In various regional or county power grids, there are more or less some small hydropower plants connected to the grid. Their main characteristics are: First, the capacity of small hydropower units varies greatly, and there are many of them. Affected by factors such as peak shaving and water inflow, the units start and stop frequently, resulting in significant variations in operating modes. Second, the output of small hydropower plants often differs greatly from the load of the grid-connected substations. They mainly rely on the main grid for operation, and sudden disconnection has little impact on their safety; in case of an accident, they are generally disconnected and shut down. Third, the relay protection and automatic devices in the grid are very simple. Each tie line mainly provides simple current and voltage protection, and reclosing devices are generally not equipped with verification devices for no-voltage and synchronizing. Most small power source sides lack protection, and many do not even have switches installed. Because the overcurrent action time of most grid-connected hydropower units is relatively long, reclosing cannot coordinate with them. To prevent asynchronous reclosing of small hydropower units after a fault trip, the automatic reclosing of each tie switch must be disabled during normal operation, reducing the reliability of power supply. Adding protection at each level would require installing switches first, which is impractical. Therefore, proactive and effective protection measures are needed to solve this problem. [b]1 Selection of Protection Method[/b] A typical primary wiring diagram for a small hydropower station connected to the grid is shown in Figure 1. For substation I, there are usually two configuration types: the first type has a voltage level of 110/35/10 kV and a wiring group of Y0/Y/Δ-11; the second type has a voltage level of 220/110/35 kV and a wiring group of Y0/Y0/Δ-11, as shown in the dotted box in the figure. The protection configuration of switch 1 DL will be analyzed using the first type of wiring diagram as an example. [img=373,108]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/45.gif[/img] Figure 1 Wiring diagram of small hydropower and system grid connection 1.1 Typical protection configuration The protection configuration is: directional current time-limited instantaneous trip, directional overcurrent (or three-stage distance protection), synchronization and no-voltage verification reclosing, low-voltage low-frequency disconnection. (1) Time-limited instantaneous trip is set in coordination with the outgoing current instantaneous trip protection on the opposite side bus of this line, and avoids the fault on the other side of the transformer connected to the bus. When the setting is set to avoid the fault on the other side of the transformer connected to the opposite side bus, since the impedance of the small hydropower unit is several times larger than the impedance of the protected line and transformer, there is generally no protection range. (2) The overcurrent is set according to the maximum load current passing through in the forward direction of the protection, and the time limit is coordinated with the highest time limit of the next level protection. The maximum load current in the forward direction should be considered after the substation II load is shed fh, but when the small hydropower is running in a small mode, the sensitivity is often insufficient. (3) The distance protection of small hydropower is a weak power source, and its short-circuit current level is low, which makes the performance of the distance protection device in an unstable zone, and the investment is high and it is rarely used. (4) Synchronization and no-voltage verification reclosing are mostly difficult to synchronize after disconnection, and the verification of synchronization reclosing cannot be successful. Because the power of small hydropower and substation II is generally difficult to balance, it eventually leads to the collapse of the small network. (5) After the low-voltage low-frequency disconnection loses the large power source, because the capacity of small hydropower is small, when the power deficit is large, the frequency and voltage drop quickly, the low-frequency relay cannot output, and it is only a disconnection device and cannot be used as line protection, so it is not advisable to use it. 1.2 Current Protection Tripping Mode: The protection configuration is as follows: Switch 1 DL direction current delay action trips switch 2 DL. In the event of a fault in tie lines XL1 or XL2, the small hydropower station is tripped. This is coordinated with the outgoing line instantaneous (or time-limited instantaneous) protection on the opposite busbar of this line, and must meet the maximum load current passing through in the forward direction. Sufficient sensitivity is required in the event of a fault in tie lines XL1 or XL2. The maximum operating current calculated based on sufficient sensitivity for each tie line fault, under the small hydropower station large mode, often