An improved strategy for out-of-step oscillation separation devices based on apparent impedance angle applied to a series compensated line

Out-of-step oscillation separation devices based on apparent impedance angle trajectory are widely used in the power grid in China, and their reliability is of great importance. In this paper, the influence mechanism of series compensation on apparent impedance angle trajectory is analyzed. It reveals that series compensation capacitors reduce the equivalent impedance angle and consequently the apparent impedance angle cannot pass through all four zones, resulting in the risk of failure for the separation devices. A revised method of apparent impedance angle based on compensation principle is discussed, and then an improved out-of-step oscillation detection criterion is proposed. In view of the fact that the apparent impedance angle at the moment of maximum current is equal to the equivalent impedance angle during an oscillation process, a practical algorithm based on the compensated apparent impedance angle is proposed. RTDS tests are conducted to verify the effectiveness of the new criterion, considering transmission lines with a high series compensation level.


Introduction
As the last defense in ensuring the stability of a power system, reliable operation of out-of-step oscillation separation devices is very important. Given their different acquisition methods, current out-of-step oscillation separation devices can be divided into wide area information-based and local information-based [1]. The outof-step oscillation separation devices based on wide area information relay on the reliability of the communication system, while the security of communication information is still a thorny problem in the field of stability control [2][3][4]. Therefore, at present, out-of-step oscillation separation devices configured in China are still mainly based on local information, and the most widely used criteria are U cos ϕ trajectory principle and apparent impedance angle principle [5][6][7]. The out-ofstep oscillation detection criterion based on the U cos ϕ trajectory can judge whether the system is out-of-step according to the voltage of the oscillation center during an oscillation process. It has the capability to accurately identify the time when the phase difference between equivalent generator electromotive forces at two ends swings to 180°, but it cannot judge the position of the oscillation center [6,8]. The out-of-step oscillation detection criterion using apparent impedance angle calculates the phase angle difference between voltage and current at a point along the oscillation line. This indirectly reflects the variation of phase difference between the two equivalent generator electromotive forces, and then whether the system is out-of-step can be judged [7]. The premise of its correct application is that the line impedance angle is close to 90° in the traditional two machine equivalent system. However, when the criterion is applied to transmission lines with a high series compensation level, the previous boundary conditions are often difficult to meet. In particular, UHV transmission lines are often interconnected with two large power grids, so the equivalent system impedances of both sides are small while the series compensation level relative to the system impedances is very large. Therefore, the equivalent line impedance angle is far away from 90°, and thus, the reliability of out-ofstep oscillation separation device based on apparent impedance angle trajectory will face challenges. Series compensation is a common practice to reduces the electrical distance of transmission lines, and this is beneficial in improving the stability of a power grid. With the gradual promotion of UHV long-distance transmission in China, the application of series compensation is more and more extensive. Therefore, it is necessary to study the influence of series compensation on out-of-step oscillation characteristics and the reliability of out-ofstep oscillation separation devices. At present, to solve the problems caused by the application of series compensation to the secondary equipment, power engineers around the world have undertaken a lot of theoretical and practical research. Most of it has focused on the field of relay protection [9][10][11], while there have been only few studies on the reliability of out-of-step oscillation separation devices.

Open Access
In research on the out-of-step oscillation detection criterion based on apparent impedance angle, reference [7] points out that when a three-phase short circuit fault occurs in the opposite direction but subsequently disappears by itself, the criterion will fail to operate, and a relevant auxiliary criterion based on measured dynamic trajectory must be taken into account to improve the reliability of the criterion. In [12,13], the criterion is improved by adding auxiliary conditions such as starting criterion and current trend criterion. In [14], an improved criterion using the threshold of voltage envelope as the unlocking condition is proposed, which can strictly distinguish between the process of short circuit and out-of-step oscillation. In [15], the influence mechanism of continuous commutation failure on the out-ofstep oscillation detection criterion is analyzed, while [16,17] consider the influence of the different voltage amplitudes and unit regulations at both sides on the oscillation center offset. However, the above studies cannot reduce the risk to the out-of-step oscillation separation device caused by the deviation of equivalent impedance angle from 90°. This paper investigates the influence of series compensation on the apparent impedance angle trajectory and out-of-step oscillation criterion, and an improved criterion is proposed to address the issues. The remainder of this paper is organized as follows. Section 2 introduces the out-of-step oscillation detection criterion using apparent impedance angle. This criterion is widely used in engineering practice. Section 3 analyses the influence of series compensation on the out-of-step oscillation separation device, and points out the risks. An improvement algorithm from the engineering application perspective is proposed in Sect. 4, while in Sect. 5, RTDS tests are carried out to verify the reliability of the proposed new criterion. Conclusions are drawn in Sect. 6.

