Abstract:
Method for detecting an islanding condition of a grid connected energy conversion system and related DC/AC converter apparatus, adapted to optimize performance in terms of preservation of the power quality, provide synchronized perturbation for all the inverters of a plant, provide shut down capability within the time requested by utilities and safety standards and provide immunity to grid frequency fluctuations.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to power conversion and generation systems, in particular, the present invention relates to methods for detecting islanding conditions in power conversion and generation systems adapted to be connected directly to the power grid. 
       STATE OF THE ART 
       [0002]    The present invention concerns a method for detecting islanding of DC/AC converters operating in grid connected mode and related DC/AC converter apparatus. 
         [0003]    Grid connected power conversion and generation systems are adapted to generate electrical power for direct delivery to the power grid, in particular converting electrical power generated by renewable sources, such as, for instance, photovoltaic panels and wind turbines. Said power conversion and generation systems usually comprise an inverter apparatus adapted to convert a DC input voltage into an AC output voltage characterized by amplitude and frequency as required by the power grid specifications. Said inverters are generally adapted to be connected directly to the power grid and typically comprise a semiconductor H-bridge circuit working as a DC-AC converter. 
         [0004]    Grid-connected power generation units need to comply with safety standards and regulations to be certified safe to connect to the public power grid. In particular, grid-connected systems need to be safe against islanding, the condition in which the power generation unit continues to power a location even though electrical grid power from the electric utility is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent automatic re-connection of devices. For that reason, power generation units must be adapted to detect islanding conditions and immediately stop producing power. Power generation units need therefore to be provided with anti-islanding capabilities that force the disconnection of the output of the DC/AC converter from the AC mains in case the mains voltage is missing. 
         [0005]    This requirement can be fulfilled if the inverter of the power generation unit is provided with means to detect islanding and is connected to the grid via a suitable automatic switching circuit, preventing unintentional connection to a sub-grid or to a stand-alone grid and therefore preventing islanding conditions to happen. 
         [0006]    Today there are various methods available to detect islanding and avoid non-safe operation in islanding mode for grid connected power systems applications, said methods being generally divided in passive and active methods. 
         [0007]    Passive methods monitor parameters such as voltage and frequency, the grid voltage change rate or the grid frequency change rate and prevent the supplying of power from the inverter to the grid if the detected working conditions differ from the nominal ones. Examples of state of the art passive methods are based on the detection and analysis of voltage phase jump, rate of change of frequency, rate of change of voltage, current harmonics. 
         [0008]    Active methods generally introduce disturbances to the connected circuit and then monitor the response to determine if the utility grid is still connected; some active methods are based on the communications between the inverter and the utility and are adapted to stop the inverter from supplying power to the grid when emergency occurs. 
         [0009]    Examples of state of the art active methods are based on detection and analysis of impedance, frequency shift, frequency drift, Variation of Active Power and/or Reactive Power, Frequency Jump, Sandia Frequency Shift, Sandia Voltage Shift. Active methods are generally preferred by the inverter manufacturers because have lower NDZ (Non Detection Zones) and faster detection times compared to passive methods. 
         [0010]    State of the art methods for detecting island conditions during inverter operation are generally affected by drawbacks concerning the immunity to frequency variations when grid connected, the stability of the system, the preservation of the power quality, the synchronization of the perturbation when there are multiple inverters in a plant. 
         [0011]    In detail, the operation of one of the most common state of the art methods for detecting island conditions, based on the periodic injection of reactive power, is hereby described. This method is based on the idea of attempting to shift the frequency of the voltage at the point of common coupling (PCC), V PCC , by injecting reactive power on a time interval. 
         [0012]    When the grid utility is connected, the frequency will not shift because the reactive power in the system is such that: 
         [0000]    
       
      
       Q 
       load 
       =Q 
       inv 
       +ΔQ  
      
     
         [0013]    Thus, when Q inv  changes, ΔQ changes accordingly in order to satisfy the reactive power Q load  requested by the load. 
         [0014]    When grid is disconnected, ΔQ is not available anymore to balance the load request, therefore: 
         [0000]      Q load =Q inv    
         [0015]    The only way to preserve the balance condition when Q inv  changes is therefore a frequency change since: 
         [0000]    
       
         
           
             
               Q 
               load 
             
             = 
             
               
                 
                   ( 
                   
                     V 
                     PCC 
                   
                   ) 
                 
