WHAT IS THE DIFERENCE BETWEEN A "PO 2

[ckavamba]

WHAT IS THE DIFERENCE BETWEEN A "POWER SWING" AND A "POWER OSCILLATION".
UNDER WHAT CONDITIONS WOULD YOU EXPERIENCE THE ABOVE IN A POWER SYSTEM.

 

[jbartos]

Comment: The power swing may be a single event. The power oscillation tends to have many oscillations that may last for considerable length of time.

 

[jack6238]

I am going to answer this question on the basis that the items your interest relates to is Transient Stability of Power Systems where these terms are frequently encountered. If this is not what you had in mind it should still be useful since you have a system with local generation. This is from your previous post involving the circulating current between the generators and the solidly grounded interconnecting transformer.

The following is not meant to be a detailed writeup on stability, but a brief review is necessary to help understand the terms power swings and power oscillations.

Stability

When you have a Power System that involves the interconnection of two or more generators or generators and a synchronous and /or induction motors, the system has the potential to become unstable. My American Heritage dictionary defines stability as "The ability of an object... to maintain equilibrium or resume its orginal position after displacement..."

In a stable Power System , this equilibrium exists between the various opposing forces acting on the rotating components. For example a motor is producing torque to turn the motor and the load is the opposing force trying to retard the rotation of the motor shaft. In a generator the accelerating force is the steam input for a turbine generator and the retarding force is the system load. Now when you have two generators operating in parallel, the average speed of rotation is essentially constant and this results in a constant frequency. So Power Engineers regard Power System stability being dependent upon the equilibrium between power flows-mechanical and electrical. When this equilibrium is upset for a sufficient amount of time, the system becomes unstable.


There are three types of stability:

Steady-state stability as described above.
Transient Stability
Dynamic

This concerns steady-state stability and transient stability.

The maximum amount of power which can be transferred between machines or a group of machines without the loss of stability is know as the power limit of the system. Operation below this limit results in a stable power system and above this limit, the system is unstable. This value of power is known as the stability limit and is a site specific value.

What Causes Instability?

Transient instability, if in fact it exists ( it must be determined by calculation for each specific system), is casued by a sudden disturbance such as a three phase fault. Suppose you have a system consisting of two generators operating in synchronism and supplying load. Suppose a three phase fault occurs on an electrically close circuit to the generator bus. Since no power can flow past a point of zero voltage, no synchronizing power can flow between the two machines.

This sudden loss of load will cause the individual generators to speed up since all of the mechanical input power will be absorbed by the machines rotors. Unless the generators and prime movers are idential and have the same exact inertia constants, they will accelerate at different rates. Upon removal of the short circuit, synchronizing power will flow between the generators. If the synchronizing power is sufficient to restore synchronism, synchronous machine stability is restored. If not, the system is unstable and the system is in trouble and headed for uncharted territority.

During fault on time, the power is swinging. This can be shown graphically by a computer plot of the generator rotor angles vs. time. When the generators are allowed to exchange synchronizing power and the sytem is stable, the generators will settle out at whatever load is left for them to supply. The generators rotor angle which results from the generator power output, will oscillate as it seeks the correct operating angle for the system conditions.

If the system is not transiently stable and this is generally due to slow acting relays seperating the fault, the game is over. I do not believe that any system which became unstable was ever able by itself to restore stability.

In an unstable system, the voltage vector of one machine will rotate with respect to the other and cause the voltage at the electrical center of the system to go from zero to two per unit. This is caused by the different speeds response of the two generators systems. By system I mean the combined prime mover and generator and the manner in which they respond to mechanical input power. This system is really "swinging."

Another way to look at the system is to realize that the coupling between the stator and rotor fields is elastic. This means that if an abrupt, rather than a gradual change occurs in one or more defining equations of synchronous machines, the rotor angle will tend to overshoot the final value determine by the changed conditions. If the conditions which caused the disturbance are not corrected or eliminated, the machine will continue to slip poles as it attempts to synchronize-in short it will pull of of step with the system to which it was connected.

Eliminating Transient Stability Concerns

The best way to eliminate transient stability concerns is to install high speed instantaneous relays on all system feeders. It has been shown that most systems will remain transiently stable if the fault can be remove in less than 15 cycles of clock time. Also you should realize that the stability considerations are based on three phase faults. While these represent the most extreme conditions, they are also the least likely to occur. A more realistic event is the line-line-ground fault. This condition still has a healthy phase and it has the ability to exchange synchronizing power. Thus the typical fault can exist for a longer period of time before the system pulls out of step-ie, becomes unstable.

The equation for power transfer between two or more machines results in a curve which is a function of the sine of the difference of the voltage angles between these machines, the voltages behind the transient reactance of the machines and inversely proportional to the transfer impedance between the two systems. Interestingly , this curve is called "The Swing Curve"

I would not go out and purchase a book on System Stability. Instead I would go after a brief writeup on Stability.

Here are several:

-Stability Considerations for Industrial Power Systems, R.H. McFadden,IEEE, Industry Application Society, Vol. IA-13, No. 2 March/April 1977
-Transient Stability Criteria of Industrial power Systems-A Qualitive Analysis, JR Dunki-Jacobs, Paper No. CP-60-1176, AIEE
-System Stability Limitations and Generator Loading, H.C. Anderson, H.C. Simmons, JR. and C.A. Woodrow, AIEE Transactions Paper 53-106

 

[jbartos]

Suggestion: References:
1. IEEE Standard 100 "The Authoritative Dictionary of IEEE Standard Terms," IEEE, 2000
2. IEEE Std 399-1997 "IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis"

Reference 1 appears to be more closely referring to the terms in original posting. It defines:
1a. "Swing" as "a transient power flow due to change in relative angles of generation on the system caused by a change in transmission or generation configuration." For example, when a transfer switch changes its position, it usually produces an abrupt change in the load of generator. The generator reacts to it in form of dynamical change described by the generator swing equation.
1b. "Oscillation" has several definitions. One that is close to the original posting is marked (3) (gas turbines). It defines ""oscillation" as the periodic variation of a function between limits above and below a mean value, for example, the periodic increase and decrease of position, speed, power output, temperature, rate of fuel input, etc. within finite limits." The roots of power oscillation may be of mechanical or electrical origin or both. Since this is an electrical forum, the electrical origin is more appropriate to address. A periodic switching of load(s) may cause the power oscillations or power distribution network parameters. A mathematical model is usually created to analyze the power generation and distribution system for oscillatory behavior. Also, some electrical power analysis software can analyze the power distribution system for oscillations. The power oscillation may also have a dc component that makes them superimposed to that dc component for a shorter period of time.
There are also hunting oscillations between two generators that are basically out of phase after some disturbance, e.g. a fault, Reference 2.