Distributed generation (DG) has much potential to improve distribution system performance and it should be encouraged. However, distribution system designs and operating practices are normally based on radial power flows and this creates a special challenge to the successful introduction of distributed generation. This paper has described a few of the issues that must be considered to insure that DG will not degrade distribution system power quality, safety or reliability. This paper focused on radial systems, although some of the issues discussed are common to low voltage distribution networks.

Determining the impact of distributed generation on power systems. I. Radial distribution systems

Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.

Grid-connected photovoltaic power systems: Technical and potential problems—A review

This paper proposes a nondetection zone (NDZ) as a performance index to evaluate different anti-islanding schemes. The NDZ for three basic passive anti-islanding schemes: under/over voltage, under/over frequency, and phase jump are derived analytically and validated by simulation. Based on the NDZ, not only can the dominant factors that influence anti-islanding protection be identified, it may also support definition of optimal combined schemes that lead to a reduced NDZ. The methodology presented in the paper can be extended to the evaluation of other anti-islanding schemes.

Evaluation of anti-islanding schemes based on nondetection zone concept

The solar energy generation has grown significantly in the past years. The importance of PV penetration in power system as a major element of renewable energy source has seen it being widely used on a global scale. Despite its promising success, PV penetration presents various issues and its impact on the distribution system has to address for seamless integration in the power system. In this paper, a comprehensive overview on important issues affecting the distribution system as a result of PV penetration is presented. Pertinent issues such as voltage fluctuation, voltage rise, voltage balance, and harmonics and their effect on the system are discussed in details. The islanding issues, which are of critical importance to the stability and integrity of the system, are also thoroughly reviewed. Details on different islanding techniques – remote and local techniques and their advantage and disadvantages are shown. Therefore, this paper can provide useful information and serve as a reference for researchers and utility engineers on issues to be considered with regards to PV penetration.

Photovoltaic penetration issues and impacts in distribution network - A review

Islanding detection is a mandatory feature for grid-connected inverters. The effectiveness of passive islanding detection methods (IDMs) is usually demonstrated by means of nondetection zones (NDZs) represented in a power mismatch space (/spl Delta/P versus /spl Delta/Q). Active frequency drifting IDMs have been shown to provide improved performance but their theoretical NDZ cannot be described in the /spl Delta/P versus /spl Delta/Q space for a general RLC load. This paper shows that a load parameter space based on the values of the quality factor and resonant frequency of the local load (Q/sub f/ versus f/sub 0/) can be used in these cases. It employs a single curve to represent the NDZ of frequency drifting IDMs for any RLC loads. Equations that represent NDZs of three common active IDMs in the Q/sub f/ versus f/sub 0/ load parameter space are derived and it is shown that the slip mode frequency shift and the Sandia frequency shift IDMs can be designed to guarantee islanding detection for equivalent RLC loads with a quality factor smaller than a design value. The accuracy of the NDZs is verified with simulation and experimental results.

Performance assessment of active frequency drifting islanding detection methods

Islanding of a utility-interactive photovoltaic (UIPV) system occurs if the UIPV system continues to power a section of the utility system after that section has been disconnected from the utility source. Since islanding creates hazards for personnel and equipment, UIPV systems are required to detect and prevent it. It is desirable to have a simplified method of determining which islanding detection methods (IDMs) are most effective. In this paper, a previously described method for finding the nondetection zones (NDZs) of IDMs is experimentally verified. This method is used to determine the NDZs of several common IDMs. These results indicate that, of the IDMs discussed in this paper: (1) Sandia Frequency Shift (SFS) is most effective; and (2) the worst-case loads are low-power loads that are near resonance at the line frequency and have a large capacitance and small inductance (a high value of the quality factor Q).

Determining the relative effectiveness of islanding detection methods using phase criteria and nondetection zones

This paper proposes a definition for a Non-Islanding Inverter. This paper also presents methods that can be used to implement such an inverter, along with references to prior work on the subject. Justification for the definition is provided on both a theoretical basis and results from tests conducted at Sandia National Laboratories and Ascension Technology, Inc.

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Results of Sandia National Laboratories grid-tied inverter testing

The level of installed photovoltaic (PV) generation has surpassed 35 GW in the United States and the solar penetration continues to increase at a high rate. Almost all installations up to now rely on utility interconnection requirements based on 2003 standards, which made distributed energy resources (DER) devices sensitive to perturbations on the utility, would quickly disconnect, and would not provide utility support when most needed. This is changing, and to minimize adverse effects on the performance of electrical power systems, PV inverters must implement voltage and frequency ride-through capabilities and provide voltage and frequency support features. These new utility support requirements have caused renewed concern of loss of utility capabilities of PV DER devices. This paper focuses on revisions to the utility interconnection requirements and investigates the impacts of these changes on the islanding detection capabilities of PV inverters.

G.A. Kern

Papers

Determining the relative effectiveness of islanding detection methods using phase criteria and nondetection zones

Results of Sandia National Laboratories grid-tied inverter testing

Loss of Utility Detection Capabilities for Today's Utility Interconnected Photovoltaic Inverters