                 2 
               
               · 
               
                 ( 
                 
                   
                     1 
                     
                       ω 
                        
                       
                           
                       
                        
                       L 
                     
                   
                   - 
                   
                     ω 
                      
                     
                         
                     
                      
                     C 
                   
                 
                 ) 
               
             
           
         
       
     
         [0016]    Thus, whenever the inverter detects a frequency shift in response to an injection of reactive power an island condition is detected. 
         [0017]    The above method is characterized by a few drawbacks namely the lack of immunity to frequency variations when grid is connected, the synchronization between multiple connected inverters and the high level of perturbation introduced in the system. 
         [0018]    Other state of the art methods for detecting island conditions during inverter operation make use of positive feedback control loops in order to start a frequency drift in a test signal. These methods usually show stability problems for instance when a weak utility is connected to the inverter, then possible instability in the power output can cause transients in the system and the positive feedback control loop to detect islanding can amplify and increase that instability. 
         [0019]    It is therefore an object of the present invention to introduce a new method for the reliable detection and management of islanding conditions in grid connected inverters to overcome the state-of-the-art methods drawbacks. 
         [0020]    In particular, it is an object of the present invention to introduce a new method, a related new AC/DC converter and a new power generation system comprising said AC/DC converter, for the reliable detection and management of islanding conditions regarding multiple grid connected inverters of the same plant. 
         [0021]    The method according to the present invention is further adapted to optimize performance in terms of preservation of the power quality; provide synchronized perturbation for all the inverters of a plant; provide inverter shut down capability within the time requested by utilities and safety standards; provide immunity to grid frequency fluctuations. 
     
    
     
