A Novel Method for Deriving Optimal Synchronism-Check Phase Angle in Transmission System

1. Introduction

Most of faults occurring in transmission system are transient faults. For such faults, automatic reclosing is economic and effective method to improve the reliability and transient stability of power system [1-8]. When 3-phase and leader-follower reclosing, especially operated at a low speed, is applied in transmission systems, it is essential to check a synchronism between two separated systems before energizing transmission lines back. Many countries make use of synchronism-check relay to limit the impact associated with automatic reclosing under Live-Bus/Live-Line (LBLL) conditions by fulfilling synchronism-check conditions. Synchronism-check relays generally respond to three conditions associated with the voltage phasors on either side of the circuit breaker which is open: differences of frequency, voltage magnitude, and phase angle [9-12]. Generally, it is well known that phase angle difference is most important condition in a synchronism-check relay.

Most of utilities conventionally use a standardized conservative setting that is in the range of 20° to 30° without any explicit basis. However, it might be not a good idea because each transmission line in system has different system conditions such as system structure, line length and load flow that might result in undesirable system impacts. If a synchronism-check phase angle setting is too high, it means that reclosing operation is allowed at high phase angle. In this case, significant system impacts could take place particularly where a transmission line is in proximity to generation and operated on EHV systems. To prevent situations which are mentioned above, it is necessary to derive the Optimal Synchronism-Check (OSC) phase angle which is appropriate for each transmission system. Fortunately, IEEE has suggested good material about this [11]. Based on it, we have derived and developed a novel method to find an OSC phase angle.

In fact, papers with respect to deriving an OSC phase angle have been rarely published although papers regarding setting of reclosing relay have been enormously published [1-6]. Most papers published focus on only impacts applied to turbine-generator like torsional stresses by evaluating changes in active power when reclosing [12-14]. They recommend that synchronism-check phase angle be selected to limit the potential for generator shaft damage. A sudden power change, ΔP, is used as a screening guide factor and a ΔP value of less than 0.5 per unit can be considered negligible in terms of the impact on the turbine-generator [13-15]. However, we propose a new method composed of three steps which consider system conditions and angle stability as well as impact on turbine-generator.

In this paper, we present a novel method composed of three steps to derive the OSC phase angle. It considers not only damage on turbine-generator but also maximum possible angle under credible operating conditions by studying generation level, load level, and etc. In Section 2, the synchronism-check is discussed, specifically. In Section 3, a novel method for deriving the OSC phase angle is presented step by step. Simulations using ElectroMagnetic Transient Program (EMTP) are performed in Section 4 under various conditions. Based on the simulation results, the OSC phase angles are derived and compared to conventional values in Korea. Finally, conclusions are discussed in Section 5.

 

2. Synchronism-Check

2.1 Basic principles of synchronism-check

Synchronism-check is used to supervise autoreclosing between two portions of a system that are connected through ties in parallel with the path being closed as shown in Fig. 1. Synchronism-check is intended to limit the impact associated with autoreclosing under LBLL conditions. Where transmission systems are operated on EHV and in proximity to generation, synchronism-check typically is utilized because impacts due to switching operation might be significant [16]. Generally, synchronism-check is not used to supervise autoreclosing when there are no parallel paths and a slip frequency exists between the systems on each side of the open breaker. As we can see before, synchronism-check provides the advantage which is limiting the system impacts by preventing reclosing when a large angular separation exists between systems. However, there is a disadvantage of utilizing synchronism-check because system restoration by reclosing and manual closing may be unnecessarily restricted if the synchronism-check settings are too conservative. Therefore, it is necessary to derive the OSC phase angle which is appropriate for each transmission system.

Fig. 1.Equivalent circuit for two systems to be reconnected

2.2 Phase angle settings applied in different countries

We have conducted a survey of phase angle settings targeting various countries applying synchronism-check relay. A report describing phase angle settings according to the country is also investigated [17]. The phase angle settings applied in many different countries are shown in Table 1 according to various voltage levels. As indicated in Table 1, it can be observed that most countries except for Indonesia and United States apply a standardized conservative setting which is in range of 20° to 30°. In cases of Indonesia and United States, it seems that their own transmission systems have specific situations.

Table 1.Phase angle settings applied in many different countries

 

3. Method for Deriving OSC Phase Angle

A method for deriving OSC phase angle consists of three steps. Each step deals with situational angular conditions and impacts which can be caused during line opening or line reclosing. Contents performed at each step are summarized in Table 2. Details of each step are discussed in this section. It is assumed that a system is connected through ties in parallel with the path which is opened as indicated in Fig. 1 and reclosing is operated under LBLL condition in this method.

Table 2.Contents performed at each step

3.1 Step-1

First step for obtaining the OSC phase angle is deriving a maximum possible angle under credible operating conditions. The impact on turbine-generator shafts resulting from LBLL reclosing depends on the angle across the open breaker and impedance in the path that is being closed. Those may vary for different operating conditions.

