Experiment no. 5 Overcurrent Protection of Transmission Line Power System Protection Laboratory Electric Power and Energy Engineering Dr. Jamal Thalji Eng. Saddam Ratrout Eng. Mageda El-Moubarak
Content of experiment: 1. Determination of reset ratio in the case of three pole short circuit. 2. Determination of reset ratio in the case of two pole short circuit. 3. Determination of reset ratio in the case of one pole short circuit. 4. Testing a circuit breaker trip characteristic in the event of a fault. Required equipment: Number Equipment Name Quantity CO3301-4J Overcurrent protection 1 CO3301-5P Power switch module 2 CO3301-3F Resistive load (3-phase, 1 kw) 1 CO3301-3A Line model 1 ST8008-4S Adjustable three-phase power supply (0-400V / 2A, 72PU) 1 CO5127-1Y Three-phase meter 2 LM 2330 Digital multimeter 1 Theory of Overcurrent Protection of Transmission line. Overcurrent protection is a selective type of overload and short circuit protection, used mainly in radial networks with single ended feeders found in medium voltage systems. Figure 1 shows the single line diagram of the radial network. Most protective devices of this kind also serve as a backup measure for differential and distance protection in the case of transformers, machines and transmission lines. The protective device is energized (excited) by a short circuit current I k or an overcurrent I > which significantly exceeds the operating current I N. To obtain an adequate measurement variable for the protective device, the current is coupled out via a current transformer and measured. If the current magnitude exceeds the set threshold, this is considered as the start command for the relay preset time delay. If the excitation is still present after the time delay, the protective device performs the desired action; the output relay is actuated, thereby triggering the circuit breaker. Otherwise, the action is cancelled. Simple overcurrent protection is non-directional, i.e. its decision criteria only involve the measured current magnitude and time length of the energized phase. For overcurrent to result in energization, the threshold value (pickup value) must lie below the minimum short circuit current occurring in the system. The reset value at which the protective relay returns to its initial position must be lower than the (minimum) operating current (pickup value). This results in a hysteresis defined by the reset ratio RR (ratio of release and response values). In modern protective relays, this ratio is approximately RR = 0.95 for overcurrent energization. A reset ratio of RR = 1 would pose a risk of chatter, i.e. uncontrolled engagement and release by the protective device. Overcurrent protection can have a directional or non-directional trip characteristic. The single stage trip characteristic of non-directional, maximum-overcurrent time protection is shown in Figure 2 and functions as described above. 1
Figure 1: Radial power system. A disadvantage of the simple trip characteristic is that the delay time is always the same, regardless of the fault current magnitude. An excessively long delay time in the event of a fault can result in considerable damage to components. For this reason, most protective devices provide a choice of two or more tripping ranges. Figure 3 shows a distinction between the "overload" and "short circuit" ranges. If the excitation level lies between amperages I> and I >>, tripping takes place at instant t> (overload stage). At very high amperages beyond I>>, such as those occurring during short circuits, tripping takes place sooner at instant t>> (short-circuit stage). If several protective devices are connected in series across the network, this leads to a graded curve (Figure 4), the nearest protective relay being tripped in the event of a fault. If a protective relay fails, the previous one acts as a backup with a longer tripping time. The disadvantage here is that a fault near the feed point (source), where the tripping time t> is longest, results in the highest current. Consequently, additional protective measures are needed here. 2
Figure 2: Simple trip characteristic of non-directional, maximum-overcurrent time protection. Figure 3: Trip characteristic of two-stage, non-directional, maximum-overcurrent time protection. 3
Figure 4: Network map with non-directional, maximum-overcurrent time protection relay. Review questions (several answers may be correct): Where is overcurrent time protection normally used? Highly meshed networks. As a backup measure for transformers, differential / distance protection etc. Simple radial networks. What is the fast tripping stage known as? Turbo stage. Short-circuit stage. Overload stage. What are the disadvantages of overcurrent time protection? This kind of protective relay is very costly compared with other protective devices. Fault currents are highest in the proximity of the feed point, the location of the overcurrent relay with the longest tripping time. The protective relay responds very slowly. A simple trip characteristic involves a constant time delay regardless of the fault current's amperage. Experimental Procedure The overhead transmission line receives a three-phase power supply and is loaded symmetrically at its end. A circuit breaker (power switch module) is located before the transmission line to disconnect the line from the power supply in the event of a fault. The time overcurrent relay measures the current in each phase via a current transformer. Set up the experiment as shown in the circuit diagram of Figure 5 and layout plan of Figure 6. 4
Figure 5: Circuit diagram Figure 6: Layout plan The potentiometers need to be set precisely and the switching times measured subsequently. The accompanying HTL-Soft 4 software is provided for this purpose. The page titled "Operating the SEG HTL Soft" later describes the software's control elements. 5
1. Set the load to its lowest level. 2. Set the relay DIP switches as indicated in the table below. (active setting = green background) Start the SEG HTL-Soft software by pressing the corresponding icon. Figure 12: Start icon The window shown below appears. Figure 13: Device selection menu Click on the appropriate device to open its user interface. If the device was not previously configured, choose a connection first. For this purpose, click on the PC COM port (1, 2, 3 or 4 as shown below) which you set. The window shown in Figure 15 then appears. 6
