Abstract : This paper introduces the relay protection for the generating units and transformers of the Dapu Hydropower Station, supplied by Welwyn Hydro GmbH of Austria. The protection configuration scheme is discussed, and Welwyn Hydro GmbH's patented Buetow generator stator single-phase grounding protection technology is explored. Keywords : Generator-transformer unit; stator single-phase grounding protection; relay protection; protection configuration; Buetow The Dapu Hydropower Station has an installed capacity of 3×30MW, with one 40MVA transformer and one 70MVA transformer. The generator-transformer unit is connected in both extended unit and unit configurations, with a circuit breaker at the generator outlet. There are two 110kV outgoing lines, and the 110kV busbar is a single-busbar segmented circuit. The main wiring diagram is shown in Figure 1. 1. Protection Panel and Configuration The generator-transformer unit protection is supplied by Welwyn Hydro GmbH, using Welwyn's DRS standard protection module. The DRS standard module is a new type of multi-functional digital relay that can be either pre-configured or user-configured. It can be installed in a 19-inch cabinet (6μm high, 42μm wide) in both recessed and protruding configurations. The dimensions are approximately 296mm × 246mm × 282mm. The DRS standard package includes all peripheral equipment, such as: optocoupler modules, alarm outputs and trip modules, binary signal input boards, DC-DC converters, and processor boards (control units or VEs) marked with local LED (light-emitting diode) displays. Communication with a personal computer is via the interface socket in front of the relay. An optical interface can be used to connect the control system (VDEW6) and the DRS COM communication software for remote data transmission. The generator-transformer unit expansion protection consists of two cabinets, while the generator-transformer unit protection consists of one cabinet. The generator-transformer unit expansion unit protection cabinet #1 (U01+JA01) consists of two protection modules, DRS-COMPACT-DGA1 and DRS-COMPACT-DGA2, providing protection for generator #1, and two protection modules, DRS-COMPACT-DTA1 and DRS-COMPACT-DTA2, providing protection for the main transformer #1. The generator-transformer unit expansion unit protection cabinet #2 (U02+JA01) consists of two protection modules, DRS-COMPACT-DGB1 and DRS-COMPACT-DGB2, providing protection for generator #2. The generator-transformer unit expansion unit protection cabinet #1 (U03+JA01) consists of two protection modules, DRS-COMPACT-DGC1 and DRS-COMPACT-DGC2, providing protection for generator #3, and two protection modules, DRS-COMPACT-DTB1 and DRS-COMPACT-DTB2, providing protection for the main transformer #2. The DRS-COMPACT-DGA1, DRS-COMPACT-DGB1, and DRS-COMPACT-DGC1 modules are equipped with generator differential protection (87G), generator stator grounding protection (64G), generator loss of excitation protection (40E), generator overvoltage protection (59), and generator overload protection (49). The DRS-COMPACT-DGA2, DRS-COMPACT-DGB2, and DRS-COMPACT-DGC2 modules are equipped with generator negative sequence protection (46), generator reverse power protection (32), undervoltage overcurrent protection with current memory (51G/27), generator overvoltage protection (59), frequency protection (81), generator starting grounding protection (64S), rotor grounding protection (64R), and shaft current protection (SC). The DRS-COMPACT-DTA1 and DRS-COMPACT-DTB1 modules are equipped with main transformer differential protection (87U), transformer discharge gap overcurrent protection (51TD), transformer low-voltage side ground fault protection (64NT), and transformer zero-sequence overvoltage protection (59N). The DRS-COMPACT-DTA2 and DRS-COMPACT-DTB2 modules are equipped with transformer composite voltage overcurrent protection (51T/27), transformer overload protection (49T), and transformer zero-sequence current protection (51NT). 2. Protection Configuration Analysis (1) The protection configuration meets the current national standards and exceeds the requirements of the standards in many places. For example, the stator single-phase grounding protection has reached 100% stator single-phase grounding protection. (2) In addition to its own longitudinal differential main protection, the generator protection is also equipped with generator-transformer group differential protection, forming a dual main protection. It is equipped with low voltage overcurrent protection with current memory as backup protection for generator-transformer group. The protection has two time limits. It disconnects the high voltage side circuit breaker of the transformer with a shorter time limit and shuts down the machine with a longer time limit, which can ensure the plant power supply when the transformer fails. (3) The transformer protection consists of gas, pressure and transformer differential protection as the main protection. The generator backup protection is used as the backup protection for the transformer. The transformer composite voltage overcurrent protection is configured on the high voltage side as the backup protection for the bus protection, so that the overall protection is mutually configured, complete and reliable. (4) The main protection and backup protection are configured separately in two modules, and generator overvoltage protection is configured in both modules. In this way, the generator will not lose the protection it should have in the event of a single module failure. 