         [0022]    Further features and advantages of the present invention will be apparent in the following description of a non-limitative embodiment with reference to the figures in the accompanying drawings, which are diagrammatic and show functional blocks which are adapted to be made according to different circuitry solutions in practice. 
           [0023]    In detail: 
           [0024]      FIG. 1  shows a block diagram of a grid connected DC/AC converter with a controller provided with an anti-islanding module according to the present invention; 
           [0025]      FIG. 2  shows a block diagram of an anti-islanding module according to the present invention; 
           [0026]      FIG. 3  shows the waveforms related to the PLL output in normal operation and after a perturbation is applied to the PLL; 
           [0027]      FIG. 4  shows a flowchart of the method for detecting islanding in grid connected power generation systems according to the present invention; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 1  shows a generic block diagram of a grid connected single-phase or multi-phase inverter comprising an energy generating system  10 —for instance a photovoltaic generator or a wind generator—and a DC/AC converter. 
         [0029]    The DC/AC converter generally comprises an input DC/DC converter  11 , a cascaded DC/AC inverter  12  and a control module  14  comprising, for instance, a current controller  22 , a voltage controller  23  and a PWM modulator  24 . 
         [0030]    The input DC/DC converter  11  is adapted to boost the output direct voltage of said energy generating system  10  to the predetermined voltage level. In case the energy generating system  10  is a photovoltaic generator, said input DC/DC converter  11  may be further adapted to track the maximum power point from the photovoltaic generator. In this case, the DC/DC converter  11  serves to perform the maximum power point tracking (MPPT) from the output of the photovoltaic generator varying according to the insolation and temperature and extract the maximum power to store it in a capacitor bank  15  at the DC-Link end, output of said DC/DC converter. 
         [0031]    The control of the DC-Link end voltage is performed by said control module  14 . When the maximum power of the photovoltaic generator is extracted from the DC/DC converter  11 , the DC-Link end voltage is boosted and the DC/AC inverter injects the energy stored in the DC-Link end into the grid  16 . 
         [0032]    The DC/AC inverter is adapted to convert direct voltage from the DC/DC converter  11  into alternating voltage to supply the inverter current to the grid  16 . 
         [0033]    Thus, the DC/AC converter turns the voltage from the energy generating system  10  into an alternating (AC) voltage synchronized with the grid phase and supplies it to the grid  16 . 
         [0034]    The control module  14  generally comprises an inner current loop adapted to control the grid current and an outer voltage loop adapted to control the dc link voltage. 
         [0035]    In general terms, the current loop may be adapted to regulate the active current I D  and the reactive current I Q  to the values I D   _   ref  and I Q   _   ref , the voltage loop is adapted to regulate the power flow balancing within the system. 
         [0036]    Said control module  14  comprises a block adapted to track the grid frequency, for instance a phase locked loop (PLL)  17 . 
         [0037]    In inverters provided with anti-islanding capability, said control module  14  further comprises an anti-islanding module  18 . 
         [0038]    Anti-islanding methods based on frequency bias are adapted to modify the waveform of the tracking signal of the PLL or the generic block adapted to track the grid phase or the grid frequency. 
         [0039]    Assuming the block adapted to track the grid phase or the grid frequency to be a PLL  17 , the enclosed  FIG. 3  shows the waveform of the PLL tracking signal. When the PLL tracking signal is added with some perturbation, the waveform of the current injected by the inverter into the grid is distorted so that the new signal zero crossing point would be before or after the zero crossing point of the actual grid signal, while the period of the waveform is left unaltered by said perturbation. 
         [0040]    In  FIG. 3  the distorted waveform shows a higher frequency with respect to the actual grid frequency picked up at the point of common coupling (PCC voltage), same considerations would apply in case the chosen perturbation of the PLL tracking signal is such as forcing the frequency to be lower than the nominal one. The distorted waveform ends each half-cycle remaining at zero amplitude for the time t z  before starting the next half-cycle. 
         [0041]    The distorted waveform may as well have a lower frequency than the PCC voltage frequency. In this latter case, when the PCC waveform is zero, the distorted waveform is zero as well. 
         [0042]    Same considerations apply in case a generic block adapted to track the grid phase or the grid frequency is employed in place of a PLL. 
         [0043]    When the utility grid is connected, the utility grid opposes any change in the grid voltage frequency. If the inverter tries to change the frequency of its output current, the utility grid keeps on providing a solid phase and frequency reference to the PLL thus stabilizing the operating point at the frequency of the grid voltage and no positive feedback within the PLL control loop occurs. 
         [0044]    On the opposite, during island condition, the frequency of the AC voltage at the PCC, V PCC , will change due to the perturbation injected by the inverter. The inverter output voltage will then follow the injected distorted current waveform and cross the zero amplitude axis in a time (T PCC /2−tz) which is different from nominal half-cycle T PCC /2. This applies also to the next half-cycle and the point of zero crossing of voltage V PCC  shifts accordingly. Then, the frequency of the PLL output signal shifts as well in order to remain in sync with the grid voltage frequency and therefore the next perturbation is applied on the PLL signal with altered frequency, thus producing a positive feedback. As a result, the PCC AC voltage, V PCC , undergoes a frequency drift, the detection of which can be used to determine an ongoing island condition. 
         [0045]    With reference to the enclosed  FIGS. 1 and 2 , the anti-islanding module  18  according to the present invention comprises three sub-modules:
       a signal injector module  19 , associated to said PLL  17 , comprising means  19   a  for generating a perturbation for the reference signal of said PLL  17 , adapted to shift the frequency of the AC output voltage of said DC/AC converter, and means  19   b  for providing said reference signal comprising said perturbation to said PLL  17  in order to modify the PLL output waveform in a way that, during an island condition, the frequency of the voltage at the PCC shifts with respect to the nominal value;       
 
