Islanding Detection Methods | BCJ Controls | Grid Protection

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Islanding Detection Methods

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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.

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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.

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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.

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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
  Hertz.
• 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.

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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
  network.

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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.

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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.

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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
  cycles)
• 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

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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.

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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.

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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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• BCJ Controls offers excellent services and
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