cannot meet the maximum load current passing through in the forward direction, limiting the operation of the small hydropower station; it can easily lead to the tripping of the small hydropower station when substation II suddenly sheds load fh. 1.3 Voltage Protection Tripping Mode: The protection configuration is as follows: Switch 1 DL direction low voltage delay action trips switch 2 DL. In the event of a fault in tie lines XL1 or XL2, the small hydropower station is tripped. When it is a transient fault, power supply to users is restored by reclosing the switches on the F side of system XL1 and XL2. Voltage protection is coordinated with the next level of protection (Stage I or Stage II) and requires sufficient sensitivity in case of faults in tie lines XL1 and XL2. Voltage protection tripping overcomes the shortcomings of current protection tripping: firstly, the smaller the operating mode of the small hydropower station, the higher the sensitivity of voltage protection; secondly, voltage protection does not limit the power output of small hydropower stations. It can improve the reliability of power supply to users and adapt to various operating modes of small hydropower stations, making it an economical and effective protection method. To prevent voltage circuit disconnection, a voltage circuit disconnection interlocking device (such as the LB-1A relay from Xuji Machinery) should be installed. In the dead zone of directional elements and when protection or switches fail to operate, to prevent asynchronous reclosing, a verification device for no-voltage and synchronization should be installed in the reclosing device of the switches on the F side of tie lines XL1 and XL2. According to the actual power grid, a directional current-voltage interlocking Stage I can be added to protect a part of the line with fast protection devices, and a current blocking circuit can be added to enhance the reliability of protection. When there is a coordination relationship between low voltage and reverse protection devices, directional elements can be omitted or disabled. 2 Voltage Analysis at Protection Installation Point During Fault In the simple power grid shown in Figure 2, let the positive sequence impedance of the system referred to the fault point be equal to the negative sequence impedance as X, the zero sequence impedance of the system referred to the fault point be X0, and the positive sequence impedance of the voltage calculation point on one side of the transformer from the fault point on the other side be ΔX. All values ​​in the calculation are taken as their per-unit modulus. Then the positive and negative sequence components are: [img=233,48]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g46-1.gif[/img]（1） [img=342,97]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/46-1.gif[/img][align=left] Figure 2 Simple power grid schematic diagram 2.1 Three-phase short circuit I=1/X, U=ΔX/X (2) 2.2 Two-phase (B and C phases) short circuit [img=231,46]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g46-2.gif[/img] (3) (1) The voltage vector diagram of the two-phase short circuit calculation point on one side of the Y/Y-12 transformer is shown in Figure 3, then: Substituting equations (1) and (3) into the above equation, we get: [/align][img=124,16]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-1.gif[/img] Since the above voltages are per-unit values ​​based on phase voltages, they need to be calculated based on line voltages, so: [img=187,17]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-2.gif[/img]（4） [img=211,198]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/46-2.gif[/img] Figure 3 Short circuit of phase BC on one side of Y／Y-12 transformer (2) The voltage vector diagram of the calculation point of the two-phase short circuit on the Δ side of Y／Δ-11 transformer is shown in Figure 4. [img=361,95]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-3.gif[/img]（5） [img=159,201]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/47-1.gif[/img] Figure 4 Short circuit of BC phase on the Δ side of Y／Δ-11 type transformer (3) The voltage vector diagram of the calculation point of the two-phase short circuit on the Y side of Y／Δ-11 type transformer is shown in Figure 5. [img=361,95]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-4.gif[/img]（6） [img=161,197]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/47-2.gif[/img] Figure 5 Y／Δ-11 type transformer Y side BC phase short circuit 2.3 Two-phase (B and C phases) short circuit to ground [img=310,136]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-5.gif[/img] (1) The voltage vector diagram of the calculation point of the two-phase short circuit to ground on the Y0 side of the Y0 type Y0/Y-12 is shown in Figure 6. [img=340,131]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-6.gif[/img]（7） [img=209,196]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/47-3.gif[/img][align=left] Figure 6 Y0／Y-12 type transformer Y0 side BC phase short circuit grounding (2) Y0／Δ-11 type Y0 side two phase short circuit grounding calculation point