         [0047]    A frequency extractor module  20 , associated to said signal injector module  19  and comprising means for measuring the frequency and the frequency change of the AC voltage at the point of common coupling between said DC/AC converter and the grid; 
         [0048]    An island detector module  21 , associated to said signal injector module  19  and comprising means for generating an alarm signal and changing the operation of said DC/AC converter in order to avoid islanding, when the frequency of the AC voltage at the point of common coupling is greater than a predetermined threshold. In greater detail, said signal injector module  19  applies a perturbation to the PLL tracking signal, that, in turn, affects the current loop control signal. Said perturbation makes the zero crossing point of the PLL waveform to be before or after the zero crossing point of the actual grid signal as if the frequency is higher or lower than the actual grid signal frequency at the PCC. Then, when the waveform reaches zero, it remains at zero for time t z  before the beginning of the successive half cycle. 
         [0049]    With reference to enclosed  FIG. 1 , said frequency extractor module  20  operates a frequency measurement on the AC voltage at the PCC obtaining frequency samples F N . In one exemplary embodiment of the present invention, said frequency measurement can be implemented through AC voltage zero crossing detection and following measurement of the time interval between two consecutive zero crossings. Measuring the frequency of the AC voltage at the point of common coupling between said DC/AC converter and the grid through detection of AC voltage zero crossing provides automatic synchronization to all the AC/DC converters belonging to the same plant and sharing the same grid. 
         [0050]    To make the measurement more accurate and evaluate the trend of the frequency of the AC voltage at the PCC, said frequency extractor module  20  further calculates the short moving average of a small number of frequency samples F N , for instance a number smaller than 10 samples, and the long moving average of a large number of frequency samples F N , for instance a number larger than 50 samples, respectively. 
         [0051]    The short moving average of a small number of the last frequency samples F N , is adapted to filter the frequency measurements F N  from noise, EMI and, in general, from inaccuracy of the measurement. The long moving average of a large number of last samples is adapted to sense the actual trend of the frequency measurement in order to speed up the detection of an islanding condition and the managing of the detected alarm. 
         [0052]    Said frequency extractor module  20  provides the calculated values to said signal injector module  19 . 
         [0053]    As explained earlier, said signal injector module  19  is adapted to apply a perturbation to the PLL tracking signal that, in turn, affects the current loop control signal. Advantageously, said signal injector module  19  can apply a reference signal to the PLL—or to the generic grid voltage phase tracker or grid voltage frequency tracker—which is increased of a predetermined quantity. Therefore, the reference signal fed by said signal injector module  19  to the PLL drives the current control loop of said control module  14  so that the inverter  12  produces an output frequency ω 0 +Δω 0  , wherein Δ 0  is the nominal grid frequency. 
         [0054]    During normal operation, when the DC/AC converter is connected to the grid and the grid voltage is present, the grid opposes any change in the grid voltage frequency. When said control module  14  drives the inverter to try and modify the frequency of its output current, the grid keeps on providing a constant phase and frequency reference to the PLL thus stabilizing the operating point at the frequency of the grid voltage and no positive feedback within said anti-islanding module  18  occurs. 
         [0055]    On the opposite, during islanding conditions, when the DC/AC converter is connected to the grid but the grid voltage is not present, the frequency of the AC voltage at the PCC, V PCC , will change due to the perturbation injected by said signal injector module  19  to the PLL tracking signal. Thus, a positive feedback establishes within the inverter current control loop: the PLL and the frequency extractor module  20  detect a PCC voltage frequency higher than ω 0  and said signal injector module  19  keeps on applying a perturbation to the PLL tracking signal until the frequency reaches a predetermined threshold, indicating an ongoing emergency, and the DC/AC converter is switched off accordingly, thus avoiding islanding. 
         [0056]    The perturbation applied by said signal injector module  19  to the PLL tracking signal is preferably calculated based on the past and present values of the frequency of the voltage at the node PCC, said past and present values of the frequency of the voltage at the node PCC being provided by said frequency extractor module  20 . 
         [0057]    In case the difference between said past and present values of the frequency of the AC voltage at the node PCC is larger than a predetermined threshold, then the next perturbation applied by said signal injector module  19  is increased, by an additional term Δω, to Δω 0 +Δω, so that in case of islanding, the inverter working frequency grows fast until a predetermined alarm threshold is reached and the system is shut down and made safe. 
         [0058]    In greater detail, in a preferred embodiment of the present invention, said frequency extractor module  20  periodically measures the frequency of the AC voltage at the PCC through the measurement of the period of the AC voltage at the PCC, that is the time interval between two consecutive zero crossings, and calculates the long moving average of frequency samples F N , and the short moving average of frequency samples F N , and finally provides the measured and calculated values to said signal injector module  19 . 
         [0059]    Said signal injector module  19  is adapted to apply a perturbation to the PLL tracking signal, that, in turn, affects the current loop control signal. 
         [0060]    The perturbation applied comprises always a constant term, Δω 0 , and may comprise an additional variable term, Δω. The constant term, Δω 0 , is aimed at triggering the frequency drift in case a loss of mains emergency occurs. 
         [0061]    Said constant term Δω 0  is chosen to be the smallest possible in order to have a negligible effect on the THD and on the cos φ of said DC/AC converter output while being enough to trigger the drift of the frequency of said DC/AC converter, according to the present invention. Furthermore, Said constant term Δω 0  can be chosen positive (adapted to cause a frequency increase) or negative (adapted to cause a frequency decrease). 
         [0062]    The additional variable term, Δω, is calculated based on the trend of the frequency measurements performed by said frequency extractor module  20  and it is aimed at speeding up the discovery of the ongoing islanding condition and the following inverter shutdown. 
         [0063]    In a preferred embodiment of the present invention said signal injector module  19  calculates said additional variable term Δω as follows: 
         [0000]      Δω=k⊖F L  wherein
 
