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The excitation device is an important component of hydro-turbine generator sets, and the accuracy of its design, calculation, and selection has a significant impact on the efficient operation of the power generation system as well as on production safety. There are many factors to consider when calculating the excitation system and selecting the excitation type; these factors are directly related to the operating parameters of the generator, as well as to the design of the excitation system itself. Based on a calculation example of the retrofitting of the excitation system for a 6.5 MW hydro-turbine generator set to 7.15 MW, this paper explores a simplified calculation process for selecting excitation systems for small and medium-sized hydro-turbine generator sets, presenting a concise yet relatively comprehensive calculation method. Selection calculations for the excitation system of generator sets: yunrun.com.cn/product/2073.html 1. Generator parameters: The original capacity of this generator set was 6.5 MW; after more than thirty years of operation, its efficiency has significantly declined, and there is serious waste of water resources, which affects the power generation efficiency of the power plant. Therefore, the power plant carried out a thorough overhaul and technical upgrade of the units, increasing their installed capacity from the originally planned 6.5 MW to 7.15 MW (an increase of 10%). The relevant parameters of the generator and excitation system after modification are as follows: rated capacity 7.15MW ； Rated voltage 6.3kV ； Rated excitation voltage: 118V ； Rated excitation current is 441A. 2. Calculation of excitation equipment parameters: Equipment configuration principle: To meet the requirements for long-term operation under the generator’s rated overload condition as well as at an excitation current 1.1 times the rated value. Sufficient margin should be considered in equipment selection. 2.1 Calculation of the excitation transformer 2.1.1 Selection of the secondary voltage of the transformer The selection principle should ensure that the requirement of a 2.0 times stronger excitation can still be met when the primary voltage is at 80% of the rated value. That is: where U2 represents the line voltage on the secondary side of the transformer ； Rated excitation voltage of UFC at 2.0 times strong excitation ； 1.35 is the three-phase full-control rectification coefficient ； αmin is the thyristor firing angle under strong excitation. Taking the arc-switching voltage drop into account, the actual voltage on the secondary side of the transformer was chosen to be 240V. 2.1.2 Selection of transformer rated capacity The transformer capacity shall meet the requirements for long-term operation at the rated excitation current, that is: where S is the transformer capacity ； Ie is the current in the secondary side winding of the transformer ； Ifn is the rated excitation current value of the generator. Taking sufficient operating margin into account, the actual rated capacity of the transformer selected is 200 kVA. Generally, a dry-type transformer with the following parameters can be used: rated capacity of 200 kVA ； Primary voltage 6.3kV ； Secondary voltage 240V ； Short-circuit impedance 4% ； Wiring group Y/Δ-11. 2.2 Calculation of thyristor components ① Reverse peak voltage: The maximum reverse voltage that each arm of the component must withstand should be less than the component’s reverse repetitive peak voltage. That is:, where Ku is the overvoltage margin factor, which is generally set at 2.0 ； Kcg is the overvoltage impulse factor, generally taken as 1.5-1.6 ； Ke is the coefficient for the increase in supply voltage, typically taken as 1.0-1.1 ； UARM is the maximum reverse operating voltage of the bridge arm. Calculation: ② Rated average on-state current of the thyristor: In the formula, (1.5–2) is the safety factor; for this calculation, 2 is used ； Kfb is the calculation coefficient for a rectifier circuit under resistive load when the control angle is 0°; it is 0.368 for a three-phase bridge rectifier circuit ； Id is the rated excitation current under the 2.0 times over-excitation condition ； Ifn is the rated excitation current of the generator. Calculation: Based on the calculations, the thyristor component parameters of 800A/1800V can be selected. 2.3 Selection of fast fuses: Calculate one fast fuse for each thyristor in series. ①The rated voltage of the fast fuse should be higher than the highest peak voltage on the low-voltage side of the excitation transformer: 240×1.414≈340V. A 500V fast-acting fuse can be used. ②The rated current of a fast fuse is calculated using the following formula: where IR is the effective value of the current flowing through each thyristor element under rated excitation conditions, and IR = Ifn × 0.577 ； K is a comprehensive coefficient that represents a combination of factors such as margin, heat dissipation, wind speed adjustments, and ambient temperature; it is typically set between 1.3 and 1.5 ； Id is the average current value during the conduction state of the thyristor element. IRN = Ifn × 0.577 × K = 441 × 0.577 × 1.5 ≈ 382A; the optional fuse parameters are 500V/500A. 2.4 Selection of the demagnetizing switch and demagnetizing resistor ① The rated voltage of the demagnetizing switch must be higher than the rated excitation voltage of the rotor circuit (118V), and its rated current must be 1.1 times higher than the rated excitation current of the rotor circuit (441A). Main parameters of the de-excitation switch: rated operating voltage 600V ； Rated operating current: 800A ； Maximum breaking current: 2000A. ②Generally, the demagnetization resistance of a hydro turbine is 5 times the rotor resistance; that is, RM≈5×0.226=1.13Ω. The principle followed is to minimize the demagnetization time while ensuring that the rotor does not experience excessive voltage during this process. Given the large voltage margin of the rotor system in this unit, and referring to the examples of the demagnetizing resistance values after capacity expansion for units of the same model, RM=1.7Ω has been adopted in this project. 