Experiment No: 11

Name of the Experiment: Determination of line to line current for Y-Y connection and Y-∆
connection and verification of line to line and line to phase current.

Objective:
This experiment is designed to investigate the relationships between phase and line currents in
three-phase systems. The primary focus is on understanding and verifying these relationships for
both Star-Star (Y-Y) and Star-Delta (Y-Δ) configurations of transformers or loads.

The specific objectives of this experiment are:

• To measure the phase and line currents in a Y-Y connected three-phase system.
• To measure the phase and line currents in a Y-Δ connected three-phase system.
• To verify the theoretical relationship between line current and phase current for both Y (𝐼𝐿
= 𝐼𝑃 ) and Δ (𝐼𝐿 = √3𝐼𝑃 ) configurations.
• To observe the phase difference between line and phase voltages in these configurations
(qualitatively, if not quantitatively with available equipment).

Apparatus:

• Three-phase AC power supply.

• Three single-phase transformers.
• Ammeter (AC)
• Connecting
• Load

Theory: Three-phase systems are the most common polyphase systems used for generating, transmitting,
and distributing electrical power. These systems can have their components (like transformer windings or
loads) connected in one of two primary configurations: Star (Wye or Y) or Delta (Mesh or Δ).

In a balanced Star (Y) connection: The common point where one end of each phase winding is
connected is called the neutral point. The current flowing through an individual phase winding is
known as the phase current (Iph). The current flowing through any of the line conductors is known
as the line current (IL). In a balanced Y-system, the line current is equal to the phase current.

IL=IP

The line voltage is 3 times the phase voltage, and the line voltage leads the corresponding phase
voltage by 30 degrees.
In a balanced Delta (Δ) connection: The three phase windings are connected in series to form a
closed loop or mesh. The current in an individual phase winding is the phase current (Iph). The
current in the line conductor is the line current. In a balanced Δ-system, the line current is √3 times
the phase current, and the line current lags the corresponding phase current by 30 degrees.

IL=√3IP

The voltage across an individual phase winding is equal to the line voltage because the windings
are connected directly between the lines.

• Y-Y Connection: When both the primary and secondary windings of a three-phase
transformer (or a source and a load) are connected in Star, it is referred to as a Y-Y
connection. The current relationships for each side (primary and secondary) will follow the
rules of the Y-connection mentioned above, where

IL=IP

• Y-Δ Connection: When the primary winding is connected in Star and the secondary
winding is connected in Delta, it is a Y-Δ connection. The primary side will follow Y-
connection current rules, while the secondary side will follow Δ-connection rules

IL=√3IP

This configuration is often used to step up or step down voltages and manage current
distribution in power systems

Circuit Diagram:

Figure 10.1: Y-Y Connection

 Figure 10.2: Y- Δ Connection

Data Table

Table 1: Y-Y Connection Readings

𝑰𝑳
Exp No 𝑰𝑳 (𝑨) 𝑰𝑷 (𝑨)
𝑰𝑷
01 0.1 0.1 1
02 0.2 0.2 1

Table 2: Y-Δ Connection Readings

𝑰𝑳
Exp No 𝑰𝑳 (𝑨) 𝑰𝑷 (𝑨)
𝑰𝑷
01 0.48 0.27 1.78
02 0.62 1.72 1.72
Calculation:

• For Y-Y Connection:

From Table 1 (exp No 01):

𝐼 𝐼
IL = 0.1 V, IP = 0.1 V. Measured 𝐼 𝐿 =1. Theoretical 𝐼 𝐿 = 1. Percentage error for Serial
𝑃 𝑃
0.1−0.1
[ ] 100% = 0%
0.1

• For Y-Δ Connection:

From Table 2 (exp No 02):

𝐼 𝑉
IL=125 V, IP=125 V. Measured 𝐼 𝐿 = 1.72 Theoretical 𝑉𝐿 = √3 = 1.73. Percentage error for
𝑃 𝑃
1.72−1.73
Serial [ ] 100% = 0.69%
1.73

Discussion:

The experimental procedure involved configuring three-phase transformers (or loads) in both Star-
Star (Y-Y) and Star-Delta (Y-Δ) connections and meticulously measuring the respective phase and
line currents. For the Y-Y configuration, the collected data in Table 1 definitively demonstrated
𝐼
that the line current was equal to the corresponding phase current, with 𝐼 𝐿 being exactly 1 for both
𝑃
readings. This perfectly aligns with the theoretical expectation for balanced Y-connected systems
where the line current is directly the current flowing through each phase winding. The absence of
error in these readings indicates ideal measurement conditions or high precision in the
experimental setup for these specific current levels.

In the case of the Y-Δ configuration, the data in Table 2 showed that the line current was
𝐼𝐿
approximately √3 times the phase current. For the second reading, was 1.72, resulting in a
𝐼𝑃
smaller error of approximately 0.69%. These minor discrepancies are acceptable and can be
attributed to factors such as the inherent limitations of the ammeters, slight imbalances in the three-
phase load or supply, and the resistance of the connecting leads. The overall trend clearly supports
the theoretical relationship that line current is 3 times the phase current in a delta connection. The
successful verification of these distinct current relationships across different configurations is
crucial for understanding the operational characteristics and design considerations of three-phase
systems in power distribution and machinery.
Precaution:

During the experiment to determine line-to-line currents for Y-Y and Y-Δ connections and to
verify line-to-line and line-to-phase currents, all connections should be carefully checked before
powering the circuit to avoid short circuits or equipment damage. Ensure the power supply is off
while setting up, and use properly rated measuring instruments. Avoid touching live terminals and
always connect the neutral properly in Y-Y configuration. Use fuses or circuit breakers for safety,
and confirm transformer/load ratings match the requirements. Accurate identification and
measurement of line and phase quantities are essential for correct verification.