Islanding basics
Electrical inverters are devices that convertQuestionable rationale
Given the activity in the field, and the large variety of methods that have been developed to detect islanding, it is important to consider whether or not the problem actually demands the amount of effort being expended. Generally speaking, the reasons for anti-islanding are given as (in no particular order): # Safety concerns: if an island forms, repair crews may be faced with unexpected live wires # End-user equipment damage: customer equipment could theoretically be damaged if operating parameters differ greatly from the norm. In this case, the utility is liable for the damage. # Ending the failure: Reclosing the circuit onto an active island may cause problems with the utility's equipment, or cause automatic reclosing systems to fail to notice the problem. # Inverter confusion: Reclosing onto an active island may cause confusion among the inverters. The first issue has been widely dismissed by many in the power industry. Line workers are already constantly exposed to unexpectedly live wires in the course of normal events (i.e. is a house blacked out because it has no power, or because the occupant pulled the main breaker inside?). Normal operating procedures under hot-line rules or dead-line rules require line workers to test for power as a matter of course, and it has been calculated that active islands would add a negligible risk. However, other emergency workers may not have time to do a line check, and these issues have been extensively explored using risk-analysis tools. A UK-based study concluded that "The risk of electric shock associated with islanding of PV systems under worst-case PV penetration scenarios to both network operators and customers is typically <10−9 per year." The second possibility is also considered extremely remote. In addition to thresholds that are designed to operate ''quickly'', islanding detection systems also have absolute thresholds that will trip long before conditions are reached that could cause end-user equipment damage. It is, generally, the last two issues that cause the most concern among utilities.Intentional islanding for backup power
Because of the greatly increased use of Public Safety Power Shutoff (PSPS) and other power grid shutdowns by utilities, the need for backup and emergency power for homes and businesses has greatly increased over the past several years. For example, some shutdowns by California utility PG&E have lasted for days as PG&E attempts to prevent wildfires from starting during dry and windy climate conditions. To fill this need to backup grid power, solar power systems with battery backup and islanding inverters are finding heavily increased demand by home and business owners. During normal operation when grid power is present, the inverters can grid tie to feed power provided by solar panels to the loads in the home or business, and thereby lower the amount of power which is consumed from the utility. If there is extra power available from the solar panels it can be used to charge batteries and/or feed power into the grid to in effect sell power to the utility. This operation can reduce the cost of power that the owner has to purchase from the utility and help offset the purchase and installation costs of the solar power system. Modern inverters can automatically grid tie when grid power is present, and when grid power is lost or not of acceptable quality these inverters operate in conjunction with a transfer switch to isolate the home or business electrical system from the grid and the inverter supplies power to that system in an island mode. While most homes or businesses can present a larger load than the inverter is capable of supplying, load shedding is accomplished by varying the frequency of the A.C. power output from the inverter (only in island mode) in response to the load on the inverter in a fashion such that the A.C. power frequency represents that loading. Load modules installed in the power feed to large loads like air conditioners and electric ovens measure the A.C. power frequency from the islanding inverter and disconnect those loads in a priority sequence as the inverter nears its maximum power output capability. For example, when the inverter power output is below 50% of inverter's maximum output capability, the A.C. power frequency is maintained at the standard frequency (e.g. 60 Hz) but as the power output increases above 50%, the frequency is lowered linearly by up to 2 Hz (e.g. from 60 Hz to 58 Hz) when the inverter output reaches its maximum power output. Because of the ease and accuracy of inverter A.C. power frequency control in islanding mode, this frequency control is an inexpensive and effective way of conveying the inverter loading to every corner of the electrical system it powers. A load module for a low priority load will measure this power frequency and if the frequency is lowered by 1 Hz or greater for example (e.g. lower than 59 Hz) the load module disconnects its load. Several load modules, each of which operates at a different frequency based on the priority of its load, can operate to keep the total load on the inverter below its maximum capability. These islanding inverter solar power systems allow all loads to potentially be powered, just not all at the same time. These systems provide a green, reliable and cost effective backup power alternative to internal combustion engine powered generators. The islanding inverter systems operate automatically when grid power fails to ensure that critical electric loads like lighting, fans for building heating systems and food storage devices continue to operate throughout the outage, even if nobody is present in the business or the home occupants are sleeping.Islanding detection methods