         [0064]    k can be a constant value or an adaptive value, k=f(ΔF L ), and 
         [0000]      Δ F   L   =F   N   −F   AVGL ,
 
         [0065]    F N  being the frequency measurement performed at time N, or at step N, and F AVGL  being the moving average of the last large number of frequency measurements F N , said large number being, for instance, larger than 50. 
         [0066]    Said signal injector module  19  applies said additional variable term, Δω, to the perturbation to the PLL tracking signal if at least one of the following conditions is true:
       1) ΔF L &gt;T1 and F AVGS &lt;F MAX  (in case the applied perturbation is positive, therefore forcing a positive frequency drift);   2) ΔF L &lt;−T1 and F AVGS &gt;F MIN  (in case the applied perturbation is negative, therefore forcing a negative frequency drift);       
 
         [0069]    wherein F AVGS  is the moving average of the last small number of frequency measurements F N , said small number being, for instance, equal to 8, F MAX  is the maximum acceptable value of the PCC voltage frequency and F MIN  is the minimum acceptable value of the PCC voltage frequency. 
         [0070]    If none of the above conditions 1) and 2) is true, said additional variable term, Δω, is not applied to the perturbation to the PLL tracking signal. 
         [0071]    If one or more of the above conditions 1) and 2) is verified true due to a faulty measurement or due to a brown-out, then Δω is applied to the perturbation to the PLL tracking signal only for a limited time without reaching the alarm threshold that triggers the inverter shut-down. In this case, after the faulty measurement or the brown-out condition is no longer present, signal injector module  19  operates in a way to decrease said additional variable term, Δω, to zero in a predetermined time interval or in a predetermined number of cycles. 
         [0072]    One example of possible decreasing procedure of said additional variable term, Δω, is the following wherein Δω is decreased according to the formula: 
         [0000]      Δω (step  n )=0.99*Δω (step  n− 1) with a certain time constant, for instance equal to a few seconds.
 