Brief introduction of the out-of-step oscillation criterion based on apparent impedance angle
When an out-of-step oscillation happens in a two-generator system, the phase angle difference between voltages at two ends of the line varies periodically between 0° and 360°. Assuming that the line impedance distribution is uniform and the impedance angle is around 90°, the apparent impedance angle along the line varies periodically between 0° and 180° or 180° and 360°. This feature is implemented in the out-of-step oscillation separation device using apparent impedance angle to identify an out-of-step oscillation [7,14]. In an actual power grid, because of the existence of transformers, reactive power compensation equipment and other components, these two assumptions are no longer justified. Thus, the apparent impedance angle ϕ does not strictly vary in the range of 180° during the out-of-step oscillation.
In view of the above, the impedance in the four-quadrant phase plane is divided into six zones as shown in Fig. 1. When ϕ varies by going through zones I-II-III-IV or I-VI-V-IV or I-IV, it can be considered that an out-of-step oscillation has occurred, and the direction of oscillation center can also be determined. At present, the boundaries of the zones are set as ϕ 1 ∈ (50 • , 60 • ) and ϕ 2 ∈ (120 • , 130 • ) in engineering practice. This is based on large amounts of field operational experience and sufficient experimental verification. It can also well be adapted to the situation where the actual line impedance angle is close to 90°.

Relationship between apparent impedance angle and equivalent impedance angle
For the two-generator system shown in Fig. 2, assuming the out-of-step oscillation separation device is installed at point M. Ė M and Ė N represent the two equivalent generator electromotive forces, Z M and Z N are the internal impedances, U M and U N are the voltages at both sides of the line, İ M is the current of point M, and Z MN represents the line impedance. The electromotive force amplitude difference of the two generators is ignored while it is assumed that the impedance angles of the whole system are the same. The phasor diagram of voltage and current on the tieline is shown in Fig. 3, where δ is the phase angle difference of generator electromotive forces at the two ends, is the equivalent line impedance angle, and θ is the complementary angle of .
The apparent impedance angle of point M is: Assuming Z = kZ M , the following result can be obtained: If point M is near the oscillation center (i.e., k = 2), then: Otherwise, ϕ can be expressed as: That is: From (3) and (5) In summary, regardless of where the oscillation center is, the maximum value of ϕ is (90 • + �) . If is less than 90°, the maximum value of ϕ will be less than 180°.

Influence of series compensation on the out-of-step oscillation separation device
Clearly, the series compensation capacitor of the transmission line further reduces the value of , as well as the measurement of the maximum value of ϕ . In severe cases, if ϕ max = (90 • + �) < ϕ 2 , ϕ will not go through all the four zones during the oscillation process. This will cause the failure of the separation device. With the different and compensation proportion k C , the value of ϕ max is shown in Table 1.
The boundary between zones III and IV is (120°-130°) in engineering practice. It can be seen from the figures in Table 1 that, when there is a high series compensation level, the out-of-step oscillation separation device based on the apparent impedance angle principle has the risk of failure. The higher the compensation level, the greater the risk. The compensation level of UHV AC lines in China is generally less than 40%, but in other countries, the compensation ratio can reach 70%, so the influence of series compensation cannot be ignored. Fig. 2 Schematic diagram of two-generator equivalence system

Experimental verification for the influence of series compensation
Using practical parameters of a regional power grid in North China, an RTDS test verifies the risk mentioned above. Figure 4 shows An out-of-step oscillation separation device is installed at BUS3 to monitor the oscillation condition of Line 5. Its parameter related to oscillation center direction is set to allow action, regardless of whether the oscillation center lies in the positive or negative direction. The lowest threshold value of the voltage profile used to judge the control action zone is set to 80 V.
During the test, all series compensations are put into operation. When Line7 is disconnected without fault, an out-of-step oscillation occurs. The oscillation center is between BUS3 and BUS4, and the equivalent impedance angle is about 68°. Some electrical quantities at the installation point of the out-of-step oscillation separation device are shown in Fig. 5, where "Phase angle difference" refers to the voltage phase angle difference between BUS3 and BUS4.
It can be seen that in the first out-of-step period the maximum value of ϕ measured by the out-of-step oscillation separation device is 123.79°. If the set value of ϕ 2 is greater than 124°, ϕ cannot complete the crossing from zone I to zone IV, and consequently the separation device cannot identify the out-of-step oscillation. Only in the second out-of-step period can the device identify effectively and the action time is delayed by about 500 ms.
In summary, the results of theoretical analysis and dynamic simulation both show that when an out-of-step oscillation occurs on a series compensated transmission line, the apparent impedance angle cannot accurately reflect the variation of phase angle difference between generator electromotive forces at two ends because the equivalent impedance angle no longer meets the requirement of approaching 90°. For this reason, the out-of-step oscillation separation device based on local apparent impedance angle has the risk of failure.