3. Stator grounding protection Stator single-phase grounding is the most common internal fault, mainly caused by the insulation damage of the winding to the iron core. The harm of a single-phase ground fault to a generator is manifested in the fact that the electric arc at the fault point will further expand the range of stator winding insulation damage. If the ground fault is not detected in time, it will develop into a phase-to-phase or turn-to-turn short circuit, and the electric arc will burn the iron core. Statistics from 1995 in my country show that there were 71 main body faults in generators of 100MW and above nationwide, of which 20 were stator winding ground faults, accounting for approximately 28.17% of the main body faults. Currently, domestic protection against single-phase stator ground faults mainly includes fundamental zero-sequence voltage grounding protection and third harmonic voltage grounding protection. The single-phase ground fault protection for the generator stator winding at the Dapu Hydropower Station consists of 85% generator starting grounding protection and 100% Buetow stator grounding protection. Unlike domestic stator single-phase ground fault protection, the Buetow stator single-phase ground fault protection can achieve 100% stator ground fault protection on its own (most domestic systems require two or more protection methods to achieve this). It is a patented technology of the Austrian company Vio. However, since the Buetow voltage transformer is installed on the upper side of the generator circuit breaker (see main wiring diagram), it cannot detect a single-phase ground fault in the generator stator before the generator is connected to the grid. During contract negotiations, we requested that the Buetow voltage transformer be installed on the lower side of the generator circuit breaker, but this would require three Buetow voltage transformers (one for each generator), which was not cost-effective. A compromise was ultimately adopted: before grid connection, 85% of the generator's start-up grounding protection is used as the stator single-phase ground fault protection; after grid connection, 100% Buetow stator single-phase ground fault protection is used. The principle of the Buetow voltage transformer protection is as follows: the 60% tap of the second coil of the secondary A, B, and C phases of the voltage transformer is connected to the other two coils in an open delta configuration. This causes a displacement of approximately 100V between the stator neutral point potential and ground, thus achieving 100% stator single-phase ground fault protection, which trips the generator circuit breaker. From a structural principle perspective, this protection is suitable for generator-transformer units, but it suffers from a lack of selectivity for generator-transformer extended unit connection types. For example, at the Dapu Hydropower Station, when a single-phase ground fault occurs in the stator winding of Unit 1, the Buetow voltage transformer connected to the 10.5kV bus cannot determine whether the fault occurred in Unit 1 or Unit 2. Therefore, both generators must be disconnected simultaneously, thus expanding the fault area. This is incompatible with the current tight power supply situation in the Guangxi power grid and will cause unnecessary losses to the owner, such as making inspection and diagnosis more complicated. When the generator neutral point is not grounded, the single-phase ground fault current is the generator's three-phase ground capacitance current Ic, which is very small; for a 30MW unit, its value is generally less than 3A. According to national standards, generators are allowed to operate for a certain period. Based on this, we propose to use time setting to ensure selectivity. The scheme is as follows: Buetow stator single-phase grounding protection is installed in the protection device of each generator. When the Buetow voltage transformer detects a stator single-phase grounding fault, the stator single-phase grounding protection of generator No. 1 trips the circuit breaker of this generator instantaneously or with a short time limit, and the single-phase grounding protection of generator No. 2 trips the circuit breaker of generator No. 2 with a long time limit. The advantages of this configuration scheme are: ① It can guarantee 50% selectivity for single-phase grounding faults. If a single-phase grounding fault occurs in generator No. 1, it can ensure the normal operation of generator No. 2; ② It can more accurately identify the faulty unit. If a single-phase grounding fault occurs in generator No. 1, generator No. 1 trips. At this time, the fault is cleared, and it can be determined that the fault is in generator No. 1. If both generators trip due to faults, it can be preliminarily determined that generator No. 2 is faulty, reducing the inspection and judgment time. 4. Opinions and suggestions (1) At present, China has made great progress in microcomputer protection. The consideration of component protection is more comprehensive and detailed than that of foreign countries, and it is more suitable for the national conditions. We should boldly carry out localization. (2) The main wiring should be as simple and clear as possible to better ensure the correctness and sensitivity of the relay protection operation. (3) Since the generator itself has a set of differential protection, and the transformer is also equipped with gas and pressure protection, and a large differential protection is also used for the generator and transformer, any short circuit from the generator neutral point to the high-voltage side of the step-up transformer has double main protection. Therefore, in terms of backup protection, the generator and transformer can share the backup protection, that is, the generator overcurrent protection completes the backup protection for the generator and transformer. The transformer's composite voltage overcurrent protection is only used as backup protection for the bus protection, thus ensuring that the protection from the generator to the bus is completely coordinated and unified.