Research and application of relay protection setting calculation for SFC system of pumped storage power station

Pumped storage is a key way to meet the regulatory needs of the power system. Currently, SFC starting has become the main starting method for large pumped storage power plants. This article introduces the typical structure and operation principle of SFC in pumped storage power plants. Based on existing guidelines, the relay protection configuration and setting principles of the SFC system in pumped storage power plants are elaborated. Starting from engineering practice, a relay protection setting calculation scheme for SFC input and output transformers is proposed and put into operation on site, ensuring the reliable and stable operation of the SFC system in pumped storage power plants.


Introduction
Under the goal of "carbon peaking and carbon neutrality", new energy will develop rapidly [1].The randomness, volatility, and anti-peak characteristics of the new energy generation output increase the difficulty of system peak regulation [2][3].Pumped storage power stations have the advantages of fast start-up and flexible adjustment, which can effectively improve the long-term peak regulation and short-term power balance capabilities of the power grid, reduce the gap in peak regulation, significantly decrease wind and solar abandonment, and increase the new energy acceptance capacity of the power grid [4].The world have 10.3 million kW of newly installed capacity for pumped storage in 2022, of which 8.8 million kW is from China, accounting for over 85%.
The generator of the pumped storage power plant is a synchronous machine and cannot start automatically.Auxiliary starting equipment is necessary for dragging the generator from static speed to synchronous speed and realizing an impact-free grid connection.There are two main starting methods for reversible units in pumped storage power plants: back-to-back starting and Static Frequency Converter (SFC) startup.For pumped storage power plants with 4 or fewer generator units, SFC startup is the main startup method, and back-to-back startup is the backup startup method; For pumped storage power plants with 6 generator units, two static frequency converters are equipped as the backup for each other [5].  1 shows a typical high-low-high, 12-6 pulse SFC structure, mainly composed of an input transformer, SFC rectifier circuit, SFC inverter circuit, output transformer, smoothing reactor, and related short circuit breakers and isolation switches [6].The input transformer reduces the input voltage of SFC, which can reduce the voltage borne by each arm of the power bridge and decrease the probability of faults occurring.Furthermore, the transformer leakage reactance is beneficial to limit the fault current level of the thyristor short circuit fault.The SFC rectifier circuit, also known as the grid bridge, serves as a rectifier and maintains DC stability through triggering pulse control.The SFC inverter circuit, also known as the machine bridge, plays an inverter role by alternately applying current to two phases of the generator's stator at gradually increasing frequencies, generating a stator rotating magnetic field that is always ahead of the rotor magnetic field.The interaction between the stator magnetic field and the rotor magnetic field enables the generator to start.

Operating method of SFC in pumped storage power plants
The SFC start-up process in pumped storage synchronous motors mainly has two stages: low-speed stage (0-5 Hz) and high-speed stage (above 5 Hz) [7].In the low-speed stage, due to the low speed and low terminal voltage, the thyristor on the inverter side cannot be turned off, and the machine bridge cannot perform automatic commutation.Forced commutation is required.In the high-speed stage, the generator motor has a certain speed, and through the excitation control system, a certain voltage level is established at the machine end.SFC can work in the natural commutation stage, and the motor enters synchronous operation mode.SFC gradually adjusts the output frequency to around 50 Hz by controlling the trigger pulses of the network bridge and machine bridge.

Relay Protection Configuration for the SFC System of the Pumped Storage Power Plant
Differential protection, high-voltage side overcurrent protection, low-voltage overcurrent protection, overfrequency protection for input transformers and output transformers, SFC converter bridge differential protection, LCI DC grounding protection, and other protection functions are equipped for the domestic SFC system relay protection device [8].
The ratio differential protection of the input transformer adopts a variable slope ratio braking curve operation characteristic.The maximum sensitivity during internal faults and the ability to avoid transient unbalanced electricity during external faults are ensured by reasonably setting the starting slope and maximum slope values [9].

Relay Protection Setting Principles for SFC System of the Pumped Storage Power Plant
Overspeed, grounding fault, magnetic flux fault, rectifier/inverter bridge over current, and power supply faults protection of SFC system are realized by SFC protection but not additional relay protection devices.Relay protection setting calculation generally only considers differential protection, high-voltage side instantaneous over current protection, and high-voltage side over current protection of input and output transformers.

