Three Phase Balanced vs unbalanced system / load

• June 13, 2020
• Gaurav J

Three phase balanced system or load and Three phase unbalanced system or load are the two most commonly used concepts in power system. But, what do we really mean by a balanced or unbalanced 3 phase system? What are the parameters that decide whether a system is balanced or unbalanced? So, if you want to get the details and the basics of a 3 phase balanced and unbalanced system, watch the video.

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When we talk about 3 phase balanced or 3 phase unbalanced system we must first know a very basic thing i.e. it is not the source that decides if a system is balanced or unbalanced (until the winding of the source generator is placed wrongly, which is a very rare case). It is the load which decides if a system is balanced or unbalanced. Now, first understand about the balanced system.

Three phase Balanced System

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As we know, in 3 phase system, phase voltages or currents are displaced from each other by 120 deg. This is because, the windings of a generator are placed 120 deg apart from each other. Consider a star connected winding as shown in the below figure.

Now, if you connect load to it, we can call it a system. Let’s say, we connected an equal load on each winding. As the load on the system is identical, current flowing through each phase is same. Therefore, phase angle between voltage and Current is perfectly 120 deg as shown in the below figure.

Also, if your load is perfectly identical on all the three phases, current flowing through neutral conductor is also zero. How? Let’s see that.

If you observe the diagram carefully, you’ll find that the sum of return currents is zero at every instance. For example, at instant corresponding to 240 deg,

Ic = Imax
Ib = Ia = -0.5 Imax.

So, if we add these return currents, we’ll get the sum =0, and this is true for every instance.

Ia + Ib + Ic = (-0.5Imax – 0.5Imax) + Imax
Ia + Ib + Ic = – Imax + Imax = 0

We can therefore, remove the neutral conductor without affecting the voltage or current in the circuit. This is only applicable for perfectly balanced system i.e. system with identical load on each phase. And we can call such system as BALANCED SYSTEM.

Properties of balanced system

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1. Waveform is perfectly sinusoidal i.e. in-terms of magnitude and phase shift of 120 deg
2. Current flowing through each phase is identical.
3. No current flows through the neutral.
4. Power loss is very low or not present.

Of course such system is ideal and existence of which is doubtful. Most of the systems are unbalanced like our distribution system. And the properties of unbalanced system are completely opposite of what we have seen just now.

Three phase Unbalanced System

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Now, let’s increase the load on one of the phase of our system. As we have increased the load on one phase, it will draw more current than the other two phases. And this will create imbalance in the system. And here you’ll need the neutral conductor. The waveform is now disturbed in terms of magnitude and phase shift. This will further cause the power loss in the system.

System gets unbalanced because of following reasons.

1. Unbalance in winding of a 3 phase equipment like 3 phase induction motor. If the reactants of the three windings are not same then it will draw unequal current from the system.
2. Unequal load on the system. This causes more current to flow through one particular phase on which the load is connected.

These are the two main and the basic causes of unbalance in a system.

Effects of unbalanced system

• Heating of the 3 phase machines
• Heating will decrease overall life of machine
• This imbalance, will also increase the I^2 R losses,
• Also, unbalanced system may cause tripping of variable frequency drives used for induction motor.

You’ll see in our distribution system a neutral is always provided. This is because, load on the phases is not defined or equal. And hence, different phases draw different current which makes the distribution system unbalanced.

Properties of an unbalanced system.

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1. Waveforms are disturbed in terms of magnitude & phase angle.
2. Current flowing through phases is not same.
3. Neutral in needed.
4. Power losses are more.

Matlab Simulation

I have also performed a Matlab simulation to check out whatever we have learned about balanced & unbalanced system is correct our not. To see the simulation watch the video given in the beginning of this tutorial.