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DL/T 684—2012 Guide of Calculating Settings of Relay Protection for Large Generator Transformer （hereinafter referred to as the “2012 Guide”） has played a significant role in improving the correct operation rate of relay protection devices for generator transformers and ensuring the safe operation of electrical equipment in power plants. However， with the increase in capacity of individual generator units in China and the emergence of new faults， the “2012 Guide” has proved inadequate， requiring revision to better guide engineering practice. Therefore， this paper analyzes the problems in the “2012 Guide” and puts forward specific revision suggestions to address problems such as misoperation of generator differential protection under sympathetic inrush current， misoperation of false power-on protection under poor synchronous conditions， and tripping of gap zero-sequence current protection of main transformer before reclosing in the case of transient single-phase ground faults on the line. The suggestions can provide reference for the revision of the “2012 Guide”.

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A spot network is a special low-voltage distribution network with multiple sources supplying it in parallel. It can significantly enhance supply reliability and is beneficial for integrating a distributed energy resource (DER). However, because traditional reverse power protection in spot networks cannot identify the reverse power cause, DERs, especially photovoltaic (PV) hosting capacity, are limited. Meanwhile, no uniform method exists for assessing the PV injection capacity in spot networks. Based on comparing the positive sequence current phase, this article presents an improved reverse power protection and PV hosting capacity assessment method for spot networks. Current variation is adopted to identify the reverse power cause, and a spot network equivalent model with PV is built. The positive sequence current phase relationship at the reverse power protection location is analysed when different types of short circuit faults occur. A short circuit fault section can be determined by comparing the positive sequence current phase. The influence of the PV output current on the improved reverse power protection performance is analysed. Then, a PV hosting capacity assessment method with improved reverse power is presented based on the PV influence on protection instead of on node voltage. A simulation model based on an actual project is constructed using PSCAD software. The simulation results show that the improved reverse power protection significantly enhances the ability to inject PV into the spot network and operates correctly under faults. The theoretical PV hosting capacity agrees with the simulation results.

The purpose of the reverse power (RP) protection is essentially to prevent severe damage to the generator prime mover. For this aim, directional relays are usually utilised. RP condition (RPC) leads to initial oscillations of power and frequency which might exceed the RP threshold and lead to mal‐operation of the relay. Therefore, assigning a time delay is essential to reduce the oscillations. However, this strategy increases the relay trip time, which leads to enhancing the motoring time and the risk of damage to the prime mover. This study proposes a novel algorithm to detect RPC in synchronous generators, based on some easily accessible quantities including active power (P), load angle (δ), internal voltage (EA), terminal voltage (V) and the phase difference between voltage and current (θ). This scheme utilises two criterions including the sign of θ and also comparing V with tangential component of EA on V (EACosδ). This algorithm is independent of the size and type of load and is also capable of discriminating RPC with other potential faults. The performance of the proposed approach has been verified with Matlab software and the obtained simulation results show the fastest operation and highest reliability of the proposed technique compared to others. [ABSTRACT FROM AUTHOR]

This article presents an improved reverse power protection for spot network with high penetration of photovoltaic (PV) interfaced with inverter. First, the current variation within a short time window is discussed respectively when fault occurs and PV operates with fluctuation. It can be used to determine the cause of reverse power. Second, the positive sequence fault component power is analysed when different types of fault occur at various locations. The fault section can be located mainly by comparing the power direction of positive sequence fault components among feeders, as well as comparing the output current of PV inverter with the current through its connecting switchgear. Finally, the improved reverse power protection for spot network with PV is presented, including the operating value and protection logic. This method can enhance the penetration of PV in spot network. The simulation model based on practical projects is constructed. The protection performance including load and PV fluctuation, and all kinds of fault is simulated with comparison to the present protection method. The simulation results prove the validity of the improved protection. [ABSTRACT FROM AUTHOR]