Example of SFC transformer protection setting calculation
The setting principle of SFC system transformer protection in pumped storage power plants is consistent with that of transformer protection in traditional thermal power plants.However, when calculating the short-circuit current of the SFC system transformer, it is necessary to consider the short-circuit current flowing through the generator bridge side winding when the grid bridge side winding is short-circuited, and the short-circuit current flowing through the grid bridge side winding when the generator bridge side winding is short-circuited.Generally speaking, the calculation of these short-circuit currents is relatively complex.For the calculation of unbalanced current, it can be measured through actual measurements.For the short-circuit current that is used for sensitivity coefficient verification, an approximate method is generally adopted, without considering the shortcircuit current passing through the rectifier bridge and inverter bridge [10].A pumped storage power plant is equipped with two domestically produced transformer protection devices and a non-electric protection device, including transformer differential protection, highvoltage side instantaneous over current protection, high-voltage side over current protection, highvoltage side overload protection, and low-voltage side over current protection.

Example of protection setting calculation 4.2.1. Protection setting calculation of differential protection.
The starting current of differential protection should be more than the maximum unbalanced current under rated load.A typical value is Iop.min=(0.4-0.6)Ie,where Ie is the rated secondary current on the reference side of the transformer. Iop.min=Krel×(Ker+ƸU+Ƹm)×Ib2n=(1.3-1.5)×(0.01×2+0.05+0.05)×Ib2n (1) In the equation, Iop.min: Starting current of differential protection; Krel: Reliability coefficient; Ker: Ratio error of current transformer; U: Error caused by transformer voltage regulation; m: Error caused by incomplete matching of voltage transformer ratio; Ib2n: Rated secondary current on the reference side of the transformer.
Instantaneous differential protection: The typical value of the operation current is (4.5-7.0)Iebased on the transformer capacity.Sensitivity verification is calculated based on the two-phase short circuit current on the power side under normal operation mode.The sensitivity coefficient is required to be greater than 2.
In the equation, Ksen: Sensitivity coefficient; Ik2: Two-phase short circuit current on the power side; n: Current transformer ratio.
Setting values of differential protection are shown in Table 2.The operation characteristic of differential protection is shown in Figure 2.
The operation time is 0.8 seconds, the SFC input circuit breaker is tripped, and the SFC output is locked.
Setting values of the high-voltage side over current protection are shown in Table 3.
where Ie is rated current.The operation time is 9 seconds, and the overload signal is tripped.Setting values of high-voltage side overload protection are shown in Table 4.
The operation time is 0.5 seconds, the SFC input circuit breaker is tripped and the SFC output is locked.
Setting values of the low-voltage side over current protection are shown in Table 5.

Output
The SFC input circuit breaker is tripped, the SFC output is locked./

Conclusion
Currently, SFC starting has become the main starting method for large pumped storage power plants, and configuring reliable relay protection devices and correct relay protection setting calculations are important ways to achieve SFC system protection.
This article introduces the typical structure and operation principle of SFC in pumped storage power plants.Based on existing guidelines, the relay protection configuration and setting principles of the SFC system in pumped storage power plants are elaborated.Starting from engineering practice, a relay protection setting calculation scheme for the SFC input transformer and output transformer is proposed and put into operation on site, ensuring the reliable and stable operation of the SFC system in pumped storage power plants.

Figure 2 .
Figure 2. Operation characteristic of differential protection.

4. 2 . 2 . 3 )
Protection setting calculation of high-voltage side overcurrent protection.The setting calculation of operation current needs to exceed the maximum load current of the high-voltage side during normal operation mode.Iop=Krel/Kr×Il.max=1.3/0.85×0.6774=1.03A(In the equation, Iop: Operation current; Kr: Return coefficient; Il.max: Maximum load current.Sensitivity verification is based on the two-phase short circuit of the low-voltage side under minimum operating mode.The sensitivity coefficient is required to be greater than 1.3.Ksen=5238/(1.05×1250)

Typical structure and operating method of SFC in pumped storage power plants 2
2 2. .1.Typical structure of SFC in pumped storage power plants Figure

Table 1 .
Parameters of a pumped storage power station SFC transformer are shown as follows: Capacity is 26.4 MVA; Connection method is Dd0y1; Rated voltage is 18/3.78/3.78kV; Rated current is 846.78A (High Voltage Side)/2016.14A (Low Voltage Side); Short circuit impedance is Uk%=8.54%.The current Transformer (CT) ratio about relay protection is shown in Table 1.Current Transformer (CT) Ratio.

Table 2 .
Differential Protection Setting Value.

Table 3 .
High-voltage Side Over Current Protection Setting Value.Protection setting calculation of high-voltage side overload protection.The setting calculation of operation current needs to exceed the rated current on each side of the transformer winding.

Table 4 .
High-voltage Side Overload Protection Setting Value.

Table 5 .
Low-voltage Side Over Current Protection Setting Value.