Detecting an islanding condition is the subject of considerable research. In general, these can be classified into passive methods, which look for transient events on the grid, and active methods, which probe the grid by sending signals of some sort from the inverter or the grid distribution point. There are also methods that the utility can use to detect the conditions that would cause the inverter-based methods to fail, and deliberately upset those conditions in order to make the inverters switch off.Passive methods
Passive methods include any system that attempts to detect transient changes on the grid, and use that information as the basis as a probabilistic determination of whether or not the grid has failed, or some other condition has resulted in a temporary change.Under/over voltage
According to Ohm's law, the voltage in an electrical circuit is a function of electric current (the supply of electrons) and the applied load (resistance). In the case of a grid interruption, the current being supplied by the local source is unlikely to match the load so perfectly as to be able to maintain a constant voltage. A system that periodically samples voltage and looks for sudden changes can be used to detect a fault condition.Bower & Ropp, pg. 17 Under/over voltage detection is normally trivial to implement in grid-interactive inverters, because the basic function of the inverter is to match the grid conditions, including voltage. That means that all grid-interactive inverters, by necessity, have the circuitry needed to ''detect'' the changes. All that is needed is an algorithm to detect sudden changes. However, sudden changes in voltage are a common occurrence on the grid as loads are attached and removed, so a threshold must be used to avoid false disconnections.Bower & Ropp, pg. 18 The range of conditions that result in non-detection with this method may be large, and these systems are generally used along with other detection systems.Bower & Ropp, pg. 19Under/over frequency
The frequency of the power being delivered to the grid is a function of the supply, one that the inverters carefully match. When the grid source is lost, the frequency of the power would fall to the natural resonant frequency of the circuits in the island. Looking for changes in this frequency, like voltage, is easy to implement using already required functionality, and for this reason almost all inverters also look for fault conditions using this method as well. Unlike changes in voltage, it is generally considered highly unlikely that a random circuit would naturally have a natural frequency the same as the grid power. However, many devices deliberately synchronize to the grid frequency, like televisions. Motors, in particular, may be able to provide a signal that is within the NDZ for some time as they "wind down". The combination of voltage and frequency shifts still results in a NDZ that is not considered adequate by all.Bower & Ropp, pg. 20Rate of change of frequency
In order to decrease the time in which an island is detected, rate of change of frequency has been adopted as a detection method. The rate of change of frequency is given by the following expression: where is the system frequency, is the time, is the power imbalance (), is the system capacity, and is the system inertia. Should the rate of change of frequency, or ROCOF value, be greater than a certain value, the embedded generation will be disconnected from the network.Voltage phase jump detection
Loads generally have power factors that are not perfect, meaning that they do not accept the voltage from the grid perfectly, but impede it slightly. Grid-tie inverters, by definition, have power factors of 1. This can lead to changes in phase when the grid fails, which can be used to detect islanding. Inverters generally track the phase of the grid signal using aHarmonics detection
Even with noisy sources, like motors, theActive methods
Active methods generally attempt to detect a grid failure by injecting small signals into the line, and then detecting whether or not the signal changes.Negative-sequence current injection
This method is an active islanding detection method which can be used by three-phase electronically coupled distributed generation (DG) units. The method is based on injecting a negative-sequence current through the voltage-sourced converter (VSC) controller and detecting and quantifying the corresponding negative-sequence voltage at the point of common coupling (PCC) of the VSC by means of a unified three-phase signal processor (UTSP). The UTSP system is an enhanced phase-locked loop (PLL) which provides a high degree of immunity to noise, and thus enables islanding detection based on injecting a small negative-sequence current. The negative-sequence current is injected by a negative-sequence controller which is adopted as the complementary of the conventional VSC current controller. The negative-sequence current injection method detects an islanding event within 60 ms (3.5 cycles) under UL1741 test conditions, 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, and is insensitive to variations of the load parameters of UL1741 test system.Impedance measurement
