Markus Dietmannsberger

Ancillary services and dynamic behavior of inverters connected to the low voltage grid

Small generators connected to the low voltage grid must be able to provide ancillary services in order to support system stability nowadays and in the future. Grid codes have been implemented that demand for load-frequency control and reactive power control. Together with anti-islanding-detection, these control algorithms mainly affect the dynamic behavior of the inverters in case of loss of mains. In this paper, an inverter model is presented that complies with the major grid codes. Islanding tests show new effects that arise because of the interaction between different control algorithms. Oscillating operating points may occur under special circumstances. This leads to further investigations on frequency dynamics. The additional benefit of rate-of-change-of-frequency (ROCOF) monitoring is numbered in a quantitative analysis and simulation. Not only steady state, but dynamic behavior is investigated with respect to the Non-Detection-Zone (NDZ). It is shown, that using frequency dynamics, scales down NDZs of passive anti-islanding methods significantly.

Impacts of Low-Voltage Distribution Grid Codes on Ancillary Services and Anti-Islanding Detection of Inverter-Based Generation

The capacity to provide ancillary services has become more and more important for generators in the distribution grid. New grid codes have been implemented which regulate inverter behavior regarding different aspects of system safety and stability. This mainly includes the frequency-dependent power reduction or the allocation of reactive power. These standards affect mains monitoring because the power control directly changes penetration after the generator has islanded. In this paper, equations for active and reactive power balance are set up and developed further. Non-Detection Zones for different control methods are shown. These are then described and interpreted in detail to show how different effects that result from grid codes interfere with one another. Frequency-dependent power reduction was found to change the size and shape of the Non-Detection Zones. Reactive power control results in a shift of the Non-Detection Zones. In fact, even the initial operating point before loss of mains has an impact on inverter behavior after islanding. Islanding operating points are sometimes undefined which leads to oscillation. An interpretation of the results and advice for future revisions of grid codes conclude the paper.

Compatibility of fault-ride-through capability and anti-islanding-detection in inverters connected to low voltage distribution grids

The amount of ancillary services provided in the low voltage (LV) grids increased in the past. Active power reduction due to over frequency and the allocation of reactive power are the most common ones including loss of mains detection. In the future, additional requirements will be necessary for generators connected to LV grids: fault-ride-through capability (FRT). But this contradicts the terms of loss of mains or anti-islanding-detection (AID). AID uses the event of voltage drops to trip the inverter and thus shuts down islands. Hence there is a potential danger that stabilizing the grid with FRT causes the AID to become ineffective. In this paper, a detailed analysis is made on that challenge. A possible solution is presented that enables a conjoint implementation of both FRT and AID. Its advantages and drawbacks are discussed in detail. For validation purposes, a model for a three phase inverter is developed which includes the proposed algorithm. It is successfully tested in simulation.

Assessment of possible Technologies for Small Scale Emergency Power Supplies

Emergency power supplies are mainly used to protect critical infrastructure in case of blackouts. For ordinary population, there are hardly any investigations on usability of different technologies for emergency power supply systems. This paper provides a detailed analysis on which requirements are relevant for laymen usage of emergency power supplies. Furthermore, a method which combines these requirements is used to evaluate a variety of technologies based on the presented criteria and to compare their individual suitability. Advantages and disadvantages of all the examined technologies are provided. With this, a quick evaluation of different technologies with respect to small scale systems is possible. As all requirements are addressed, this helps the reader to get a quite but differentiated overview. Finally, a benchmarking is presented for each criterion and each technology.

Different load types and their effect on islanding detection and control in low voltage grids

Unintentional islanding is a well known problem for distributed generation units. Due to several hazards, grid-codes regarding anti-islanding-detection have been introduced and adapted in the near past. Those standards have been implemented with respect to parallel RLC load behaviour, which is assumed to represent worst-case conditions. But today, more and more loads do not show a linear behaviour any more. In fact, switch-mode power supplies are applied especially in consumer products connected to the low voltage grid. In this paper, the behaviour of non-linear loads is scrutinized by using IEEE standards and synthetic models representing a broad variety of different loads. It was found, that load composition influences the operating voltage in islanded grids and thus has a direct effect on the effectiveness of anti-islanding-detection. The results of the investigation show, that with an increasing share of non-linear loads the NDZ is reduced i.e. effectiveness of anti-islanding-detection is increased and in turn stable control in intended islanding is becoming more complicated. The findings indicate that the changes in low voltage load composition should be considered, when grid codes are revised in the future.

