuit  60 . Control circuit  60  reads signals  61 ,  62 ,  63  and  68  for a number of line-frequency cycles and stores averaged values, peak values, waveforms, phase shift and/or frequency data which characterize the non-perturbed grid. When command  69  is given to change the tap switch, the terminal  32  circuit changes from terminal  72 A, the balanced tap, to terminal  72 B and signals  61 ,  62 ,  63  and  68  are read (observed) and this perturbed grid data is compared to the non-perturbed baseline data. If grid  90  is present and an island does not exist, the line  2 -to-neutral voltage and the power into line  2  will be reduced by the turns ratio of the tap change. In addition, the voltage at transformer neutral point  74 , calculated by control circuit as the sum of signals  61 ,  62  and  63 , will become imbalanced by a known amount. If an island does exist, the line  2 -to-neutral voltage will be the reflected voltage of the current through load  80  rescaled by the perturbed turns ration and the delta-to-wye transform. With power converter  30  sourcing an island, the line  2 -to-neutral voltage and the power into line  2  will be reduced by an amount approximately twice that of power converter  30  sourcing grid  90 . During the short perturbation period, the power converter current is constant because the change in energy out of the power converter is small compared with the DC buss energy storage of any known power converter topology. Power quality is maintained by timing the open and close transitions of switch  59  with line  2  current zero crossings. In practice, switch  59  may use bi-directional blocking semiconductor devices, such as back-to-back (emitter-to-emitter or collector-to-collector) insulated gate bipolar transistors to configure the single-pole-double-throw switch  59 . 
     In  FIG. 3  one method is disclosed to perturb the system by using a transformer tap change. Other methods may include, providing a single-pole-three-throw switch function with nominal+, nominal and nominal-voltage taps, so that perturbation events would alternate between momentarily increasing power and decreasing power. Another method may include a multi-tier perturbation where the power is perturbed by successively large power change increments if island detection is indeterminate after testing with smaller incremental power changes. Another method may include tap change capability on all three phases, used in rotation to force a higher or lower frequency or relative phase shift at the point of common coupling. Another method may include, a controllable switch to momentarily open one low-voltage winding of a delta-configured transformer. Another method may include a controllable switch used to short-circuit a small number of turns on a low-voltage transformer winding. All methods in this paragraph are considered novel and are part of the invention. 
     The invention is targeted for use in solar photovoltaic power plants which have a number of distributed DC-to-AC power converters. The invention eliminates the interaction of anti-islanding algorithms between multiple power converters, determines islanding conditions with a greater degree of certainty by using adaptive and relative as well as absolute trip points, reduces current distortion over prior art anti-islanding methods and provides a least-cost anti-islanding solution for systems with distributed power converters.