How is reactive power related to the problem of voltage collapse?

In terms of this water-carrying analogy, the frequency of going up and down the ladder should be nearly constant (that, is like our 60 cycles per second electrical frequency). So, when more water is needed, the amount that each person carries up the ladder must get bigger (since they are not allowed to go faster or slower). Well, if this water gets too heavy, either the ladder might break, or the person might get too tired to carry it. We could argue that if the ladder breaks, that is like the outage of a transmission line that either sags or breaks under the stress of too much current. There are devices called relays in an electrical system that are supposed to sense when the load is too much and send a signal to a “circuit breaker” to remove the line from service (like removing the set of three ladders). If the person gets too tired, we could again stretch this analogy to say that this is like not having enough reactive power (resulting in low

1 Another analogy that says that reactive power is the “foam on the beer” is fairly good here because the space in the glass is taken up by the useless foam - leaving less room for the “real” beer.

voltage). In the extreme case, the person might “collapse” under the weight of the water that the person is being asked to carry. If it happens to one person, it will probably happen to many of them. In the electrical system this could be considered a “voltage collapse”. While there are “undervoltage relays,” there are no relays in the system to directly sense the problem that the voltage is about to collapse.

Remember, the people going up and down the ladders do not absorb or produce energy over a complete cycle and are therefore analogous to reactive power. It is the water going up the ladder to fill the tank that absorbs real power that must be paid for. But, the real power cannot be delivered without the reactive power. And, if there is not enough reactive power (like with people going up and down the ladders), the real power delivery will eventually fail.

In summary, a voltage collapse occurs when the system is trying to serve more load than the voltage can support. A simulation has been prepared to illustrate voltage collapse by simply using a system with an Eastern generator and customer load, a Western generator and customer load, and East to West transmission lines. In the simulation, the Eastern generator has a constrained supply of reactive power and progressive line outages for unspecified reasons lead to a voltage collapse even when reactive power supply is ample at the Western generator. The results of the simulation are available in Power Point slides. (If you are connected to the Internet, click here to view the slides.)

In contrast to all of this, you could route a hose up the side of the water tower and simply turn on the water and let the water flow in the hose to fill up the tank. The water pressure is like voltage, and the water flow is like current. This type of system would be a direct current (DC) system and would not involve reactive power at all. However, the concept of voltage collapse is not unique to AC systems. A simple DC system consisting of a battery serving light bulbs can be used to illustrate how too much load on a system can lead to a condition where voltages drop to a critical point where “adding more load” results in less power transmission - a form of voltage collapse. (If you are connected to the Internet, click here to view Power Point slides illustrating DC voltage collapse.)

A physical analogy for reactive power...

While there are numerous physical analogies for this quantity called reactive power, one that is reasonably accurate is the process of filling a water tower tank with water - one bucket at a time. Suppose you want to fill a water tower tank with water, and the only way that you can do that is by climbing up a ladder carrying a bucket of water and then dumping the water into the tank. You then have to go back down the ladder to get more water. Strictly speaking, if you simply go up a ladder (not carrying anything) and come back down (not carrying anything), you have not done any work in the process. But, since it did take work to go up the ladder, you must have gotten all that energy back when you came down. While you may not feel that coming down the ladder completely restores you to the condition you were in before you went up, ideally, from an energy conversion viewpoint, you should! If you don’t agree, get out your physics book and check out the official definition of doing work.

OK, if you still don’t agree that walking up a ladder and coming back down does not require any net work, then think of it this way. Would you pay anyone to walk up a ladder and back down without doing anything at the top? Probably not. But, if they dumped a bucket of water in the tank while they were at the top, then that would be something worth paying for.

When you carry a bucket of water up the ladder you do a certain amount of work. If you dump the water at the top and carry an empty bucket down, then you have not gotten all your energy back (because your total weight coming down is less than going up), and you have done work during that process. The energy that it takes to go up and down a ladder carrying nothing either way requires reactive power, but no real power. The energy that it takes to go up a ladder carrying something and come down without carrying anything requires both real power and reactive power.

A reminder here is that power is the time rate of energy consumption, so consuming 500 Watts of real power for 30 minutes uses 250 Watt-hours of energy (or 0.25 kilowatt-

hours which costs about 2.5 cents to generate in the U.S.). The analogy is that voltage in an AC electrical system is like the person going up and down the ladder. The movement of the water up the ladder and then down into the tank is like the current in an AC electrical system.