Principle of apparent impedance angle compensation
To accurately reflect the variation of phase angle difference between generator electromotive forces at the two ends of the line during the oscillation process, the apparent impedance angle must be revised. As shown in Fig. 3, for any , ϕ + θ can accurately reflect the variation law of δ . Therefore, ϕ ′ = ϕ + θ can be used to identify the out-of-step    oscillation instead of ϕ , where θ is defined as the compensation impedance angle. The value of can be obtained from field measurement, and can also be calculated according to the structural parameters of the power grid. Assuming the system equivalent impedance is R + jX , � = arctan(X/R) and the compensation impedance angle is: Considering the series compensation capacitor, the calculation of can be modified as: In this way, the apparent impedance angle can be compensated as: Using the compensated apparent impedance angle to identify the out-of-step oscillation can avoid the failure due to the inaccurate assumption.

Practical algorithm of the compensated apparent impedance angle
The calculation of ϕ ′ requires the equivalent impedance angle information, but a power system oscillation is complex and volatile, which increases the difficulty in the setting in real applications. In addition, the series compensation capacitors may be switched on/off or adjusted at any time during operation. The out-of-step oscillation separation device needs to be adjusted with the compensation parameters in real time according to the change of operational mode. This increases the risk of operation mismatch in application. As can be seen from Fig. 3, when the power angle swings to 180°, is equal to ϕ , which is also the time when the line current reaches its maximum. Therefore, in the process of oscillation, the out-of-step oscillation separation device can record the apparent impedance angle ϕ i max at the point of maximum current, and the complementary angle can be acquired by θ = 90 • − ϕ i max . Consequently, the compensated apparent impedance angle ϕ ′ can be calculated according to (8), and the out-of-step oscillation separation device can be adapted to the real operational condition based on the trajectory of ϕ ′ .
For the previous RTDS test, Fig. 6 shows the apparent impedance angle trajectory after compensation. It can be seen that the compensated apparent impedance angle gradually increases to 145° in the first out-of-step period, goes through zones I-II-III-IV, and the separation device can identify the occurrence of out-of-step oscillation in time.

Verification tests of the new criterion
In order to further verify the reliability of the improved out-of-step oscillation identification criterion, further tests are conducted based on the previous RTDS model in Fig. 4. The tests are designed according to the requirements raised by relevant standards [18] and by taking into consideration practical operational environments, such as synchronous and out-of-step oscillations, various types of short circuit faults and other disturbance scenarios. There are four out-of-step oscillation separation devices, namely D1, D2, D3 and D4. D1 and D2 are installed on the left and right sides of Line3, whereas D3 and D4 are installed on the left and right sides of Line5.
The tests and performance of the devices are presented below.

Out-of-step oscillation
Out-of-step oscillations are simulated under different on/ off strategies of series compensation capacitors.
Scenario 1: The parameters related to oscillation center direction are set to allow action only when the oscillation center lies in forward direction. The performance of the four devices is shown in Table 5.
Scenario 2: The parameters related to oscillation center direction are set to allow action, regardless of whether the oscillation center lies in the positive or negative direction. The performance of the four devices is shown in Table 6. (2) Incomplete phase oscillations caused by circuit breaker single phase tripping. In these scenarios, all four out-of-step oscillation separation devices do not act. These results satisfy the relevant specification requirements The above tests show that the performance of the outof-step oscillation separation devices based on the principle of compensated apparent impedance angle are consistent with the design principle.

Other disturbances
(1) Various short-circuit faults at different fault points.
The fault scenarios include: temporary and permanent single-phase-to-ground faults, phase-to-phase faults and three-phase faults at various locations along Line1, Line5 and Line9, while system faults disappear spontaneously with no change of network topology (simulating faults occurring at and cleared by other lines). TV/TA single-phase or three-phase broken wires are simulated. (5) Switching on/off the device power supply.
Under the above five disturbance scenarios, all four out-of-step oscillation separation devices operate correctly. All the test results demonstrate that the functions and electrical performance of the out-of-step oscillation separation device based on the compensated apparent impedance angle meet the requirements of relevant technical specifications and standards.

Conclusion
In this paper, the influence mechanism of series compensation on the apparent impedance angle trajectory and out-of-step oscillation criterion is discussed. An improved criterion from the engineering application perspective is proposed. The effectiveness and validity of the proposed criterion have also been extensively tested using RTDS simulation. The conclusions of this investigation are summarized as follows: (1) When an out-of-step oscillation happens in a twomachine system, the series compensation capacitor reduces the equivalent impedance angle while the maximum value of apparent impedance angle at each end of the line is less than 180°, and the higher the proportion of compensation, the smaller maximum value of apparent impedance angle. In severe