Star (Wye) and Delta-Connected Loads | What Is 3-Phase Power?, Part 2
From the series: What Is 3-Phase Power?
In 3-phase electrical power systems, there are two fundamental connection options for load components: Star (Wye)-Connected Loads and Delta-Connected Loads. Understanding how these connection options affect voltage and current is fundamentally important for AC electrical system analysis and operation.
You will learn:
- How Star (Wye) and Delta Connection are made in a 3-phase system
- The definition of line measurements and phase measurements for both voltage and current
- The vector relationship between line and phase measurements for both Star- and Delta-Connected Loads
- Some practical considerations for a certain connection choice
Published: 19 May 2022
Hello, everyone. My name is Graham Dudgeon, and welcome to the second in a series of videos that I will be giving on three-phase power. The aim of the video series will be to build up our engineering knowledge on the design, analysis, and operation of three-phase electrical power systems.
Today, we'll explore star and delta connected loads. Note that star is also referred to as wye.
With a three-phase system, we have two fundamental connection options for components. We can connect from a phase to a point of common coupling or neutral point. This is known as a star or wye connection. And we can also connect from one phase to another phase. This is known as a delta connection.
Now, we'll look at how we measure voltage and current. We have line voltage and current, which are measurements made from the supply lines feeding a load, and phase voltage and current, which are measurements at the components of a load. Line voltage is measured across two lines. Line current is measured through a line. Phase voltage is measured across a load component. Phase current is measured through a load component.
For a star connected load, phase current is equal to line current. Line voltage is square root of 3 times the magnitude of phase voltage. Line voltage leads phase voltage by 30 degrees.
Now, we can't readily see the magnitude and phase relationships from this diagram, but we'll explore these relationships in more depth in just a moment.
For a delta connected load, phase voltage is equal to line voltage. Line current is square root of 3 times the magnitude of phase current. Line current lags phase current by 30 degrees.
We'll now consider the magnitude and phase relationships of voltage and current in more detail. We'll use simulation to gain insights into the behavior of star and delta connected loads. The visualizations I'm going to show in the rest of the video are driven from simulated data.
Here, we are seeing the phase voltage and line voltage for a star connected load. On the left, we see the instantaneous voltage as time progresses, and on the right, we see a vector representation of the voltages.
For phase voltage, we see that the vectors are tracking the instantaneous values as expected.
But for the line voltages, we observe two things. First, the line voltage magnitude is greater than phase voltage magnitude. And second, we do not see a clean tracking of the vector values against the instantaneous values. The increase in magnitude is because we're now measuring across two lines, and you can see that the vectors for line voltage are the correct magnitude, being square of 3 times larger than the phase voltage vectors.
The main difference with a line voltage measurement is that the vectors do not have a fixed reference like they do with phase voltages in a star configuration. You can see the tail end of each vector is rotating.
For the purpose of waveform measurement, we need to bring the tails of the line voltage vectors to a common reference. So what we do is we slide vector CA so that it is referenced to the origin, we slide vector AB so that it's referenced to the origin, and finally slide vector BC so that it too is referenced to the origin.
You can now see that the vectors are tracking the instantaneous values. Note also that the line vectors are 30 degrees ahead of the phase vectors. The vectors we are seeing are balanced. In the trigonometric form, we express the vectors as shown. V mag is the magnitude of the phase voltage. Note again that the line voltages are square root of 3 times larger in magnitude and lead the phase voltages by 30 degrees. The angular shifts are measures in radians, and so we multiply the angle in degrees by pi divided by 180.
Next, we'll compare star and delta voltage profiles. What we're showing here is voltage vectors that cover our source, which is a star connected voltage source in this case, phase voltage of the load, and line voltage. On the top is a star connected load and on the bottom is a delta connected load. Note we use the same source in both cases.
For the star connected load, line voltage is square root of 3 times the magnitude of phase voltage, and line voltage leads phase voltage by 30 degrees. For the delta connected load, line voltage is equal to phase voltage. You may be wondering why I have a larger circle for the line measurement. We'll see why when we look at current.
For this example, the loads consist of 1 ohm resistances for both the star and delta connected loads. For a star connected load, line current is equal to phase current. For a delta connected load, line current is square root of 3 times the magnitude of phase current, and line current lags phase current by 30 degrees.
Also notice that line current for a delta connected load is 3 times the magnitude of line current for a star connected load when we have equivalent source voltage and load impedance. This means that the power delivered to a delta connected load will be 3 times the power delivered to a star connected load, again, given that we have equivalent source voltage and load impedance.
There are a number of factors that influence the choice of load connection. Some factors include that star connections offer a neutral point, which can be grounded to increase safety. For a given supply voltage, delta connections provide higher voltage and so may be used, for example, to increase the speed of three-phase motors.
Delta connections provide higher reliability in some respects. The disconnection of a delta connected component does not compromise the supply to the remaining two components, but the line current will become unbalanced.
For a given load power, supply voltage can be reduced for a delta connected load. Some of these considerations will be explored further in future videos.
So in summary, phase voltage is measured across a component in a load. Phase current is measured through a component in a load.
Line voltage is measured across two supply lines. Line current is measured through a supply line.
In a star connected load, phase current is equal to line current. In a delta connected load, phase voltage is equal to line voltage.
In a star connected load, line voltage is square root of 3 times the magnitude of phase voltage, and line voltage leads phase voltage by 30 degrees. In a delta connected load, line current is square root of 3 times a magnitude of phase current, and line current lags phase current by 30 degrees.
A star connected load has a neutral point. A delta connected look has no neutral point.
Given the same source voltage and load impedance, a delta connected load will consume 3 times the power of a star connected load.
I hope you found this information useful. Thank you for listening.