Now, this pulsating power is not good in an electrical system because it causes pulsations on the shafts of motors and generators which can fatigue them. So, the answer to this pulsation problem is to have three ladders going up to the water tower and have three people climb up in sequence (the first person on the first ladder, then the second person on the second ladder, then the third person on the third ladder) such that there is always a steady stream of water going into the tank. While the power required from each person is pulsating, the total result of all three working together in perfect balanced, symmetrical sequence results in a constant flow of water into the tank – this is why we use “3-phase” electrical systems where voltages go up and down in “sequence” – (first A phase, then B phase, and finally C phase).

In AC electrical systems, this sequential up/down pulsation of power in each line is the heart of the transmission of electrical energy. As in the water tower analogy, having plenty of water at ground level will not help you if you cannot get it up into the tower. While you may certainly be strong enough to carry the bucket, you cannot get it there without the ladder. In contrast, there may be a ladder, but you may not be strong enough to carry the water. However, the people do take up room around the water tower and limit how much water can go up and down over a period of time - just as reactive power flow in an electrical system requires a larger current which limits how much real power can be transmitted.1

To make the system more reliable, we might put two sets of three ladders leading up to the tank on the tower. Then, if one set fails (maybe the water plus the person get too heavy and the ladder breaks), the other set picks up the slack (that is, has to carry more water). But, this could eventually overload the second set so that it too fails. This is a cascading outage due to the overloading of ladders.

What is Reactive Power?

Reactive power is a quantity that is normally only defined for alternating current (AC) electrical systems. Our U.S. interconnected grid is almost entirely an AC system where the voltages and currents alternate up and down 60 times per second (not necessarily at the same time). In that sense, these are pulsating quantities. Because of this, the power being transmitted down a single line also “pulsates” - although it goes up and down 120 times per second rather than 60. This power goes up and down around some “average” value - this average value is called the “real” power and over time you pay for this in kilowatt-hours of energy. If this average value is zero, then all of the power being transmitted is called “reactive” power. You would not normally be charged for using reactive power because you are consuming some energy half the time, and giving it all back the other half of the time - for a net use of zero. To distinguish reactive power from real power, we use the reactive power unit called “VAR” - which stands for Volt-Ampere-Reactive. Voltage in an electrical system is analogous to pressure in a water system. Current in an electrical system is analogous to the flow of water in a water system.
Let’s go back to this notion that voltage and current may not go up and down at the same time. When the voltage and current do go up and down at the same time, only real power is transmitted. When the voltage and current go up and down at different times, reactive power is being transmitted. How much reactive power and which direction it is flowing on a transmission line depends on how different these two times are.
Two extreme examples of the time relationship between voltage and current are found in inductors and capacitors. An inductor is a coil of wire that is used to make motors. A capacitor is made of parallel conductive plates separated by an insulating material. The electrical properties of these two devices are such that if they are both connected to the
same AC voltage source, the inductor absorbs energy during the same “half cycle” that the capacitor is giving energy. And similarly, the inductor produces energy during the same “half cycle” that the capacitor absorbs energy. Neither of them absorbs any real power over one complete cycle. Thus, when a motor needs reactive power, it is not necessary to go all the way back to electric power generators on the transmission grid to get it. You can simply put a capacitor at the location of the motor and it will provide the VARs needed by the motor. This relieves the generator and all the lines between the generator and the motor of having to transmit those VARs. They are provided “locally” by the capacitor. This means that with the capacitors installed, the current in the lines will be smaller than when the capacitors are not installed. This is a good thing because current in the lines causes heat and every line can only handle a limited amount of current. Since the line current is smaller when the capacitors are installed, the voltage drop along all the lines is also less, making it more likely that the motor will have a voltage closer to the desired value. When there are not enough VARs flowing locally to the loads, the generators must supply them remotely, causing unnecessarily large currents and a resulting drop in voltage everywhere along the path.

What is load angle?

In synchronous system while generator generates rated power and load increase in system, Than frequency start goes to down respectively same as rpm of the all generator start reduce but the speed of the rotating magnetic field is same, So the angle created between rotating magnetic field and generator rotor, This angle are known as load angle of generator.
(In isolated system there is same as synchronous system, Load angle very sharply increase in isolated system than synchronous system cause in the isolated system the access load share only by the particular machine while in the synchronous system there is share with all connected machine, which is very small other than)

Detailed Description of a DC Generator

A generator is the opposite of a motor; while a motor turns electrical energy into mechanical energy, a generator does exactly the reverse. Unlike alternating current generators, which produce current that periodically changes direction, a DC generator produces unidirectional current.