voltage vector diagram is shown in Figure 7. [/align][img=332,221]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g47-7.gif[/img]（8）[img=189,194]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/47-4.gif[/img] Figure 7 Y0／Δ-11 type Y0 side BC phase short circuit grounding 2.4 Single phase (A phase) grounding IA1=IA2=1／（2X＋X0） UA1=（X＋X0）／（2X＋X0） UA2=X／（2X＋X0） （1） Figure 8 shows the voltage vector diagram of the calculation point for single-phase grounding on the Y0 side of the Y0 type transformer. [img=236,126]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g48-1.gif[/img]（9） [img=198,135]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/47-5.gif[/img] Figure 8 Y0/Y-12 type transformer Y0 side A phase grounding (2) Figure 9 shows the voltage vector diagram of the calculation point for single-phase grounding on the Y0 side of the Y0 type transformer. [img=212,133]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g48-2.gif[/img] （10） [img=206,209]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/48-1.gif[/img] Figure 9 Y0／Δ-11 type transformer Y0 side A phase grounding [b]3 Voltage protection disconnection setting calculation[/b] 3.1 Operating voltage is set according to protection sensitivity (1) When the fault is three-phase short circuit, two-phase short circuit and two-phase short circuit grounding, we can get from equations (2), (4) and (7): [img=276,37]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g48-3.gif[/img] Where Udz.j is the operating voltage of the low-voltage relay; Zbh is the impedance from the protection installation point to the end of the protected line; Zxtf.max is the impedance of the small hydropower system on the back side of the protection installation point under the maximum operating mode; KE is the ratio of the generator potential to the primary rated voltage of the PT, taken as 1.1; KK is the reliability coefficient, taken as 1.3. Since the impedance of small hydropower units is relatively large, if Zxtf.max is 3 times Zbh, then Udz.j = 35.75V. Taking the minimum scale of the DY series low-voltage relay with a rated voltage of 100V as 40V can fully meet the setting requirements. If the setting value requirement is smaller, a static voltage relay can be used. (2) When the fault is a single-phase grounding of line XL1, it can be seen from equation (9) that the relay voltage at the protection installation point is greater than 50 V, so the low voltage protection tripping is impossible. However, when the single-phase grounding protection of line XL1 near the F-side switch of the system trips, for substation I: if the neutral point of the high voltage side of the main transformer is not grounded, its gap protection clears the fault with a certain delay (generally 0.5 s); if the neutral point of the high voltage side of the main transformer is grounded, its zero-sequence protection clears the fault with a certain delay. Therefore, when a single-phase grounding occurs, the main transformer gap protection and zero-sequence protection are coordinated with the reclosing of the F-side switch of the system to make up for the deficiency of low voltage protection tripping. 3.2 Coordination of operating voltage with each outgoing line of the opposite busbar Taking the wiring shown in Figure 1 as an example, let the reactance of the protection extended to the lower-level line be X. When the operating voltage of the low voltage relay is Udz.j, there is the following equation. After obtaining the extension range X, the coordination relationship with each outgoing line of the opposite busbar can be determined. [img=273,39]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g48-4.gif[/img] Where KZ is the auxiliary coefficient, equal to (XMF.min + XXL2 + XNf.min) / XMF.min; XNf.min is the reactance of the small hydropower station f referred to bus N under the minimum mode; XMF.min is the reactance of the system F referred to bus M under the minimum mode; XXL2 is the reactance of line XL2. 3.3 Coordination of Operating Voltage with the Third Side of the Main Transformer In the wiring diagram shown in Figure 10, bus M is the location where the low-voltage protection is disconnected. Assume the zero-sequence reactance of the line belonging to bus N is n times its positive-sequence reactance, and the per-unit value of the low-voltage operating voltage is Udz. By determining the range X1 extending from bus N under various fault conditions, the coordination relationship with each outgoing protection of bus N can be easily verified. [img=321,90]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/48-2.gif[/img] Figure 10 Schematic diagram of wiring coordination with the third side of the main transformer. Referring to Section 2, the following correspondence exists: ΔX=K1fz.X1M＋X1N＋X1 X=X1NΣ． min＋X1 X0=X0NΣ． In the formula max＋n.X1, X1NΣ． min——i.e. (X1f．min＋X1M)∥(X1F．min＋X1N), the comprehensive positive sequence reactance referred to bus N under the minimum operating mode; X0NΣ． min——the comprehensive zero sequence reactance referred to bus N under the zero sequence large operating mode; K1fz——i.e. X1F． min／(X1f．