Islanding Detection MethodsSUBTITLE

Islanding Detection Methods

▪ These can be classified into passive methods, which look forevents on the grid, and active methods, testing the networkfrom the inverter or the grid distribution point. There arealso methods that the utility can use to detect the conditionsthat would deliberately upset those conditions in order topower down the inverters. These methods are summarizedbelow.

Passive and Active Methods

▪ Passive methods include any system that detects anomaliesto usual network condition, indicating the need to disconnect.

▪ Active methods attempt to detect a network failure byinjecting small signals into the line, and then detectingwhether or not the signal reading changes.

Under/Over Voltage

▪ Under/over voltage detection is normally simple to implementin mains connected inverters, because the basic function ofthe inverter is to mimic the grid conditions, including voltage.That means that all mains connected inverters have thehardware required to detect the changes. All that is needed isa program to detect sudden changes. However, suddenchanges in voltage are common on the grid as high loads areattached and removed, so a limit must be used to avoidnuisance tripping.

Under/Over Frequency

▪ The frequency of the power to the grid is critical to thefunctionality of any mains powered devices, as all inductiveloads are calibrated to run at a nominal frequency of 50Hertz.

▪ Over and under frequency conditions can cause circulatingovercurrent faults between sources of supply, leading toequipment damage and injury from overheating and possiblefires.

▪ Unlike variations in voltage, it is unlikely that a randomcircuit would have a frequency the same as the grid. Mostmodern generating devices and inverters sync to the networkfrequency.

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.

Voltage Phase Jump Detection

▪ Loads generally have power factors that are lagging, meaningthat they do not accept the voltage from the grid perfectly,but impede it slightly. Grid-tie inverters are usually set tohave power factors of unity. This will change in phase whenthe network fails, which we can use to detect islanding.

▪ Inverters track the phase of the grid by tracking when thesignal crosses zero volts, varying the current output to thecircuit to produce the proper voltage waveform. When thegrid disconnects, the power factor suddenly shifts from thegrid’s unity compared to the load’s not quite unity. As thecircuit is still driving a current that would produce a smoothvoltage output given the known loads, this will result in asudden change in voltage. By the time the waveform iscompleted 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 thepractically infinite capacity of the grid that filters theseevents out. Inverters though, generally have much largerdistortions, as much as 5% THD. This is due to theirconstruction; some THD is a side-effect of the switched-modepower supply circuits most inverters are based on.

▪ So when the grid disconnects, the THD of the local circuit willincrease to that of the inverters themselves. This provides avery secure method of detecting islanding, because there aregenerally no other sources of THD that would equal theinverter. Also, interactions within the inverters, notably thetransformers, have non-linear effects that produce unique2nd and 3rd harmonics that are easily measured.

Negative-Sequence Current Injection

▪ This is an active islanding detection method which can beused by three-phase inverters. The method is based oninjecting a negative-sequence current and detecting andquantifying the corresponding negative-sequence voltage atthe point of common coupling. The negative-sequence currentinjection method:

▪ detects an islanding event within 60 ms (3 cycles)

▪ requires 2% to 3% negative-sequence current injection forislanding detection;

▪ can correctly detect an islanding event for the grid short circuitratio of 2 or higher

Impedance Measurement

▪ Impedance Measurement measures the overall impedance of thecircuit being fed by the inverter. It does this by slightly increasingthe current amplitude, presenting too much current at a giventime. Usually this would have no effect on the measured voltage, asthe extra current is soaked up by the grid. In the event of adisconnection, even the small increase would result in a noticeablerise in voltage, allowing detection of the island.

▪ The main advantage of this method is that it has a very small NDZfor any given single inverter. However, in the case of multipleinverters, each one would be increasing a slightly different signalinto the line, hiding the effects on any one inverter. It is possible toaddress this problem by communication between the inverters toensure they all increase on the same schedule.

Slip Mode Frequency Shift

▪ This is one of the newest methods of islanding detection. It is basedon changing the phase of the inverter’s output to be slightly mis-aligned with the grid, with the expectation that the grid willoverwhelm this signal. The system relies on the actions of a finelytuned phase-locked loop to become unstable when the grid does notoverwhelm the signal; in this case, the PLL attempts to adjust thesignal back to itself, which is programmed to continue to drift. Inthe case of grid loss, the system will drift away from the designfrequency, causing the inverter to shut down.

▪ The good thing is that it can be done using existing hardware thatis already in the inverter. The disadvantage is that it needs theinverter to always be slightly out of sync with the grid, a loweredpower factor. Generally speaking, the system has a very small NDZand will quickly disconnect.

Frequency Bias

▪ Frequency bias produces a slightly off-frequency signal intothe grid, but resets this at the end of every cycle by jumpingback into phase when the voltage passes zero. This is similarto Slip Mode, but the power factor remains closer to the grid,and resets itself every cycle. Moreover, the signal is less likelyto be filtered out.

▪ There are numerous variations to this basic scheme. TheFrequency Jump version, also known as the “zebra method”,inserts forcing only on a specific number of cycles in a setpattern. This reduces the possibility that external circuitsmay filter out the signal.

BCJ Controls

▪ At BCJ Controls we produce Secondary ProtectionSwitchboards with solar protection relays for CommercialSolar over 30kW grid connections.

▪ BCJ Controls offers excellent services and quality instrumentin Electric Power Grid. Such as, testing and commissioning,installation and quality products like protection relay. GoodQuality we offer to our valued customer in Australia.

BCJ Controls

Need Specialist?

Testing and Commissioning

Products? High Quality?

Contact us:

(02) 9534 8465

2,17 Lorraine St

Peakhurst NSW 2210

Visit us for more about Islanding:www.bcjcontrols.com.au