Therefore, it is important to evaluate a range of system conditions to identify the worst potential system impact for credible operating conditions. If the synchronism-check phase angle is greater than a maximum possible angle under credible operating conditions, reclosing operation can’t be controlled by synchronism-check relay and it means that there is no need to make use of synchronism-check relay. Thus, synchronism-check phase angle should be determined as value which is below a maximum possible angle.

In general, an outage of one transmission line or generator causes change of power flow and leads to overloads in other transmission lines, so that a difference of phase angle between two systems can be larger than that prior to an outage [18]. System dynamic study can be conducted to find a range of angle which can be possible under credible operation conditions during an outage. When the system dynamic study is conducted, factors considered are as follows:

- Generation level - Load level - Seasonal load power factor - Machines out - Lines out

All factors mentioned above can change system power flow conditions so that we can derive a maximum possible angle under credible operating conditions by performing the system dynamic study using power flow program such as EMTP according to those factors. In all steps, it is assumed that a transmission line is already out, thereby considering four factors except for ‘Lines out’ in this step. Fig. 2 shows a flow chart to find out the maximum possible angle in step-1 which is called δ1 . As shown in Fig. 2, it is derived by taking a maximum value of four values (δA, δB, δC, δD) to consider the worst system condition. Each process is conducted iteratively by slowly changing conditions regarding four factors until getting the four values. The OSC phase angle should be determined below δ1 because phase angle difference above δ1 will never happen under credible operating conditions.

Fig. 2.Flowchart for deriving maximum angle δ1 in step-1

3.2 Step-2

Second step for obtaining the OSC phase angle is deriving a maximum possible angle considering impact on turbine-generator shafts. Unlike the step-1, the step-2 focuses on the impact when a reclosing operation occurs. It has been researched that the impact on the turbine generator can be evaluated by a sudden power change, ΔP, generated by a switching operation [19-23]. An IEEE Committee recommends that a ΔP value equal to 0.5 per unit be considered an acceptable screening level for evaluating steady-state switching if a turbine generator is operating under the allowable load condition [14-15]. If the ΔP value generated by a switching operation is less than 0.5 per unit, the loss-of-life of the turbine generator would generally be expected to be negligible and can be quantified as less than 0.01% per incident. Otherwise, it is recommended that the turbine generator manufacturer be consulted to determine the damage to the turbine generator.

To restrict the impact on turbine-generator shafts, it is needed to prevent the reclosing operation which can result in ΔP exceeding 0.5 per unit. Fig. 3 shows a flow chart to derive the maximum possible angle in step-2 which is called δ2. As indicated in Fig. 3, δ2 is the maximum value of angles that can result in ΔP less than 0.5 per unit in reclosing the line and it is determined through an iterative process like the step-1. In Fig. 3, α is an index for representing the degree of accuracy and if α is small, it suggests high accuracy of a process whereas time for simulations to find out an optimal setting takes longer.

Fig. 3.Flowchart for deriving maximum angle δ2 in step-2

3.3 Step-3

In final step, a maximum possible angle which can make system restored normally without any problem on system stability is derived. It is necessary to check the system stability following reclosing operation although this step, in fact, is not a dominant than the others. Integral Square Error (ISE), that is one of assessment methods for system performance, is used to check the system restored safely [24]. ISE can be expressed as follows :

where y (t) is an instantaneous angle across an open breaker and y (∞) is a convergent angle across an open breaker after reclosing. The y (∞) is the same as the angle before line trips so that it is regarded as constant value. Therefore, it is obvious that if the y (t) is divergent, ISE also becomes divergent and it means system gets unstable. The opposite case is valid too. ISE curves according to the system stability are illustrated in Fig. 4 and 5. As mentioned above, we can see that ISE gets divergent in case of system unstable but, convergent in case of system stable. Fig. 6 shows a flow chart for step-3. The flow chart is quite similar to that of step-2.

Fig. 4.ISE curve in case of system stable

Fig. 5.ISE curve in case of system unstable

Fig. 6.Flowchart for deriving maximum angle δ3 in step-3

3.4 OSC phase angle

To satisfy the factors considered in all steps, the OSC phase angle (δopt) should be determined as minimum value of angles which is derived through all steps. If it is decided that the OSC phase angle is the maximum value of angles δ1, δ2 and δ3 , it means that at least two factors are not satisfied. Fig. 7 shows a flow chart for obtaining the OSC phase angle and includes all of steps discussed above.

Fig. 7.Flowchart for obtaining the OSC phase angle

 

4. Simulation

4.1 Simulation Model

Fig. 8 shows models of the 154kV and 345 kV transmission systems in Korea used to verify the proposed method. As shown in Fig. 8, 154kV and 345kV transmission systems are composed of two and eight generators of which total capacities are 492MVA and 4896MVA, respectively. Generators of each system are identical and capacities of those are indicated in Fig. 8. Transmission lines in proximity to generation facilities are quite short and it leads to large impacts on system when reclosing. The systems are modeled using the EMTP based on actual data from Korea [25-26].