Modellierung und Bewertung einer autarken Notstromversorgung der Bevölkerung unterhalb der KRITIS-Schwelle unter Einsatz von Photovoltaik-Systemen

Im Rahmen dieser Publikation wurde analysiert, wie eine Notstromversorgung nicht-kritischer Infrastrukturen durch Photovoltaik-Systeme verwirklicht werden kann. Dazu wurden vier Szenarien entwickelt und modelliert, die eine Abschatzung des Leistungs- und Energiebedarfs ermoglichen. Dies beinhaltete die Untersuchung relevanter Verbraucher in den einzelnen Szenarien, die Grose des Solargenerators sowie eines Zwischenspeichers. Es wurde zwischen Sommer und Winter sowie der verfugbaren Solarstrahlung in drei Sektoren Deutschlands differenziert. Abschliesend fand eine Analyse des Flachen- und des Kostenbedarfs sowie eine Diskussion der Realisierbarkeit in Anbetracht der raumlichen und finanziellen Moglichkeiten in den einzelnen Szenarien statt. Es wurde ermittelt, dass Unterschiede zwischen dem nordwestlichen Sektor und dem sudlichen Sektor von bis zu 45 % beim Flachenbedarf existieren. Eine Bewertung der benotigten Flachen ergab, dass Notstromversorgungssysteme allein mit Photovoltaik-Systemen oft aus Platzgrunden nicht realisierbar sind. Im Anschluss wurden verschiedene Kostenpunkte einer Photovoltaik-Anlage separat betrachtet. Die Kosten variieren bis zu 41 % zwischen den betrachteten Bereichen Deutschlands.

Restoration of Low-Voltage Distribution Systems With Inverter-Interfaced DG Units

The increasing share of distributed generation (DG) offers new chances in grid restoration of low-voltage distribution grids. Instead of relying on the transmission or high- and medium-voltage levels, establishing islanding operation in low-voltage grids might be a good option after a wide-area voltage collapse. This paper proposes a restoration strategy from zero-voltage conditions for inverter-interfaced DG under islanded conditions. In the approach, a flexible and scalable master DG inverter concept is introduced for DGs, where no communication is needed and an outage of the master can be balanced by other DG inverters. The control strategy ensures the tracking of nominal values of the system voltage and frequency without zero steady-state error. The influences of noncontrollable DG are also taken into account in the strategy with an effective countermeasure developed. Experimental results verify the effectiveness of the proposed approach.

Implementation and Compatibility Analysis of Dynamic Voltage Support and Unintentional Islanding Capability of Power-Electronic Generators

One reason for the trend towards microgrid operation is the increased security of supply for the island itself. But if more and more generation units disconnect from the grid subsequent to a fault event, the overall grid stability might be weakened because there are less generators that provide dynamic valtage support. In this paper we propose a control strategy that combines a low-voltage ride through capability with a subsequent islanding operation. With the proposed approach, inverters are able to supply a maximum amount of grid support in the first place. If there is a collapse of the grid, the inverter disconnects and switches to islanding operation. The designed valtage and frequency control are implemented in an ordinary grid parallel inverter. Thus the method is suitable for a wide range of inverter units whether they are used in microgrids or not. The proposed control strategy was tested in simulation and experimentally validated. It was shown that the control is robust under all relevant load conditions.

Simultaneous Implementation of LVRT Capability and Anti-Islanding Detection in Three-Phase Inverters Connected to Low-Voltage Grids

In the future, the provision of ancillary services in low-voltage grids will become more and more important. Low-voltage ride-through capability has been a requirement in medium- and high-voltage grids but will become mandatory even in low-voltage grids. There may be conflicts with existing requirements such as anti-islanding detection. This paper proposes a method that allows for both low-voltage ride-through capability and anti-islanding protection to be implemented simultaneously in three-phase inverters connected to low-voltage grids. This method uses constant voltage and frequency monitoring and reacts according to the relevant specific requirements. With the proposed method, inverters can contribute to system stability without the risk of unintentional islanding operation. The implementation of low-voltage ride-through capability is shown to even facilitate the detection of islanded grids. To prove the merit of this proposal, a model for a three-phase inverter is developed, which incorporates the proposed method and is successfully tested both in simulation and in experiments in the laboratory.