Impedance Measurement attempts to measure the overall impedance of the circuit being fed by the inverter. It does this by slightly "forcing" the current amplitude through the AC cycle, presenting too much current at a given time. Normally this would have no effect on the measured voltage, as the grid is an effectively infinitely stiff voltage source. In the event of a disconnection, even the small forcing would result in a noticeable change in voltage, allowing detection of the island.Bower & Ropp, pg. 24 The main advantage of this method is that it has a vanishingly small NDZ for any given single inverter. However, the inverse is also the main weakness of this method; in the case of multiple inverters, each one would be forcing 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 force on the same schedule, but in a non-homogeneous install (multiple installations on a single branch) this becomes difficult or impossible in practice. Additionally, the method only works if the grid is effectively infinite, and in practice many real-world grid connections do not sufficiently meet this criterion.Impedance measurement at a specific frequency
Although the methodology is similar to Impedance Measurement, this method, also known as "harmonic amplitude jump", is actually closer to Harmonics Detection. In this case, the inverter deliberately introduces harmonics at a given frequency, and as in the case of Impedance Measurement, expects the signal from the grid to overwhelm it until the grid fails. Like Harmonics Detection, the signal may be filtered out by real-world circuits.Bower & Ropp, pg. 26Slip mode frequency shift
This is one of the newest methods of islanding detection, and in theory, one of the best. It is based on forcing the phase of the inverter's 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 signal is missing; in this case, the PLL attempts to adjust the signal back to itself, which is tuned to continue to drift. In the case of grid failure, the system will quickly drift away from the design frequency, eventually causing the inverter to shut down.Bower & Ropp, pg. 28 The major advantage of this approach is that it can be implemented using circuitry that is already present in the inverter. The main disadvantage is that it requires the inverter to always be slightly out of time with the grid, a lowered power factor. Generally speaking, the system has a vanishingly small NDZ and will quickly disconnect, but it is known that there are some loads that will react to offset the detection.Frequency bias
Frequency bias forces a slightly off-frequency signal into the grid, but "fixes" this at the end of every cycle by jumping back into phase when the voltage passes zero. This creates a signal similar to Slip Mode, but the power factor remains closer to that of the grid's, and resets itself every cycle. Moreover, the signal is less likely to be filtered out by known loads. The main disadvantage is that every inverter would have to agree to shift the signal back to zero at the same point on the cycle, say as the voltage crosses back to zero, otherwise different inverters will force the signal in different directions and filter it out.Bower & Ropp, pg. 29 There are numerous possible 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 dramatically reduces the chance that external circuits may filter the signal out. This advantage disappears with multiple inverters, unless some way of synchronizing the patterns is used.Bower & Ropp, pg. 34Utility-based methods
The utility also has a variety of methods available to it to force systems offline in the event of a failure.Manual disconnection
Most small generator connections require a mechanical disconnect switch, so at a minimum the utility could send a repairman to pull them all. For very large sources, one might simply install a dedicated telephone hotline that can be used to have an operator manually shut down the generator. In either case, the reaction time is likely to be on the order of minutes, or hours.Automated disconnection
Manual disconnection could be automated through the use of signals sent through the grid, or on secondary means. For instance,Transfer-trip method
As the utility can be reasonably assured that they will always have a method for discovering a fault, whether that be automated or simply looking at the recloser, it is possible for the utility to use this information and transmit it down the line. This can be used to force the tripping of properly equipped DG systems by deliberately opening a series of recloser in the grid to force the DG system to be isolated in a way that forces it out of the NDZ. This method can be guaranteed to work, but requires the grid to be equipped with automated recloser systems, and external communications systems that guarantee the signal will make it through to the reclosers.CANMET, pg. 12-13Impedance insertion
A related concept is to deliberately force a section of the grid into a condition that will guarantee the DG systems will disconnect. This is similar to the transfer-trip method, but uses active systems at the head-end of the utility, as opposed to relying on the topology of the network. A simple example is a large bank ofSCADA
Anti-islanding protection can be improved through the use of the Supervisory Control and Data Acquisition (SCADA) systems already widely used in the utility market. For instance, an alarm could sound if the SCADA system detects voltage on a line where a failure is known to be in progress. This does not affect the anti-islanding systems, but may allow any of the systems noted above to be quickly implemented.References
Bibliography
* Ward Bower and Michael RoppStandards
Further reading
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