Fig. 8.Transmission system model

Fig. 9 depicts the multi-mass model of the turbine generator used to simulate the mechanical torques generated in a turbine generator shaft. Generally, the turbine generator is modeled as a single mass; however, a multi-mass model is necessary to perform time-domain simulations for the shaft torsional torques. The multi-mass model shown in Fig. 9 is composed of six masses with five shafts connecting each mass; those represent the exciter(EXC), the generator (GEN), two low pressure turbines(LPA and LPB), an intermediate pressure turbine(IP) and a high pressure turbine(HP).

Fig. 9.Multi-mass model of the turbine generator used in transmission system.

The multi-mass models of the turbine generators in both the 154kV and 345 kV systems are structurally identical. The parameters associated with the turbine generator are shown in Table 3 and 4. The parameter values shown in Table 3 and 4 are based upon the physical dimensions of the shaft system and its material properties. The most important parameter in Table 3 and 4 is the spring constant and it pertains to the elastic connection between adjacent masses and is directly involved with a torsional interaction.

Table 3.Parameters of the turbine generator in the 154 kV system

Table 4.Parameters of the turbine generator in the 345 kV system

4.2 Simulation conditions

When the simulation is conducted, it is assumed that a system is connected through ties in parallel with the path being closed and reclosing is operated under LBLL condition. Simulations are performed in various system conditions to verify the proposed method. Table 5 indicates locations of open line where those are in proximity to generation facilities or a large angular separation is more likely to happen.

Table 5.Locations of open line according to various cases

In step-2, ΔP as per unit quantity is calculated by using (2).

where Ppre is the maximum power output at the instant of switching and the Ppost is the initial power output of the unit before the reclosing. We present the torsional torque between the GEN and LPB only because it is generally the largest. Torsional torque is calculated by (3) [27].

where Tmax and Tmin are the maximum and minimum torques generated in the shaft, respectively. Fig. 10 illustrates a simple example of the maximum and minimum torque.

Fig. 10.Shaft torque generated between GEN and LPB.

4.3 Simulation results and discussion

Simulation results for step-1 in case 1 is shown in Table 6. Step-1 is conducted by changing values of various factors and maximum possible angles for each factor are derived. As we can see from Table 6, the maximum possible angle which represents the worst system condition in case 1 is 31.9°. It means that possible angle under credible operating conditions is 31.9° and angle separation above it will never happen. Therefore, we can say that OSC phase angle should be less than 31.9°. Table 7 and 8 show the simulation results for step-2 and step-3 in case 1, respectively. In case 1, there is no angle separation to cause the ΔP exceeding 0.5 per unit and it means a limit for step-2 doesn’t exist. In contrast to the step-2, a limit for step-3 exists clearly and it is 31°.

Table 6.Simulation results for step-1 in case 1

Table 7.Simulation results for step-2 in case 1

Table 8.Simulation results for step-3 in case 1

Fig. 11 shows the OSC phase angle for case 1. When comparing the OSC phase angle with conventional synchronism-check phase angle in Korea shown in Table 1, it is obvious that there is possibility to make the phase angle setting higher than the angle which is used currently.

Fig. 11.Simulation results in case 1.

Table 9 to 11 shows the results for all steps in case 2. 39.9° is derived by process of the step-1. It implies that angle separation of the 345kV transmission system is larger than that of the 154kV transmission system. It can possibly lead to serious impacts on system when reclosing in 345kV transmission system. In this case, as opposed to case 1, there exists a limit for step-2, so that we can’t choose δopt as above δ2 considering impact on turbine-generator. The δ2 for case 2 is derived as 32° from the detailed simulation results. A limit for step-3 also exists and it is 39°. Therefore, it can be concluded that OSC phase angle for 345kV transmission system should be less than 32° based on the simulation results.

Table 9.Simulation results for step-1 in case 2

Table 10.Simulation results for step-2 in case 2

Table 11.Simulation results for step-3 in case 2

Fig. 12 shows the OSC phase angle for case 2. From the results of case 2, we can see that a difference between δopt and conventional angle is not too much. Thus, we can conclude that conventional synchronism-check phase angle setting in Korea is quite appropriate and reasonable for case 2. If a system operator need to get some margin, the phase angle setting can be higher a little bit.

Fig. 12.Simulation results in case 2.

 

5. Conclusions

This paper proposes a novel method to derive the OSC phase angle, accomplished by using the three steps considering various factors. Transmission system models based on actual data and structure of Korean transmission lines which are in service are used to apply the proposed method by using EMTP. Simulations are performed in transmission lines which are in proximity to the generation, especially, in order to consider the worst conditions which can cause enormous impacts on the system. To obtain the OSC phase angle, various system conditions including generation level, load level and damage on shaft torsional stress of turbine generator are considered in each step. The simulation results show that the proposed method is valid and effective to find out the synchronism-check phase angle in transmission system. Based on simulation results, we propose the synchronism-check phase angle setting which is appropriate for Korean transmission systems. At the same time, we discuss adequacy of settings currently used in Korea. Finally, we can conclude that if the proposed method is applied especially where a transmission system is in proximity to generation facilities or a large angular separation is more likely to happen, we can solve problems due to impacts on system, effectively. In other words, we can avoid undesirable reclosing which can have a bad influence on turbine-generator and system stability