         [0073]    Furthermore, if F AVGS &gt;F MAX  or F AVGS &lt;F MIN , then said signal injector module  19  does not increase said additional variable term, Δω, anymore, even in case of ongoing islanding condition. 
         [0074]    In a preferred embodiment of the present invention, in order to speed up the detection of islanding conditions to be compliant with the safety normative concerning the matter, said constant value k, such as Δω=kΔF L , may have two different values: a first value, k1 is applied when the frequency drift is in the starting phase, a second value, k2&gt;k1 is applied to speed up the frequency drift in order to make the detection of the islanding condition fast enough. 
         [0075]    In greater detail, said signal injector module  19  checks the term ΔF L =F N −F AVGL . 
         [0076]    If ΔF L  is greater than a first threshold T1 (ΔF L &gt;T1 or ΔF L &lt;−T1) then k=k1. 
         [0077]    If ΔF L &gt;T1 (or ΔF L &lt;−T1) constantly within a predetermined time interval and ΔF L &gt;T2 (or ΔF L &lt;−T2), then k=k2. 
         [0078]    The above checks of the value of ΔF L  are to establish if ΔF L  is really increasing and there is no reading errors or jitters in the detected zero crossing of the amplitude of the AC voltage. 
         [0079]    In a further preferred embodiment of the present invention, said constant term Δω 0  is chosen alternatively positive for a certain number n of cycles and negative for a certain number m of cycles with n and m being integer numbers, until a frequency drift is detected, then the sign of said constant term Δω 0  is kept constant and equal to the sign said constant term Δω 0  had when the frequency drift started. 
         [0080]    In situations where a plurality of DC/AC converters belong to the same plant and share the same grid, they might have different loads or different point of operation and input power. As a result, said DC/AC converters might react different to the same perturbation Δω 0  possibly cancelling out the overall effect that should be detected on the frequency of the AC voltage at the PCC. In case this happens, an ongoing islanding condition might not be detected correctly. 
         [0081]    Applying the above described sign shift to said constant term Δω 0  it is possible to avoid the possibility of the undesired cancelling effect described above. Advantageously, said number of cycles n and m can be chosen to be different integer numbers and they can be further chosen to be different prime integer numbers. 
         [0082]    Said island detector module  21  is associated to said frequency extractor module  20 , and it is adapted to detect the ongoing islanding condition and signal the alarm condition in case of islanding detection. In case the frequency of the PCC voltage is detected to be drifting, said island detector module  21  changes the operation of the inverter  12  thus guaranteeing the safety of the converter. 
         [0083]    In a preferred embodiment of the present invention, said island detector module  21  checks the PCC voltage actual frequency against a predetermined alarm threshold F T  and signals an emergency due to islanding if the detected frequency is greater (or smaller) than said threshold F T . 
         [0084]    In another preferred embodiment of the present invention, said island detector module  21  checks the PCC voltage frequency against a predetermined alarm threshold F T  and signals an emergency due to islanding if the detected frequency is greater (or smaller) than said threshold for a predetermined number of consecutive measurements. 
         [0085]    Furthermore, said island detector module  21  is adapted to signal an emergency due to islanding within a predetermined time interval in compliance with the safety normative that applies. 
         [0086]    According to the description above and with reference to enclosed  FIG. 4 , the anti-islanding module  18  of the present invention performs a method for detecting islanding in grid connected power generation systems wherein the following steps are taken: 
         [0087]    a) measuring  40  the frequency of the AC voltage at the point of common coupling between said DC/AC converter and the grid; 
         [0088]    b) generating  41  a perturbation (Δω 0 ) for the reference signal of said means for tracking the grid frequency, in order to shift the frequency of the AC output voltage of said DC/AC converter; 
         [0089]    c) providing  42  said means for tracking the grid frequency with a reference signal comprising said perturbation (Δω 0 , Δω 0 +Δω), 
         [0090]    d) measuring  43  the change in the frequency of the AC voltage at the point of common coupling; 
         [0091]    e) if  44  the change in the frequency of the AC voltage at the point of common coupling is smaller than a first predetermined threshold then repeat the preceding steps starting from step b); if  44  the change in the frequency of the AC voltage at the point of common coupling is greater than a first predetermined threshold: 
         [0092]    f) if  45  the frequency of the AC voltage at the point of common coupling is greater than a second predetermined threshold then changing  47  the operation of said DC/AC converter in order to avoid islanding; 
         [0093]    g) if  45  the frequency of the AC voltage at the point of common coupling is smaller than a second predetermined threshold, then calculating  46  a new additional term (Δω) and generating a perturbation (Δω 0 +Δω), for the reference signal of said means for tracking the grid frequency, comprising said additional term (Δω) and then repeat from step c). 
         [0094]    Preferably, said measuring the frequency of the AC voltage at the point of common coupling between said DC/AC converter and the grid of previous steps a) and d) is performed through detection of AC voltage zero crossing. 
         [0095]    The apparatus and method for detecting islanding in grid connected power generation systems according to the present invention introduces several advantages with respect to the state-of-the-art: 
         [0096]    In terms of automatic synchronization, measuring the frequency of the AC voltage at the point of common coupling between said DC/AC converter and the grid through detection of AC voltage zero crossing, provides automatic synchronization of the perturbation provided to all the converters installed in the same plant, since all the inverters in a plant share the same grid. This way the zero-crossing time of the grid voltage represents the intrinsic synchronization event of the apparatus and method according to the present invention. 
         [0097]    In terms of immunity to grid frequency variations, the present method and apparatus are adapted to detect islanding after starting a frequency drift and checking the PCC frequency against thresholds comprising a maximum and a minimum frequency thresholds, thus achieving greater immunity to grid frequency variations if compared with other state-of-the-art methods that apply a periodic perturbation and then measure variations in the PCC frequency. 
         [0098]    In terms of stability and power quality, the present method and apparatus are adapted to apply to the PLL, or equivalent device within the control module of the converter, a fixed perturbation that is so small that power quality is not compromised. The fixed perturbation is kept very small because it just serves to trigger the frequency drift for positive feedback. Moreover, the present invention provides safety and stability in case a weak utility grid is connected. In fact, the instability at the power output of the converter will not cause undesirable transients in the converter, thanks to the control of the perturbation according to the present invention that checks the PCC frequency against a preliminary frequency threshold and checks the number of consecutive cycles of frequency drift in the same direction. 
         [0099]    In case the control module  14  comprises a pcontroller or a DSP, said signal injector module  19 , said frequency extractor module  20 , said island detector module  21  and the method according to the present invention can be implemented within the firmware of said pcontroller or DSP.