Figure 14: Connection options The user interface has four different color-coded display groups: ALARM / MEASUREMENT VALUE / STATUS / PARAMETER The settings and measurement values are polled continuously, so that any change is indicated immediately in the presence of a connection between the protective relay and the PC. Use the scroll bar on the right-hand side to move up and down the displayed list. Make sure that the parameter settings is PANEL Figure 15: Display parameters For the purpose of setting the potentiometers, the values in this view are indicated to an accuracy of two decimal places (Figure 16). Figure 16: Potentiometer settings 7
1. Determination of the reset ratio in the case of a three-pole short circuit. Connect the power switch module as shown in Figure 7 so that the right-hand side is bridged, and the left-hand side connected to all three phases at the overhead line's end. For this experiment, disconnect the relay output from power switch 1 to prevent premature tripping. Figure 7: Three-pole short circuit Figure 8: Potentiometer On relay XI1-I, set the overcurrent level I> to 0.5 A and all other potentiometers to 0. Make sure that the source voltage is 0 V. Turn on both power switches and slowly increase the voltage until the relay is energized (red upper LED comes on). Read the amperage on the three phase meter and note the pickup value Ipickup in the table below. Reduce the voltage on the three phase power supply and note the release (reset) value Ireset at which the relay is deenergized again (red upper LED goes off). Calculate the reset ratio RR from the pickup value Ipickup and reset value Ireset as follows; RR = Ireset / Ipickup Repeat this procedure at overcurrent (I>) levels of 0.8, 1 and 1.2A and enter the corresponding values in the table below. Open power switches 1 and 2 (OFF buttons) and turn the voltage back to 0 V. 8
2. Determination of the reset ratio in the case of a two pole short circuit. We will now simulate a two-pole short circuit and determine the associated reset ratio. Close power switch 2 as shown in Figure 9, so that the right-hand side is bridged, and the left-hand side connected to two phases at the overhead line's end. Figure 9: Two-pole short circuit Leave the potentiometer settings from the first experiment unchanged. Turn on both power switches and slowly increase the voltage until the relay is energized. Read the amperage on the three-phase meter and note the pickup value Ipickup in the table below. Reduce the voltage on the three-phase power supply and note the release value Ireset at which the relay is de-energized again. Calculate the reset ratio RR from the pickup value Ipickup and release (reset) value Ireset as follows; RR = Ireset / Ipickup Repeat this procedure at overcurrent (I> levels) 0.8, 1 and 1.2A and enter the corresponding values in the table below. Open power switches 1 and 2 (OFF buttons) and turn the voltage back to 0 V. 9
3. Determination of the reset ratio in the case of a single pole short circuit. We will now simulate one-pole short circuit and determine the associated reset ratio. Connect the power switch (2) as shown in Figure10, so that the right-hand side is bridged and the left hand side connected to one phase and the N-conductor. Figure 10: One-pole short circuit On relay XI1-I, set the overcurrent level I> to 0.5 A and all other potentiometers respectively to their lowest values. Turn on both power switches and slowly increase the voltage until the relay is energized. Read the amperage on the three phase meter and note the pickup value Ipickup in the table below. Reduce the voltage on the three-phase power supply and note the release (reset) value Ireset at which the relay is de-energized again. Calculate the reset ratio RR from the pickup value Ipickup and release value Ireset as follows; RR = Ireset/ Ipickup Repeat this procedure at overcurrent (I>) levels 0.8, 1 and 1.2A and enter the corresponding values in the table below. 10
Review questions What is the average reset ratio RR in all three experiments? RR = % Why does the reset ratio deviate from 1, thereby resulting in a hysteresis between the pickup and release values? Digital protective relays are often imprecise and have an indeterminate response. A constant hysteresis prevents chatter and ensures a stable response. The transformers in the network cause delays which result in the hysteresis. 4. Testing a circuit breaker trip characteristic in the event of a fault. Set up the experiment as shown in Figure 11. Reestablish the connection between the OFF button and the 24 VDC power supply of power switch 1 as well as relay outputs 24/21 and 14/11. Figure 1.11 - Layout plan Set the relay's DIP switches as shown in the table below (active setting = green background). 11
A. Simulation of an overload test Open power switch 2 (OFF button). The potentiometers should remain at the settings shown in the table above. Leave the load setting unchanged. Line-to-line voltage ULL = 220V. Turn on power switch 1 (ON button). The configuration is back in its normal operating mode. Decrease the load resistance slowly until the relay responds. At which stage does pick-up occur? Stage is: At which stage does tripping occur? Stage is: What is the trip delay? Trip delay is: B. Simulation of a three-pole short circuit Make sure that power switch 2 is open (OFF button). Check the potentiometer settings once again. Turn the load control up to the halfway mark, so that the white arrow points straight up. Set the line-to-line voltage ULL = 220 V. Turn on power switch 1 (ON button). The configuration is now in its normal operating mode. Check the connections for power switch 2. Three phases at the end of the line must be linked together. Produce a three-pole short circuit by closing power switch 2 (ON button). At which stage does pick-up occur? Stage is: At which stage does tripping occur? Stage is: What is the trip delay? Trip delay is: 12