Distance Relaying with Power Swing Detection based on Voltage and Reactive Power Sensitivity

3.1 Problem description

During the fault condition, the voltage at the bus reduces and current through the line increases thus impedance seen by the distance relay decreases. Distance relay operates when measured apparent impedance (Z) enters in predefined zones (may be Zone-1, Zone-2 or Zone-3) of distance relay and stays therein for the longer duration than the set value of operating time. The possibility of mal-operation of distance relay due to tripping of Zone-3 element in the event of power swing is more as Zone-3 covers highest reach on R-X plane. Hence, functionality of Zone-3 of distance relay is considered in this study during severe system disturbances.

Figure 2 represents the MHO relay characteristic (Zone-3), inner and outer Power Swing Detection (PSD) Zones and impedance trajectory seen by the relay installed on line L2. The system shown in Figure 1 was initially loaded and the locus of measured impedance falls away from the outer PSD Zone at point X as shown in Figure 2.

Figure 2:

Impedance locus of line L2 relay at SEB during 25% load increase at REB.

The conventional power swing detection technique is shown in Figure 2 which operates on the rate of change of the impedance (dz/dt) during its travel between blinders (PSD Zone) and Zone-3. This rate of change of the impedance (dz/dt) is too slow and takes more time during power swing condition to cross the distance between blinder and Zone 3. On the other hand, its movement is faster during fault. However, during major system disturbances, this scheme may fail due to unwanted tripping of relay in third Zone [23].

The system disturbance is created at time, t=2 s by applying sudden increase in load of 25% at REB. This causes the power swing in a system and apparent impedance seen by the relay travels from original operating point - X towards Zone-3 of relay. If it stays therein for longer duration, then the relay issues trip signal and finally line L2 gets disconnected unnecessarily which results into system instability as shown in Figure 3. On the other hand, during third zone weak fault (at far end of next line), the trajectory of impedance some time remains outside the Zone-3 characteristic and relay remain inoperative after a predefine time elapse for backup protection.

Figure 3:

Distance relay operation during heavy load fall on system.

Hence, blinder characteristic based power swing blocking technique is not competent to discriminate between load encroachments and short circuit in all the way. Relay mal-operation during system disturbance other than actual fault leads to redundant system separation and affect system stability. Hence, a new method is suggested in order to prevent relay mal-operation and securing power system stability.

3.2 Proposed method

For discrimination between power swing and actual fault condition, a simple but effective method has been proposed in this work. The method utilizes the rate of change of line voltage and reactive power as a decisive factor for the required discrimination.

Any change in the power system network in terms of short circuit or change in load accompanies the variation of bus voltages and power transfer through the transmission lines. These variations in generator/bus voltages and power (active and reactive) depend on the nature of change in system configuration. With the measurement of rate of change of bus voltage V_S and line reactive power Q_S at SEB, an appropriate index is determined which can be effectively used to discriminate between fault and power swing condition.

It is observed that whenever short circuit occurs on transmission line, the voltage magnitude is considerably decreases in the faulted phase [28]. However, it is also observed that the magnitude of voltage get reduces during generator outage or sudden load increase and excitation failure which can result in negative rate of change of voltage (dV/dt). Thus, the technique based on rate of change of voltage alone may not be capable to discriminate the said two power system conditions of transmission line.

To overcome the limitations of decision criteria made on the basis of only rate of change of voltage magnitude, a new algorithm is proposed which perfectly distinguishes fault and power swing conditions. The proposed algorithm is based on combination of rate of change of bus voltages (dV/dt) and line reactive power (dQ/dt) measured at relaying point at SEB. The proposed fault and power swing discriminating algorithm is shown in Figure 4.

Figure 4:

Proposed Algorithm to Discriminate Power Swing and Fault Condition.

The occurrence of both fault and power swing brings the impedance locus seen by the relay into the operating region of Zone-3 of distance relay. But the dV/dt and dQ/dt based decision logic precisely identify the actual condition due to which the impedance locus is brought in the operating zone of the distance relay. Decision about the fault or power swing conditions depends on the following criteria:

A fault is declared, when:

Otherwise power swing is declared and activates Power Swing Blocking (PSB).

