3.2.2. Hierarchical Out-of-Step Protection

In order to solve the protection principle defect and the action coordination problem under the multi-generators connected at one bus out-of-step conditions, combined with the existing three-component out-of-step protection criterion, the hierarchical out-of-step protection is constructed based on multi-generators information fusion. The out-of-step fault form can be identified by the cooperation of the station and local protection criterions, and the protection fixed value can be adaptively adjusted according to the operating mode of the power plant and the power system. The hierarchical out-of-step protection scheme is shown in Figure 9.

**Figure 9.** Hierarchical out-of-step protection.

The basic protection configuration, action characteristics and tripping strategy are set as follows:

(1) The basic protection configuration. In order to detect the multi-generators out-ofstep fault condition, the station layer out-of-step protection criterion is set at the

bus. The local out-of-step protection criterion of each generator is retained to detect the single generator out-of-step fault. Through information interaction with the station layer protection, the local out-of-step protection criterion can adapt to different out-of-step modes and operating conditions.


## 3.2.3. Simulation Analysis of Protection Action Conditions under Multi-Generators Out-of-Step Fault

In order to verify the effectiveness of the hierarchical out-of-step protection scheme, the equivalent model shown in Figure 10 is constructed based on PSCAD/EMTDC software. Figure 10 shows that in large hydropower plant 1, 1# generator–transformer group and 2# generator–transformer group operate in parallel at the same bus M1. They are connected to the infinite system S through the 500 kV double-loop AC lines with the length of 200 km. Among them, G2 and T2 are formed by equivalent generator–transformer units. They are under the same working conditions and with the same capacity. The actual number of the generator–transformer units depends on the specific simulation condition, and the maximum is three. In hydropower plant 2, G3 and T3 are formed by equivalent generator–transformer units with four parallel operating generator–transformer units. All the generator–transformer units in the model are equipped with prime mover, governor and excitation systems. The model can reproduce the operation condition and control process of the actual multi-generators system to some extent and meet the basic requirements of the multi-generators out-of-step fault simulation.

Four generator–transformer units are set to operate in parallel at bus M1. Before the fault occurs, each generator sends out the active power as P = 600 MW and reactive power as Q = 167 Mvar. Four generator-transformer units are set to operate in parallel at bus M2. Before the fault occurs, each generator sends out the active power as P = 400 MW and reactive power as Q = 116 Mvar. The three-phase short-circuit fault is set at 0.5 s on Line1 II near the bus M1, and the fault line is removed at 0.61 s by protection action. The power angle changes of the three equivalent generators are shown in Figure 11.

**Figure 10.** Schematic diagram of the out-of-step fault simulation model system.

**Figure 11.** Power angle curve of the equivalent generators.

In Figure 11, after the fault is removed for a period of time, the equivalent generators G1 and G2 are out-of-step, and the equivalent generator G3 is oscillating synchronously. Therefore, the equivalent generators G1 and G2 constitute the situation of multi-generators connected at one bus out-of-step fault, and the equivalent generator G3 wobbles. According to the variation of the measured impedance trajectory at each bus during the simulation period, the action characteristics of the station layer out-of-step protection are shown in Figure 12.

**Figure 12.** *Cont*.

**Figure 12.** Measured impedance trajectory and the station layer protection action characteristics at each bus: (**a**) measured impedance trajectory at M1 and the station layer out-of-step protection action characteristics and (**b**) measured impedance trajectory at M2 and the station layer out-of-step protection action characteristics.

In Figure 12, the measured impedance trajectory at M1 can stably enter the action area of the station layer out-of-step protection. In order to verify the judgment of the station layer out-of-step protection, the local layer action areas of the generators G1 and G2 are defined as the multi-generators out-of-step mode. However, since the measured impedance trajectory at M2 does not enter the action area of the station layer protection, it can be judged that the equivalent generator G3 is only synchronous oscillation. Therefore, the station layer judges that there is not the multi-generators out-of-step fault at M2, so the local layer action area of G3 is defined according to the single generator out-of-step mode, which is the same as the existing out-of-step protection action area. The impedance trajectory measured at the terminal of each equivalent generator and the corresponding out-of-step protection action characteristics are shown in Figure 13.