2.5 Excitation System Design: DC excitation is used, with the excitation current being 12.5% of the no-load rated excitation current. According to the electromagnetic calculation sheet, the no-load rated excitation current is 221 A; therefore: IQL = 221 × 0.125 = 27.625 A. The current-limiting resistor value is: 220 / 27.625 ≈ 7.9638 Ω. The DC resistance of the rotor is 0.2338Ω; therefore, the excitation current-limiting resistance is RQL = 7.73Ω, which is connected to the rotor circuit via a contactor. Capacity of the excitation current-limiting resistor: At a maximum voltage of 220×(1+20%)=264V, the current in the excitation circuit is 264÷7.9638≈33.15A, and the power is PQL=33.152×7.73≈8.495kW. Since the excitation operation lasts for a short period of time (1 second), a resistor capacity of 1/2 (4.2kW) is sufficient. The optional excitation current-limiting resistor model is RX-7.5Ω/5kW. 2.6 Calculation of strong excitation and strong de-excitation 2.6.1 Calculation of the maximum excitation voltage during strong excitation: Excitation multiplier: KF=250.4÷118=2.12. 2.6.2 Calculation of the minimum excitation voltage during strong reduction: Ufd-min=1.35×UL×cos150°=1.35×240×(-0.866)≈-280V. During strong reduction, the magnetic field energy is rapidly released through inverter-based demagnetization. 2.7 Calculation for cable selection: The cable selected shall meet the requirements for long-term operation at 1.1 times the excitation current, and at the same time satisfy the requirements of ease of on-site installation and cost-effectiveness. The current density of the cable is 2.5 A/mm2. Cross-sectional area of the rotor-side cable conductor: SZ = (Ile × 1.1) / 2.5 = (441 × 1.1) / 2.5 ≈ 194 mm2; a YJV-240 cable can be used in practice. Cross-sectional area of the low-voltage side cable of the excitation transformer: SJ = (Ile × 1.1 × 0.816) / 2.5 = (441 × 1.1 × 0.816) / 2.5 ≈ 158 mm2. A YJV-185 cable can be used in practice. The rated voltage of the cable is 0.6–1 kV. 3. Excitation regulator 3.1 Main functional requirements: A microcomputer-based excitation regulator is selected due to its mature technology and superior performance; it generally fits well with the aforementioned parameters and meets the requirements of the excitation system. Some specific functions and parameter values that require special consideration and corresponding action requests are as follows: 3.1.1 Regulation and control functions ① Automatic operation mode refers to PID regulation of the generator’s terminal voltage, while manual operation mode refers to PID regulation of the generator’s excitation current ； ②Reactive power regulation for load sharing; the direction of positive or negative load sharing can be set arbitrarily, and the load sharing coefficient can be adjusted within the range of 0–15%, with increments of 1% ； ③Excitation control in self-excitation mode ； ④Ensure the stable operation of the generator under no-load, loaded, and other operating conditions, as well as smooth regulation of reactive power ； ⑤Generator operates at constant power factor (optional) ； ⑥Generator operates at constant reactive power (optional). 3.1.2 Limiting and protection functions ① PT open-circuit protection ； ②Overexcitation limitation and protection ； ③Under-excitation limitation and protection ； ④No-load overvoltage protection ； ⑤Thyristor fault detection ； ⑥Excitation current limitation is applied when the rectifier cabinet loses air supply or when some of the rectifier cabinets are taken out of service ； ⑦All limitation and protection setting values, as well as the activation and deactivation of each protection, can be adjusted on-site. 3.2 Main technical parameter requirements ① Voltage regulation range: 5%-130%. ②Voltage regulation accuracy < 0.5%. ③Phase shift range: 0-150°, with upper and lower limits that can be set programmatically. ④Reactive power regulation: The software enables reactive power adjustment, with the level difference available in either positive or negative direction at a value of ±1%. ⑤Frequency characteristic: For every 1% change in frequency, the voltage at the generator terminal changes by no more than ±0.25% of the rated value. By leveraging new technologies and materials, and carrying out technical upgrades to increase the capacity of the units during the plant’s major repairs, it is possible to tap into the plant’s potential to a certain extent and improve power generation efficiency, which is in line with the principles of green energy conservation. Based on an example of the capacity expansion technical renovation of a unit in a medium-sized hydropower station, this paper introduces the calculation process for selecting the excitation system of generator sets during such renovations, providing useful references for the operation, maintenance, and technical upgrades of small and medium-sized hydro-generator sets. Author: Nong Shaoan