Once the fault condition is declared, the relay logic waits until the timer times out. The final check for the status of impedance locus to stay within the operating region of distance relay will be ascertained. The relay issues the trip signal if the impedance locus still present within the zone-3 boundary of the distance relay for the longer duration than timer out time.

The reactive power measured by the distance relay can be represented by:

where,

Q_s=Sending end reactive power,

V_s=Sending end voltage

V_r=Receiving end voltage (reference),

δ=angle difference between Vs and Vr,

X=Line reactance

For smaller values of angle difference (δ) and load impedance, the amount of reactive power delivered is very low. During fault condition, the value of angle difference (δ) may change drastically within a cycle. Thus, the rate of change of δ is faster in event of faults as compared to power swing condition. Moreover, during fault on line, the relay encountered only the line reactance (X) present in the system up to a fault point. The variation in active power (P_E) and reactive power (Q_S) with the change in δ is depicted in Figure 5. The reactive power increment is very high during fault as compared that of swing condition.

Figure 5:

Real and reactive power flow in transmission line during change in δ.

4.1 Power swing cases due to electrical load switching

Application of the electrical load brings the impedance locus seen by the distance relay much closer to operating region of the relay whereas the rejection of the electrical load shifts it away. Thus, the distance relay faces the problem of overreaching and may mal-operate with the application of heavy electrical load if the Zone 3 characteristic covers larger portion on R-X plane. In this work, several cases for electrical load changes are considered by increasing the load at REB by 25%, 50%, 100% and 125% above its original loading as given in Figure 1. First two window of Figure 6 shows the waveforms of voltage and current measured at the relay location in Figure 1. Also, the magnitude of rate of change of voltage and reactive power for the case when the load is increased suddenly by 25% above its original value at t=3 s (1.2e4 samples) is shown in Figure 6. The rate of change of voltage and reactive power are compared against their preset values to take decision about the power swing. The threshold values for the rate of change of voltage and reactive power are set at 0.6 pu/s and 0.9 pu/s, respectively, based on the several simulations studies for power swing conditions. It can be observed from Figure 6 that during power swing, dV/dt and dQ/dt do not cross their respective threshold values, thus trip signal of the relay is not generated. This clearly reveals that the proposed method can effectively identify the power swing condition and prevent the mal-operation of the distance relay.

Figure 6:

Variations in voltage and reactive power during 25% overloading.

4.2 Power swing cases due to mechanical disturbances

In this case, power swing is generated by changing the mechanical input to the turbine-generator system. The imbalance between mechanical power input to the turbine and electrical power output of generator results in acceleration or deceleration of the generator rotor. The rotor experiences the heavy oscillations till the imbalance between input and output power persists [29]. In this case, the mechanical power input to turbine is reduced by 20% below its original value at t=3 s. The dV/dt and dQ/dt are determined for this case and it is observed in Figure 7 that the values of these parameters are well below the set threshold value. Hence, for this case also, the relay remains inoperative against power swing conditions.

Figure 7:

Power swing due to change in mechanical power input to the turbine of generator G1.

4.3 Fault Simulation on protected line (L2)

Various fault cases are simulated on line L2 by considering different types of faults and variations in system parameters such as Fault Inception Angle (FIA), fault resistance (R_F), Fault Locations (FL) and Power Flow Angle (δ). For all these fault cases, distance relay must identify the faulty condition and operate as per the set operating time. Depending upon the zone of the fault, the operating time may be different. Figure 8 shows the variations in different parameters for the single-line-ground fault (L-g) which is created in line L2. The FIA, R_F, FL and δ are set equal to 0°, 15 Ω, 30 km and 5° respectively. With the occurrence of the fault, the voltage and reactive power changes very rapidly this can be clearly noticed from Figure 8. The dV/dt and dQ/dt cross the threshold values and results in trip signal which has been initiated after waiting upto the operating time of the relay. Hence, in Figure 8, the trip signal for relay has been generated after certain samples once the fault condition is correctly declared by the proposed method. The substantial changes in dV/dt and dQ/dt for the fault condition as illustrated in Figure 8 are defensible as compared to those shown in Figures 6 and 7 for power swing. These significant changes have been successfully utilized for the discrimination between fault and power swing conditions.

Figure 8:

Relay response during fault on line to be protected (L2).