Features

• All direct current generators include at the minimum a rotating coil of wire; a split ring (with two breaks in it) that rotates together with the coil; two stationary metal brushes that transfer current from the rotating split ring to the external circuit and magnets that generate a magnetic field. The magnets may be permanent magnets or electromagnets. The combination of split ring and brushes is called a commutator.

Function

• As the coil of wire turns in the magnetic field, the magnetic flux (the total number of magnetic lines of force) passing through the loop of wire changes, because the angle between the coil and the magnetic field is always changing. In keeping with Faraday's Law, this change in magnetic field creates a voltage in the coil of wire and thereby generates electric current.

Considerations

• In an AC generator the EMF (electromotive force) if plotted as a function of time is a smooth wave-shaped graph (a sine wave), where the EMF is sometimes positive and sometimes negative. In a DC generator, the EMF rises from zero to max and falls back to zero, but never becomes negative. Some DC generators use more than one coil to smooth out this bumpy wave pattern and produce a more constant current.

Parts of a Dynamo Motor.

A dynamo motor, also known as an generator, is a device that can convert mechanical energy into an electric current. The concept of the dynamo is often attributed to Michael Faraday, who discovered that moving a magnet around a closed electric circuit can induce an electric current to flow in it. Modern dynamos or electric generators are made of several components that are essential for their function.

Magnetic Circuit

• On a modern dynamo motor, the magnetic circuit is made of several parts that include an armature core, a yoke, an air gap, and poles. An armature core is composed of sheet steel with a blanking die and thin magnetic steel laminates. These laminated sheets are either welded or bonded together to keep them from splitting apart.

  The poles are either steel laminates or solid steel, and they have field coils that set up the magnetic fields in the machine. The top and bottom poles are also called a pole head and a pole shoe, respectively. These poles usually fan out and are designed to smooth the flow of air on the air gap. The yoke is the casing where the completed poles are mounted, and it provides the magnetic path between the poles.

Electric Circuit

• The electric circuit is made of an armature winding, a commutator, a field winding and brushes. The armature winding is made up of copper wires that are insulated from the armature core. In a dynamo motor, the armature winding receives the voltage generated by the motor. Armature windings are connected to the commutator. The commutator acts as the mechanical rectifier that converts AC voltage to DC voltage, and it's usually made of silver-bearing copper. The commutator conducts the current to an external circuit through the brushes.

Mechanical Support

• The mechanical support of a dynamo motor is composed of a shaft, a frame, end bells, and bearings. In most cases, the yoke and the frame are the same, and they are usually made of steel or aluminum that encases the whole dynamo motor. The armature is placed inside the frame by using a steel shaft supported by two lubricated bearings that can either be a ball, a roller, or a sleeve type. Once the armature is placed inside the frame, both ends of the frame are enclosed using the end bells. The end bells are usually made of the same material used on frames and yokes.

How to Assemble a Dynamo With a DC Motor?

Electric motors and power generators are often identical in design. An AC motor, for example, may also be used as an alternator. The major difference in function depends upon whether the device is used to generate electricity such as an alternator or generator, or whether electricity is applied to the device to do work.

A DC motor contains a device called a commutator. This device allows a DC motor to produce DC power without adding any other electronics to the motor leads. When force is applied to a DC motor's rotor, the DC motor will work as a DC generator, also known as a dynamo.

* Use the electrical pliers to cut two lengths of electrical wire, with each length being six inches long. Strip 1/2 inch of insulation from the ends of each wire segment.

* Place one end of the first wire on the motor's positive electrical terminal. Melt a small drop of solder to both the electrical terminal and to the end of the first wire. Smooth out the soldered joint with the tip of the soldering iron, making sure that the electrical joint is shiny and free of lumps. Use this procedure to solder one end of the second wire to the negative electrical terminal.

* Attach the crank to the rotor shaft on the motor.

* Turn on the multimeter and set the measurement scale to "Volts DC." Connect the red probe to the loose end of the first wire, which should be attached to the positive electrical terminal. Connect the black probe to the loose end of the second wire attached to the negative terminal.

* Turn the crank and observe the multimeter display. If the crank is turned slowly, the voltage will fluctuate greatly. However, if the crank is turned quickly, the voltage will stabilize.