**Figure 13.** *Cont*.

**Figure 13.** Measured impedance trajectory and the local layer protection action characteristics: (**a**) measured impedance trajectory of G1 and the local layer out-of-step protection action characteristics, (**b**) measured impedance trajectory of G2 and the local layer out-of-step protection action characteristics and (**c**) measured impedance trajectory of G3 and the local layer out-of-step protection action characteristics.

In Figure 13, the red dotted line represents the action characteristic of the proposed hierarchical local layer out-of-step protection and the blue dotted line represents the action characteristic of the existing out-of-step protection. According to the simulation results in Figure 13a,b, the measured impedance trajectory of the out-of-step generator only enters the action area of the existing out-of-step protection action characteristic within a short time, and the impedance trajectory of the stable out-of-step oscillation is completely outside the action area. Thus, the existing out-of-step protection refuses to act. Since the hierarchical out-of-step protection has considered the increasing effect of the oscillating current on the system impedance consisted by other generators, the protection action area has been adjusted. The simulation results in Figure 13a,b show that the multi-generators connected at one bus out-of-step fault can be accurately identified, and the crossing time of the measured impedance trajectory meets the requirements. In Figure 13c, since station layer has judged that the equivalent generator G3 is only synchronous oscillation, the protection action characteristics of the hierarchical local layer out-of-step protection and the existing out-ofstep protection are the same. The simulation results show that the measured impedance trajectory does not enter the action area, and the protection will not act.

Hierarchical out-of-step protection can realize the coordination between the station and local layer protection through real-time information interaction between different layers and can effectively solve the problem that the existing protection scheme cannot accomplish the identification of multi-generators connected in one bus out-of-step fault conditions. The protection area boundary can be adaptively adjusted according to the actual operation mode of the system and the power plant, so that the protection criterion can adapt to the real-time operating condition. In addition, by fusing the protection information from the stability control system of the grid-side, a reasonable generator tripping strategy can be constructed to guarantee the safe operation of the generators and the power grid.

#### **4. Key Problems of the Hierarchical Power Generation-Side Protection System**

There are many problems which need to be studied to construct a complete multiinformation fusion-based hierarchical power generation-side protection system. The key problems include the following aspects shown in Figure 14.

**Figure 14.** Key problems of the hierarchical power generation-side protection system.

#### *4.1. Construction Mode of the Hierarchical Power Generation-Side Protection System*

The local and station layer protection function should be determined based on the security and coordinated operation requirements of the protection performance and the information acquisition requirements of the hierarchical protection system. In order to determine the information organization and interaction mode among the hierarchical protection layers between generation units and between the generation and grid sides, the information from the CMS and the protection layers need to be analyzed. Moreover, the coordination mechanism between the station layer protection and the local layer protection needs to be studied.

#### *4.2. Communication Technology of the Hierarchical Power Generation-Side Protection System*

According to the data interaction requirements of the hierarchical protection system, the communication network mode and the data protocol need to be designed. In addition, the communication interaction method between the system generation and grid sides needs to be determined. The information redundancy and fault tolerant identification technology need to be studied to analyze the reliability of the communication system. Furthermore, the big data technologies need to be studied such as information mining and application, data organization and management, etc.

#### *4.3. Research Approaches of New Hierarchical Power Generation-Side Protection Methods*

The multi-information fusion-based hierarchical protection system opens a new way to improve the power generation-side protection. The research of the new hierarchical protection power generation-side protection methods can be started from the following aspects:

(1) Research on the supplement and improvement of the existing protection principle. This includes the improvement of the existing local protection and grid-related protection. The improved protection principle should fully adapt to the protection requirements under the complex power network operating environment.

