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Islanding Detection Methods | BCJ Controls | Grid Protection


Islanding Detection can be classified into passive methods, which look for events on the grid, and active methods, testing the network from the inverter or the grid distribution point. There are also methods that the utility can use to detect the conditions that would deliberately upset those conditions in order to power down the inverters. – PowerPoint PPT presentation

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Title: Islanding Detection Methods | BCJ Controls | Grid Protection

Islanding Detection Methods
  • subtitle

Islanding Detection Methods
  • These can be classified into passive methods,
    which look for events on the grid, and active
    methods, testing the network from the inverter or
    the grid distribution point. There are also
    methods that the utility can use to detect the
    conditions that would deliberately upset those
    conditions in order to power down the inverters.
    These methods are summarized below.

Passive and Active Methods
  • Passive methods include any system that detects
    anomalies to usual network condition, indicating
    the need to disconnect.
  • Active methods attempt to detect a network
    failure by injecting small signals into the line,
    and then detecting whether or not the signal
    reading changes.

Under/Over Voltage
  • Under/over voltage detection is normally simple
    to implement in mains connected inverters,
    because the basic function of the inverter is to
    mimic the grid conditions, including voltage.
    That means that all mains connected inverters
    have the hardware required to detect the changes.
    All that is needed is a program to detect sudden
    changes. However, sudden changes in voltage are
    common on the grid as high loads are attached and
    removed, so a limit must be used to avoid
    nuisance tripping.

Under/Over Frequency
  • The frequency of the power to the grid is
    critical to the functionality of any mains
    powered devices, as all inductive loads are
    calibrated to run at a nominal frequency of 50
  • Over and under frequency conditions can cause
    circulating overcurrent faults between sources of
    supply, leading to equipment damage and injury
    from overheating and possible fires.
  • Unlike variations in voltage, it is unlikely that
    a random circuit would have a frequency the same
    as the grid. Most modern generating devices and
    inverters sync to the network frequency.

Rate of Change of Frequency
  • Rate of change of frequency is given by the
    following expression
  • where
  • f   is the system frequency,
  • t   is the time,
  • ?P   is the power imbalance,
  • G   is the system capacity, and
  • H   is the system inertia.
  • If the rate of change of frequency, or ROCOF
    value, becomes greater than a certain value, the
    embedded generation will disconnect from the

Voltage Phase Jump Detection
  • Loads generally have power factors that are
    lagging, meaning that they do not accept the
    voltage from the grid perfectly, but impede it
    slightly. Grid-tie inverters are usually set to
    have power factors of unity. This will change in
    phase when the network fails, which we can use to
    detect islanding.
  • Inverters track the phase of the grid by tracking
    when the signal crosses zero volts, varying the
    current output to the circuit to produce the
    proper voltage waveform. When the grid
    disconnects, the power factor suddenly shifts
    from the grids unity compared to the loads not
    quite unity. As the circuit is still driving a
    current that would produce a smooth voltage
    output given the known loads, this will result in
    a sudden change in voltage. By the time the
    waveform is completed and returns to zero, it
    will be out of phase.

Harmonics Detection
  • Even with noisy sources, the total harmonic
    distortion (THD) of a grid-tied circuit is
    generally immeasurable due to the practically
    infinite capacity of the grid that filters these
    events out. Inverters though, generally have much
    larger distortions, as much as 5 THD. This is
    due to their construction some THD is a
    side-effect of the switched-mode power supply
    circuits most inverters are based on.
  • So when the grid disconnects, the THD of the
    local circuit will increase to that of the
    inverters themselves. This provides a very secure
    method of detecting islanding, because there are
    generally no other sources of THD that would
    equal the inverter. Also, interactions within the
    inverters, notably the transformers, have
    non-linear effects that produce unique 2nd and
    3rd harmonics that are easily measured.

Negative-Sequence Current Injection
  • This is an active islanding detection method
    which can be used by three-phase inverters. The
    method is based on injecting a negative-sequence
    current and detecting and quantifying the
    corresponding negative-sequence voltage at the
    point of common coupling. The negative-sequence
    current injection method
  • detects an islanding event within 60 ms (3
  • requires 2 to 3 negative-sequence current
    injection for islanding detection
  • can correctly detect an islanding event for the
    grid short circuit ratio of 2 or higher

Impedance Measurement
  • Impedance Measurement measures the overall
    impedance of the circuit being fed by the
    inverter. It does this by slightly increasing the
    current amplitude, presenting too much current at
    a given time. Usually this would have no effect
    on the measured voltage, as the extra current is
    soaked up by the grid. In the event of a
    disconnection, even the small increase would
    result in a noticeable rise in voltage, allowing
    detection of the island.
  • The main advantage of this method is that it has
    a very small NDZ for any given single inverter.
    However, in the case of multiple inverters, each
    one would be increasing a slightly different
    signal into the line, hiding the effects on any
    one inverter. It is possible to address this
    problem by communication between the inverters to
    ensure they all increase on the same schedule.

Slip Mode Frequency Shift
  • This is one of the newest methods of islanding
    detection. It is based on changing the phase of
    the inverters output to be slightly mis-aligned
    with the grid, with the expectation that the grid
    will overwhelm this signal. The system relies on
    the actions of a finely tuned phase-locked loop
    to become unstable when the grid does not
    overwhelm the signal in this case, the PLL
    attempts to adjust the signal back to itself,
    which is programmed to continue to drift. In the
    case of grid loss, the system will drift away
    from the design frequency, causing the inverter
    to shut down.
  • The good thing is that it can be done using
    existing hardware that is already in the
    inverter. The disadvantage is that it needs the
    inverter to always be slightly out of sync with
    the grid, a lowered power factor. Generally
    speaking, the system has a very small NDZ and
    will quickly disconnect.

Frequency Bias
  • Frequency bias produces a slightly off-frequency
    signal into the grid, but resets this at the end
    of every cycle by jumping back into phase when
    the voltage passes zero. This is similar to Slip
    Mode, but the power factor remains closer to the
    grid, and resets itself every cycle. Moreover,
    the signal is less likely to be filtered out.
  • There are numerous variations to this basic
    scheme. The Frequency Jump version, also known as
    the zebra method, inserts forcing only on a
    specific number of cycles in a set pattern. This
    reduces the possibility that external circuits
    may filter out the signal.

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