UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE. Department of Electrical and Computer Engineering

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1 UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE Department of Electrical and Computer Engineering Experiment No Balanced Three-Phase Networks Overview: Three-phase networks can be connected either in a Delta configuration such as in Figure 1 or a Wye configuration such as in Figure 3. For balanced loads, Z 1 =Z 2 =Z 3. For the balanced delta-connection with an ABC phase sequence we have V-phase = V- line (V AB, V BC, V CA ) I-phase = V- phase/ Z-phase (I AB, I BC, I CA ) I- line = 3 I-phase 30 degrees (I A, I B, I C ) Figure 1: Delta connected load The line (phase) voltages are 120 electrical degrees apart. So are the line currents and the phase currents. A typical phasor-diagram for a delta-connection is shown in the Figure 2 below. Figure 2: Phasor diagram for a delta-connected network

2 For the balanced Wye-connection with an ABC phase sequence we have V-phase = (V-line/ 3 ) 30 degrees (V AN, V BN, V CN ) I-phase = V- phase/ Z-phase (I AN, I BN, I CN ) I-line = I-phase (I A, I B, I C ) Figure 3: Wye-connected load Again the line (phase) currents are 120 electrical degrees apart. So are the line voltages and the phase voltages. A typical phasor-diagram for a Wye-connection is shown in the Figure 4 below. Figure 4: Phasor diagram for a wye-connected network Total power for either connection would be, S = 3 V-phase *I-phase = P + JQ = P 2 + Q 2 a tan Q P 2

3 S = 3 V line I line P = 3 V phase I phase cos Vp & Ip = S cos Vp & Ip Q = 3 V phase I phase sin Vp & Ip = S sin Vp & Ip P = 3 I 2 phase R phase Q = 3 I 2 phase X phase 3

4 Pre-Lab - Balanced Three-Phase Networks 1. For the balanced Y-connected load in Figure 5 below, prove mathematically that the sum of the two watt-meter readings in Figure 6 will give the total real power, P. Hint: Assume the phase sequence is ABC and show mathematically that, I A + I B + I C = 0. Now, remove the common meter connection from point N and reconnect it to a point X of arbitrary potential. Prove that the total real power is still given by W 1 + W 2 + W 3. With the arbitrary potential allowed to be V C, W 3 will be zero and the total power will be W 1 + W 2. Figure 5: Wye connected load with three power meters attached (INSTRUCTOR'S SIGNATURE DATE_ ) 4

5 Lab-Session - Balanced Three-Phase Networks 1. After checking with the instructor, connect one of the circuits below. Be careful not to exceed 120 V per phase. With the voltage initially adjusted to its minimum increase it slowly until power meter number 1 reads a line voltage of 120 V. R L series resistance of the inductor ω frequency in radians/seconds CAUTION: DO NOT MAKE CHANGES IN THE NETWORK UNLESS THE CIRCUIT BREAKER IS OPEN! The two meters (power analyzers) should be connected as shown in Figure 6. Figure 6: Circuit setup with two watt meters 2. Record the two power readings (with sign), the two line voltage readings, and the two line current readings. 5

6 3. Insert one of the wattmeters into one of the phases. Record the phase voltage, phase current, and phase power readings. See Figure 7 on the next page. Figure 7: Single phase wattmeter measurement 6

7 Lab-Session - Balanced Three-Phase Networks (Data Sheet) Table 1: Network Network Number Connection Impedance (w=377) Table 2: Power, current and voltage readings from watt meters Power Meter Line Power (Watts) Line Current (A) Line Voltage (V) 1 2 Table 3: Power, current and voltage readings from watt meter Power Meter Phase Power (Watts) Phase Current (A) Phase Voltage (V) 1 INSTRUCTOR'S INITIALS DATE: 7

8 Post-Lab - Balanced Three-Phase Networks 1- Using data from step 3 of lab-session, verify the per-phase load. 2- Calculate the phase voltage, phase current, line current, and total power (S, P, and Q) For the network tested using the given, measured line voltage and the theoretical given load. 3- In a table, compare S, P, Q, V-phase, I-phase, V-line, and I-line using the calculation in the previous step, the data in step 2 from the lab-session, and the data in step 3 of the lab session. Use the load found in the first step of the post-lab to find S and Q from the data. Be sure the powers are total three-phase powers. Discuss any differences. 8