Continuously expanding deployments of distributed power-generation systems (DPGSs) are transforming the conventional centralized power grid into a mixed distributed electrical network. The modern power grid requires flexible energy utilization but presents challenges in the case of a high penetration degree of renewable energy, among which wind and solar photovoltaics are typical sources. The integration level of the DPGS into the grid plays a critical role in developing sustainable and resilient power systems, especially with highly intermittent renewable energy resources. To address the challenging issues and, more importantly, to leverage the energy generation, stringent demands from both utility operators and consumers have been imposed on the DPGS. Furthermore, as the core of energy conversion, numerous power electronic converters employing advanced control techniques have been developed for the DPGS to consolidate the integration. In light of the above, this paper reviews the power-conversion and control technologies used for DPGSs. The impacts of the DPGS on the distributed grid are also examined, and more importantly, strategies for enhancing the connection and protection of the DPGS are discussed.

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Distributed Power-Generation Systems and Protection

Among different sources of alternate energy, wind and solar are two prominent and promising alternatives to meet the future electricity needs for mankind. Generally, these sources are integrated at the distribution utilities to supply the local distribution customers. If the power generated by these sources is bulk, then they are either integrated at the distribution/transmission level or may be operated in an island mode if feasible. The integration of these renewables in the power network will change the fault level and network topologies. These fault levels are intermittent in nature and existing protection schemes may fail to operate because of their pre-set condition. Therefore, the design and selection of a proper protection scheme is very much essential for reliable control and operation of renewable integrated power systems. Depending upon the level of infeed and location of the renewable integration, the protection requirements are different. For low renewable infeed at the distribution level, the existing relay settings are immune from any small change in the network fault current from new incoming renewables. However, bulk renewable infeed requires modification in the existing protection schemes to accommodate the fault current variation from the incoming renewables. For bulk penetration of the renewable, the requirement of modified/additional protection schemes is unavoidable. Adaptive relaying and non-adaptive relaying schemes are discussed in the literature for protection of power networks, which are experiencing dynamic fault currents and frequent changing network topologies. This article presents a detailed review of protection schemes for renewable integrated power networks which includes distribution, transmission and microgrid systems. The merits and demerits of these protection schemes are also identified in this article for the added interest of the readers. The visible scope of advance protection schemes which may be suitable for providing reliable protection for dynamic fault current networks is also explored.

Protection challenges under bulk penetration of renewable energy resources in power systems: A review

The ever growing penetration of distributed generation (DG) in a distribution network has a profound impact on network protection and stability. Traditional protection schemes and algorithms need to be extensively investigated as more and more DGs get introduced into the network. The current version of IEEE Standard 1547 does not present a comprehensive solution for fault current detection in the presence of various kinds of DGs. Power electronic inverter-based DGs (IBDGs) are of special concern in distribution network protection as they are often incapable of providing sufficient fault current and their controllers play a principal role in the DG behavior. In this paper, the effects of voltage and current controllers for IBDGs on industrial and commercial power system protection schemes are investigated. It is shown that the inverter control mode has a direct impact on its fault current levels and duration. A simplified distribution network model with IBDG operating under voltage and current control modes was tested to verify the effects of these controllers. This paper also proposes an adaptive relaying algorithm to detect the faults in the presence of IBDGs with various types of controllers.

The Influence of Inverter-Based DGs and Their Controllers on Distribution Network Protection

A main challenge in the practical implementation of a microgrid is the design of an adequate protection scheme in both grid-connected and islanded modes of operation. Microgrid protection requires a fast, reliable, and robust communication system to adjust relay settings to the appropriate current level according to the operation mode. Various approaches have been implemented to deal with this problem, yet the most promising ones are the use of adaptive protection techniques abiding by the International Electrotechnical Commission (IEC) 61850 communication standard. This paper presents a review of the technical challenges for the use of classical protection techniques and the need for an enhanced, adaptive, smart protection system. However, the risk of communication link failures and cyber security threats still remain a challenge in implementing a reliable adaptive protection scheme. A contingency is essential in the event that a communication failure prevents a relay from adjusting between grid-connected and islanded mode. An adaptive protection scheme is proposed, which utilizes energy storage devices to enhance resiliency against communication outages. The proposed solution does not incur additional costs to the network, as the energy storage devices are already utilized for other purposes. A case study experimentally verifies this contingency under a worst-case scenario, where a fault occurs simultaneously with a pulsed load while operating in islanded mode with a communication failure.

A Review of Communication Failure Impacts on Adaptive Microgrid Protection Schemes and the Use of Energy Storage as a Contingency

In this paper, an adaptive directional overcurrent relaying technique based on the positive-sequence (PSQ) and negative-sequence (NSQ) superimposed currents is proposed for microgrid protection. Due to the change in mode of operation of microgrid, the level and direction of prefault current are altered. On occurrence of a fault in any line section of the microgrid, coordination between the primary and backup overcurrent relays is lost due to variations in level and direction of fault current. With inception of faults, the current contributed by the inverter-based distributed energy resource is in the range of 2 p.u. To solve protection coordination issue, this paper presents an adaptive overcurrent relay settings using PSQ and NSQ superimposed currents those are contributed during fault conditions. Further, the direction of the fault is obtained using the new phase change between prefault and superimposed sequence fault currents. The proposed technique is implemented on a dSPACE processor board, which is connected to a real-time digital simulator to carry out the hardware-in-the-loop test. Extensive simulation and hardware results obtained for several microgrid operating modes indicate the effectiveness of the proposed technique. This proposed approach is independent of the voltage information.

Superimposed Adaptive Sequence Current Based Microgrid Protection: A New Technique

The fault characteristic of inverter interfaced DG is very different from that of synchronous rotating generators. Traditional protection methods based on large fault current are no longer applicable to inverter-only supplied microgrid. This paper analyzes the fault behaviour of inverter based microgrid, and proposes an adaptive fault current protection algorithm for this type of microgrid. According to the fault characteristic of inverter interfaced DG, it uses fault component current which only presents in fault condition. The result is simulated and proved by DigSilent.

Yi Han

Papers

Study of adaptive fault current algorithm for microgrid dominated by inverter based distributed generators