Wencong Wang

A Power Line Signaling Based Technique for Anti-Islanding Protection of Distributed Generators—Part I: Scheme and Analysis

Anti-islanding protection of distributed generators (DG) is a significant technical barrier to the emerging DG industry. This paper presents an innovative power line signaling based anti-islanding scheme developed in response to the challenge. The scheme broadcasts a signal from a substation to the DG sites using the distribution feeders as the signal paths. A DG is considered as islanded from the upstream system if the signal cannot be detected at the DG site. The proposed scheme has been evaluated using analytical, simulation and field tests. The results are very promising. This paper presents the main ideas of the scheme and its design considerations. Methods to create and detect the signals are shown and their performances are analyzed.

A Power Line Signaling Based Scheme for Anti-Islanding Protection of Distributed Generators—Part II: Field Test Results

Anti-islanding protection of distributed generators (DG) is a significant technical barrier to the emerging DG industry. In response to this challenge, a power line signaling based anti-islanding protection scheme has been proposed in a companion paper. The scheme broadcasts a signal from a substation to the DG sites using the distribution feeders as signal paths. A DG is considered as islanded from the upstream system if the signal is not detected at the DG site. This paper presents extensive field test results and experiences obtained for the proposed scheme. The results confirm that the proposed scheme is a very promising and economical method to satisfy anti-islanding protection requirements for synchronous DG interconnections.

Interharmonic-flicker curves

Flickers and interharmonics have an inherent relationship. The magnitude of a voltage can fluctuate if it contains interharmonics. This paper proposes a limit for interharmonics by utilizing the published limits on voltage flicker. The limit is presented as an interharmonic-magnitude versus interharmonic-frequency curve and is called interharmonic-flicker curve. The interharmonic-flicker curves based on the rms and peak magnitude fluctuations and on the IEC flickermeter are derived. A systematic method to derive limits for interharmonics from the flicker limits is presented. The paper also reveals that the IEC flickermeter needs improvement when dealing with interharmonic-caused flickers. Potential applications of the proposed interharmonic-flicker curves include the designing and evaluation of mitigation measures to reduce the flicker impact of interharmonic producing sources.

A Scalable Power-Line-Signaling-Based Scheme for Islanding Detection of Distributed Generators

Power-line-signaling-based schemes are regarded as promising solutions to the problem of islanding detection of distributed generators (DGs). These schemes broadcast dedicated signals from the distribution substation to the downstream DGs and, therefore, transform the detection of islanded DGs into the determination of the signal presence at the DG sites. This paper presents an improvement to a published power-line-signaling method. With this improvement, the signaling device can be located anywhere between the substation and the DG sites, creating the opportunity for designing customized islanding detection arrangements for various DG interconnection scenarios. Simulations and laboratory tests show that the improved scheme can work reliably.

Identification of the Faulted Distribution Line Using Thyristor-Controlled Grounding

For ungrounded systems, identifying the line experiencing a single-phase-to-ground fault is very difficult because an ungrounded system produces very low fault current. This paper presents a novel method that can help to overcome this difficulty. The idea is to temporarily convert an ungrounded grounded system into a grounded system through a controlled grounding of the system neutral. The result is a controllable ground fault current that is large enough for identifying the faulted line and yet small enough not to cause system problems. Theoretical analysis, computer simulation and lab experiments verified the effectiveness of the proposed method. The proposed method also applies to high-resistance grounded and resonant grounded systems.

Power Electronic Signaling Technology—A New Class of Power Electronics Applications

The application of power electronics to facilitate the transmission or conversion of electric energy has been well known. This paper presents a different class of power electronic applications-the power electronic circuits are deployed to create small but discernible signals online. The signals are utilized for monitoring, power line communication and other information-oriented purposes. We use the term “power electronic (PE) signaling technology” to designate the technologies involved in these applications. The objective of this paper is to survey and review the developments in this fascinating field. Several highly successful PE signaling technologies and their applications are illustrated. It is believed that signaling-oriented power electronic techniques will have many potential applications in power systems and can be a major source of innovation for the smart grid initiative.

Local anti-islanding protection for distributed generators based on impedance measurements

Distributed generators (DG), while working in conjunction with the main electricity grid, must have antiislanding protection. This paper presents a local detection scheme for distributed generators by using an SCR based device to draw a current pulse around the zero- crossing of the voltage waveform and analyzing the pulse to determine whether islanding has occurred. This method is based on the fact that the system impedance will change when islanding occurs, and therefore the current pulsepsilas characteristics will change. The peak of the current pulse is used to determine the impedance change and the decision to trip the DG. Theoretical analysis along with extensive computer simulations have shown that this method is largely independent on the system parameters and the proposed method was found capable of islanding detection very reliably. The proposed method has a negligible impact on the voltage waveform, thus reducing the power quality impact the device introduces.

An Improved Signal Detection Algorithm for TWACS based Power Line Signals

TWACS (two-way automatic communication system) is a powerful technology to send signals on power lines. The authors have developed an innovative new approach to detecting TWACS signals while applying the technology to check the continuity of a distribution feeder and detect the formation of islands in power distribution systems. This paper presents a new algorithm to detect TWACS signals which increases the reliability and accuracy of the detection process while reducing the cost of signal detection and transmission. It will take advantage of a DSP capability and has been tested using extensive field measured signals. The results show signals as low as 1.2% of the supply voltage can be detected as compared to 3% with the current analog method. Thus the cost and negative impact of the signal on power quality can be reduced. This paper presents detailed research results on the proposed algorithm and extensive field data tests have shown that the proposed algorithm is much more reliable than the traditional method

Controlled Closing of PT Delta Winding for Identifying Faulted Lines

It is very difficult to identify the line experiencing a single-phase to ground fault in ungrounded and resonant grounded systems. This paper presents a scheme for faulted line identification based on the concept of converting the ungrounded system into a grounded system in a controlled manner. This conversion or controlled grounding is done through the delta-winding of the potential transformers commonly available in substations. The delta-winding is inserted with a pair of SCRs and the controlled conduction of the SCRs results in a temporarily closed delta and thus creating a temporarily grounded system. This scheme is very easy to implement and does not involve high voltage components. Results of theoretical analysis, computer simulation and lab experiments have confirmed the effectiveness of the proposed method.

A Power Line Signaling Based Technique for Anti-islanding Protection of Distributed Generators: Part I: Scheme and Analysis

Summary form only given. Anti-islanding protection of distributed generators (DG) is a significant technical barrier to the emerging DG industry. This paper presents an innovative power line signaling based anti-islanding scheme developed in response to the challenge. The scheme broadcasts a signal from a substation to the DG sites using the distribution feeders as the signal paths. A DG is considered as islanded from the upstream system if the signal cannot be detected at the DG site. The proposed scheme has been evaluated using analytical, simulation and field tests. The results are very promising. This paper presents the main ideas of the scheme and its design considerations. Methods to create and detect the signals are shown and their performances are analyzed.