Dynamic and Transient Stability

Steady state is, by definition, an expression of stability. In order to maintain stability, a dynamic system has to be introduced to limit the deviation from steady state as a result of a load change.

Can't anybody use a search engine any more?

Dynamic and static voltage stability enhancement of power ... -

Search engine is good. But some technical paper say dynamic stability is almost the same to steady state stability. but some reference say dynamic is almost same to transient stability. Im quite confused. Hope some knowledgeable engrs can give more technical opinion on this subject. Thanks

It isn't a matter of opinion.

There have been some good answers already, to help further........

If you are unsure of the meaning of a word, look it up in a dictionary - ""Wiktionary" on the "web" is easy to use. It comes in many languages which may help.

As an example, a generator flexible mounting on a warship would have

1. static stability limits due to movement of the ship like a list or roll (fixed or slowly changing load)
2. dynamic limits due to self-induced and ship vibration (continuously fluctuating loads at distinct frequencies)
3. Transient loads due to shock from weapon firing or hostile weapons or load shedding (impulse loads of short duration)

Any or a combination of these loads could exceed the range of flexibility of pipes or cables from the ship or cause contact "generator to ship" if neglected in the specification, design and testing process.

According to the area of engineering, even within "electrical" the meaning will change a little. However, I would never equate dynamic with transient. See.......

http://en.wiktionary.org/wiki/transient

http://en.wiktionary.org/wiki/dynamic

It's a matter of definition:

"...The limiting value of power is called the transient stability limit or the steady-state stability limit, according to whether the point of instability is reached by a sudden or gradual change in conditions of the system..."

From "Elements of Power System Analysis" by Stevenson, a book that is a must read for anyone who wants to call him/herself a Power Systems Engineer.

Every electrical system has "reactances" known as steady state, Transient State and"Sub-transient State reactances. Machines behave (rise or decay of current, voltage etc) differently during these three stages. Various periods for these behaviors (different time constants) are governed by various machine parameters. For example, the power transmitted during any disturbance is given by P = EVSin(delta)/X. The value of X changes with time from sub-transient to steady state. Accordingly the value of P will change. For steady state. if value of delta crosses 90 degree, the generator will fall out of step, but for dynamic stability, because of greater difference between transient and sub-transient values of reactance another term which is proportional to the product of difference if Xd" and Xd' and Sin (2*delta) is added.Then the delta value can be theoretically taken to 135 degree with out loosing stability.

Therefore if we are planning to add more loads in the system the maximum power transfer can be allowed is at 90degrees power angle? This is the steady state stability limit? If the angle overshoot more than 90degress this is called dynamic stability? Please correct me if my undestanding is incorrect. Thanks.

Maximum power transfer happens when the impedance of the source matches the impedance of the load. Both impedances are a function of frequency.

This thread is wandering into areas other than the practical ones of operating a generator.

"...Therefore if we are planning to add more loads in the system the maximum power transfer can be allowed is at 90degrees power angle?" In theory this is true, in practice you would never operate at this point.

"...This is the steady state stability limit?" Yes, for any synchronous machine.

"...If the angle overshoot more than 90degress this is called dynamic stability?" No, this is an unstable system. Reread the definition provided in post #5.

"...Please correct me if my undestanding is incorrect." More study will provide greater insight into this complex topic. If you are willing to tell us more about the nature of your inquiry you will probably get more appropriate information.

Well, OP it depends what you want to learn, and how you want to learn the subiect matter.

Power engineering and analog electronics are using different language, and using the same language with different meaning and implications. I favor analog, but appreciate both, that they have a reason for being.

Having disposed of that. There is no static or dynamic stability in the general world. There are stability margins (or the lack of it) under specific conditions and frequencies.

There is a fundamental difference between the analog and power world, that keeps shaping them.

In the analog world I have contol over the signal, the equipment, and mostly the load too.

In the power system the 1.st and 3.rd is outside of my control. Add to that an extensive transmission system.

General system dynamics is a tough enough subiect to make your own. Power system engineers have the added task to translate the results into the commonly used lingo in the profession.

This is power system stability! Im going to share what i have searched. Steady state stability is making sure the increase load will not exceed 90degrees limit. Why? Further increase may cause pole slip causing to trip your out of step relay. When trip occurs your system is unstable. Transient stability is a sudden loss of load or fault or switching may cause the rotor angle to swing. We have to satisfy the area criterion to make sure system can return to stability. Dynamic stability is caused by poor damping due to fast acting avr. Solution is to use PSS POWER SYSTEM STABLE. if someone is more knowledgeable in this topic hope you can add more explanation. Thanks

Thank you for a prompt and able response. You gave an excellent demonstration, that you talk Powerese with a Singaporean accent, and I speak General Dynamics with a Hungarian background.

The two have a real difficulty meeting, and agreeing even on the language.

I for example consider generator cogging a minor case in General Dynamics, half electrical, half mechanical inertia. Oh, it has real life consequences all right. And NO, when the cogging is finished, it ought to be stable again. While the trouble caused by the phase reversal, or a half wave loss propagates to the end user. Causing whatever.

Running a power plant without an ample safety margin is generally irresponsible. So is running it with the wrong phase relationship between Voltage and Current.

I happened not to know of 4 quadrant controllers for generators, and in normal operation I see no urgent need for it. Most modern setups are for 2 quadrants, and are good enough, mostly. There are still plenty of old generators, that can handle only resistive / inductive loads within limits (1 quadrant). The controllers are designed by my kind of people, electronics R&D. Unless you make effort to understand their behaviour beyond the basics, you will be hampered in your professional life.

As you see, I speak powerese only haltingly, having difficulty translating some concepts.

On the other hand, you are a student. Who has difficulty understanding stuff coming his way in a somewhat unfamiliar packaging.

Too bad. I found learning foreign languages always rewarding.

This is a topic that requires a deep understanding of the underlying principles of power system analysis/operation. Your questions indicate that you are researching a higher level question at the same time that you are learning the basic subject matter. I will do my best to point you in the right direction because this forum is not appropriate for the lengthy tutorial that is required.

"...We have to satisfy the area criterion to make sure system can return to stability...". This refers to the Power Angle Diagram and looking at the areas representing the change in load vs the change in power angle which represents the reserve capacity available to meet the increasing load.

"...Dynamic stability is caused by poor damping due to fast acting avr..." This should be rewritten as "Dynamic instability may be the result of poorly tuned AVRs..." The root cause for the possible instability is the difference in response times between mechanical and electric systems, which are orders of magnitude different.

"...Solution is to use PSS POWER SYSTEM STABLE (sic)..." A Power System Stabilizer is a highly tuned feedback system that takes inputs from the governor, AVR, and the system and coordinates their reaction rather than having them act independently, sort of like the Electronic Stability System in modern cars that controls the brakes and the throttle faster/better than the driver can.

For more info see the Electrical Transmission and Distribution Reference Handbook, Chapter 13, Power System Stability Basic Elements of Theory and Application. There are also some decent references to be found by Googling on, of all things, Equal Area Criteria. Also consider reading some manufacturers' literature on PSS. Class dismissed.

A practical point, jonald. No-one would operate, steady state, at 90 degrees. There would be no margin for the dynamic swings described by RAMCONSULT. It is usual practice to allow a margin of minimum 10% rated active power above a steady generator operating point. Power system shorts reduce voltage at generator in-feeds. This has an immediate effect on synchronising torque, so generator rotors accelerate in a "leading" direction. If there were no power margin, the set could immediately be in "static" instability, not just dynamic.