min＋X1M＋X1F．min), the positive sequence branch coefficient on the side of bus M; X1f．min——the positive sequence reactance of small hydropower f referred to bus M under the minimum operating mode; X1F． min——the positive sequence reactance of system F referred to the main transformer O point under the minimum operating mode; X1M——the positive sequence reactance between bus M and the main transformer O point; X1N——the reactance on the third side of the main transformer; X1——the positive sequence reactance of the low voltage protection extending from bus N. Then, the equations for solving X1 are as follows: (1) For a single-phase ground fault, substituting the values ​​of ΔX, X, and X0 into equation (10), we get: [img=400,45]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g49-1.gif[/img] (2) For a two-phase short-circuit ground fault, substituting the values ​​of ΔX, X, and X0 into equation (8), we get: [img=389,92]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g49-2.gif[/img] The above equation is a quartic equation in one variable, which can be solved quickly by using a microcomputer and the optimization method or the bisection method. (3) Two-phase short circuit From equations (5) and (6), it can be seen that the relay voltage at the protection installation point is greater than 50 V, so the low voltage protection cannot extend out of the main transformer. (4) Three-phase short circuit Substituting the values ​​of ΔX and X into equation (2), we get: [img=227,42]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g49-3.gif[/img] 3.4 Coordination of operating voltage and back-side protection Taking the wiring shown in Figure 1 as an example, let the reactance of the low voltage protection of switch 1DL extending in reverse to the bus N be X, when the operating voltage of the low voltage relay is Udz. When j, the following equation holds: [img=263,38]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g49-4.gif[/img] Where XfNΣ.min——the reactance of the small hydropower station referred to bus N under the minimum operating mode; XFNΣ． min——the reactance of the system F referred to bus N under the minimum operating mode. Based on the calculated X, if there is a coordination relationship with the protection of each switch belonging to the back-side bus, the directional element can be disabled to eliminate its dead zone. 3.5 Setting of Operating Time Based on the above calculations, if the low-voltage protection has a coordination relationship with the instantaneous time period of other related protections, its operating time tdz = Δt; if it needs to coordinate with the II stage of related protections, the operating time tdz = tII + Δt; the reclosing time tzch of each tie switch on the opposite side refers to the delay in reclosing after successful verification of no voltage and synchronization. If the time relay and switch have excellent time performance, the minimum time difference Δt of the protection can be taken as 0.3s to quickly clear the fault point. Under the premise that the insulation strength of the fault point is restored, the minimum reclosing time tzch can also be taken as 0.3s to quickly restore power supply. [b]4 Application Example[/b] In May 1998, the newly built 2×3000 kW units of the Zhanghe Hydropower Station were put into operation. Its primary system diagram and impedance per unit value are shown in Figure 11. [img=390,113]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/49.gif[/img] Figure 11 Wiring diagram of Zhanghe Hydropower Station and its grid connection. At 4:35 AM on June 10, 1998, the Yanhua Hydropower Station, without reporting to the dispatch center, unilaterally reduced its load and shut down for maintenance, causing the Zhang33 switch to trip due to directional overcurrent. At 10:08 AM on the same day, the same protection tripped again due to excessive power output from the hydropower station. At 4:15 PM on the 11th, the same protection tripped again due to the hydropower station's failure to control its power output. In previous operations, there have been several instances where the tripping of the Zhang35 switch simultaneously caused the tripping of the Zhang34 switch. The original operating settings were based on the operation of a single 800 kW unit, providing sufficient sensitivity for faults on the Zao side of the Zhanghe line: [img=357,115]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hubeidl/hbdl99/hbdl9901/image1/g49-5.gif[/img] However, if the settings were based on avoiding the maximum load current, it would be obvious that even with all three 800 kW units operating at full capacity, the protection sensitivity requirements would not be met. Therefore, other components were needed to replace the current-sensitive components. On June 28, 1998, based on the above principle, the directional overcurrent disconnection of the Zhanghe small hydropower station in the Zhanghe substation grid was changed to directional undervoltage disconnection. Since then, similar problems have not occurred, and the station has been operating normally.