Distributed generators (DG) provide means for generating power locally. Instead of centrally producing power to a main grid in a power plant, the power grid may be powered by a plurality of distributed generators.
A distributed generator may include, for instance, a power source, such as a solar panel array or a wind turbine, and means for converting the power produced by the power source into a form in which it can be fed to the power grid. A frequency converter, for instance, may be used as the means for connecting the power source to the power grid.
Islanding conditions refer to a state of a power grid in which a part of the electrical power grid is separated from the rest of the power grid. In this disclosure, a part which is separable from the rest of the power grid is referred to as a “distributed power grid”, a “distributed grid”, or simply a “grid”. The part of the power grid from which the distributed grid may be separated is referred to as a “main grid”.
A distributed generator may continue its operation under islanding conditions, thus, producing power to the distributed grid. This may be a part of the function of the distributed generator. However, it may be important to stop the power production if an unintentional island has been produced. An unintentional island may, for instance, be caused by opening a breaker circuit either by maintenance personnel or simply because it has automatically tripped.
Unintentional islanding can be dangerous to maintenance personnel. They may be unaware that a part of the power grid is still powered even though the connection to the main grid has been cut. Further, unintentional islanding may damage customer equipment because of uncontrolled voltage and frequency transient excursions during the islanding conditions and during the reclosing into an island.
Therefore, it can be extremely important, for instance, in photovoltaic (PV) inverters to have a mechanism which immediately reacts upon detection of loss of main grid power and stops the power production.
In many cases, detecting the loss of main grid power may be relatively easy, as islanding can create under/over voltage and under/over frequency conditions. These conditions can be detected and used by, for instance, relays of a generator in the distributed grid to disconnect, and, thus, stop the power production.
However, when the power (both active and reactive components) generated by a distributed generator closely matches the power consumed by load or loads, detection of the islanding may become extremely difficult. Thus, the distributed generator may continue operation without detecting the islanding conditions [1].
The specification for detecting unintentional islanding in distributed power generation systems, and, for example, in PV inverters, has motivated an intensive research and development of different detection methods. Several standards have been established to specify the conditions for disconnection during unintentional islanding. For instance, the Underwriters Laboratories Standard UL 1741 calls for tripping of a distributed generator within 2 seconds after loss of the main grid power [4]. The German Standard DIN VDE 0126-1-1 proposes several islanding detection methods [5].
Methods for detecting islanding conditions may be divided in three categories: passive methods, communication-based methods, and active methods [1, 2, 3]. These methods can also be divided into internal and external methods where an internal method may, for instance, reside inside a distributed generator, and an external method may, for instance, be implemented as an external device, between the distributed generator and the main grid.
Passive methods can monitor variables of the distributed grid in order to find abnormal changes in, for instance, frequency, voltage amplitude, phase angle, harmonics contents, etc. Passive methods can be effective in most situations. However, their non-detection zone (NDZ), e.g., the range of loads for which the islanding detection method may fail, can be large.
Communication-based methods usually operate on the basis of establishing communication channels between distributed generators and the main grid. Communications-based methods can detect islanding conditions even when the power produced matches the power consumed. However, the communications devices can be expensive. Implementing a communication-based method may also call for co-operation of the main grid provider.
Active methods can detect a main grid power disconnection on the basis of observations on the response of the distributed grid to a disturbance intentionally introduced by the method. The response, or its magnitude, depends on the presence of the main grid power. Thus, islanding conditions may be determined on the basis of the response. In this manner, the NDZ can be minimized.
However, the injected disturbances may decrease the quality of the power produced. The disturbance introduced to the network for islanding detection purposes may have a predefined maximum limit in order to keep the quality of the produced power at an acceptable level. For instance, according to standard UL 1741, the variations of the active and reactive power injected to the network should not exceed ±3% of the rated apparent power of the distributed generator.
An active method may, for instance, be implemented by introducing a small reactive current component in the current reference of a current controller of a distributed generator. Then, the method may monitor changes in, for instance, the load voltage, frequency and/or phase and detect islanding conditions on the basis of the changes [7, 8, 9].
Another approach for an active method is to implement a positive feedback. A positive feedback of a distributed grid quantity, such as voltage, frequency or phase, may be added to a control reference controlling the produced power [10, 11]. For instance, in a voltage feedback scheme, the inverter may command more real power (or active current) when the distributed grid voltage amplitude is increased. As a result, the voltage keeps increasing to balance the real power. This continues until the voltage amplitude exceeds the protection limits, and thus the islanding can be detected. Similar approaches may be applied for a frequency feedback or a phase feedback.
The positive feedback can be a very effective method in detecting islanding, as it forces the trajectories of voltage and frequency to abandon their monitored protection limits, thus producing an imminent detection of abnormal operation. Some authors propose to maintain continuous operation of these positive feedback schemes, under the assumption that the effect will be negligible during normal operation, e.g., during main grid connection, and will become unstable only if an islanding condition arise.
However, there may be situations where the positive feedback methods can cause instabilities, even during normal operation conditions, e.g., under main grid connection.