4.4 Various fault cases during power swing

Distance relays must be blocked during power swing to ensure reliability of the power system. However, if a fault occurs during a power swing, it should be detected and the blocking function should be removed to clear the fault as soon as possible. Due to the symmetric nature of signals during the power swing, symmetrical faults are difficult to be detected. To test the proposed method, several asymmetrical and symmetrical faults are simulated on the line to be protected during different swing conditions persisting in system.

As shown in Figure 9, a power swing case is simulated at t=3 s by connecting extra load of 25% at REB and at the same time by disconnected parallel line (L1). However, the impedance trajectory doesn’t enter into third zone characteristic and hence, relay remain unresponsive. Later on, in continue power swing situation a symmetrical 3-phase fault is applied at t=4 s to validate the proposed scheme.

Figure 9:

Performance of proposed method for fault discrimination during power swing.

It has been observed from Figure 9 that the dV/dt and dQ/dt value during power swing (at 3 s) are well below the threshold and hence no trip signal has been issued. On the other hand, during fault on power swing, these values crosses respective thresholds (at 4 s) and hence relay sends trip signal as shown in Figure 9. Thus, the proposed method successfully identifies symmetrical faults during power swings for all different power system conditions. In order to test the proposed scheme and further to compare with existing scheme, few extra fault cases are simulated during power swing condition. Table 1 shows the relay operation criterion and result in term of time of operation of the proposed scheme.

The proposed algorithm is validated for the symmetrical and asymmetrical fault occurring during the power swing including variation in power system parameters like Fault Type, Fault Location and Fault resistance. A comparison between existing method [26] and proposed method is also outlined in above Table 1. It is to be observed from Table 1 that the proposed scheme operates faster than the existing scheme during various low resistance fault generated in presence of power swing. However, during high resistance fault (R_F=200 Ω) as describe in case-7 of Table 1, existing and proposed scheme both remain stable as the impedance seen by relay is greater than the reach of zone-3. In addition, during the fault at far end of transmission line (95% of line length), as per case-8, in Table 1, the existing method fails to detect the fault during swing condition whereas the proposed scheme operates and issues delayed trip signal.

4.5 Power swing cases due to post fault isolation on line L1

Various faults were simulated on line L1 and effect of power swing was observed by quick isolation of fault on line L1. For this simulation case, a L-g fault is simulated at 2.9 s on L1 and thereafter power swing is generated by isolation of same fault at 3 s by opening of circuit breakers B1 and B2 of line L1.

Figure 10 shows the value of voltage and current of line L2 during fault and on isolation of fault from parallel line. The removal of fault at 3 s on adjacent line L1 produces power swing effect. The apparent impedance enters into Zone-3 setting of relay just after the instant of fault; hence, Zone-3 timer is initiated. Conversely, as fault is removed at 3 s (by local protection of line L1), the magnitude of dV/dt and dQ/dt goes down to that of the set value before the timer times out. Thus, the proposed relay remains stable and doesn’t issue any trip signal as shown in Figure 10.

Figure 10:

System Condition during fault isolation on adjacent line.

4.6 Relay backup for fault on parallel transmission line

Since, Zone 3 relay mainly serves as a backup relay, the dynamic operation of the parallel or adjacent transmission lines should be considered. In case of a single line to ground fault on parallel line, the dynamic process, including the initiation of the fault, single pole tripping, and the correlative voltage and reactive power changing, should all be considered. The removal of fault on adjacent line L1 produces power swing effect, however the proposed relay (located on line L2 in Figure 1) remain stable and will not issue any trip signal as discussed in previous section. On the other end, if the fault on line L1 is not isolated by relay of parallel line L1, then the proposed relay located on line L2 will issue a trip signal and serve as backup protection.

To validate the algorithm, a solid L-g fault at 70 km from REB on parallel line L1 is created and parameter variations for the same are shown in Figure 11. It is observed that the impedance enters into Zone-3 region of relay characteristic. Moreover, the magnitude of rate of change of voltage i.e. dV/dt is higher than that of set value of threshold at the time of fault. Also, slightly high value of rate of change of reactive power (dQ/dt) is observed during said fault condition. It is to be noted that the impedance spot still remain in third zone after a set time elapsed by timer during said fault in parallel line. Thus, the proposed relay issues delayed trip signal and will serve as backup protection as shown in Figure 11.

Figure 11:

Performance of proposed method for fault on parallel transmission line.