How to define the "weak" power system 1

[ciy]

Hi,
Happy Holiday to Everyone!
I heard about the term of “weak power system” from somewhere.
How to define the said “weak”?
Please help!
Thanks!

 

[rbulsara]

You need to define "somewhere". Provide more context as to where did you hear that.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Lower than normal available fault current. More than normal voltage fluctuation with load.
Weak versus strong:
Strong, Stiff = Low source impedance.
Weak = Higher source impedance.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricuwe]

You always have to consider also the load connected. A good measure to judge on this is the available SC power (short circuit power) vs. the nominal power of the equipment. See for example IEEE 519 for guidelines on converter harmonics.

 

[m3ntosan]

I would say the health of a system is characterized by frequency and voltage, both in normal and fault situation.

Like warros said, fault level is very important, affecting voltage step change, protection operation, commutation failures … and inertia of the system, given by rotational masses, affecting the speed in frequency change. Both are affecting the ability of the system to withstand faults.

Level of interconnection might be another factor…


May you grow up to be righteous, may you grow up to be true...

 

[m3ntosan]

like rbulsara said, the context is very important; should we consider a system, with high fault levels and high inertia, crashing at N-1 strong in the first place?

every system in restoration phase would be considered weak and prone to crash.



waross,
apologies for misspelling your nickname


May you grow up to be righteous, may you grow up to be true...

 

[ciy]

Dear All,
I am appreciated your reply.
About the “somewhere”, that maybe just oral form somebody or paper/material as “Integration of a hydraulic production plant in a weak power system on a long radial line” found on the website.

 

[cuky2000]

The system strength could be characterized by the Short Circuit Ratio (SCR) defined as the SC available divided equipment size or load connected to the AC system.

SCR= S_sc/P_load

For a system considered weak, the index SCR<2

 

[m3ntosan]

@cuky2000

can you explain further more the SCR, what should we expect with SCR lower than 2?


@ciy

volts will go through the roof when offloading and charging the circuit and stability problems for the generator. I am not sure what's the english term for "when a generator goes over the leading limit forced by the system" open ended long circuit generating lots of MVARs and nothing to absorbe ... autoexcitation - selfexcitation?


May you grow up to be righteous, may you grow up to be true...

 

[Samiyoussif]

It depends on how the grid responds to the shocks and disturbances (protection maloperation or true faults), the number of partial or full blackouts per an interval of time should be considered.
The grids with more options is considered stable, or the term smart grids is used more likely

 

[cuky2000]

The concept of the short circuit ratio (SCR) is an indicator used to characterize the strength of a power system. This indicator is also associated with the system short circuit capacity.

The common concept of “infinite bus” (equivalent to infinite strong) is a special case of SCR = [&infin;]. This bus is able to maintain constant voltage for all fault location outside itself. With the same rational, busses with finite strength may be classified in the range of, 0<SCR<<[&infin;].

The SCR may be explained in the paragraphs below quoted from various sources:

• “A power system with high short circuit capacity results in high short circuit ratio. This means good performance in voltage regulation and vice versa.”

• “The short circuit capacity is defined as the product of the magnitude of pre-fault voltage and post fault current. The strength of a bus is directly related to its short circuit capacity. The higher the short circuit capacity of the bus the more it is able to maintain its voltage in case of a fault on any other bus. Also it can be seen that higher the short circuit capacity, lower will be the Thevenin’s equivalent impedance as seen between the faulted bus and the zero potential bus.”

• “A low short circuit capacity means a weak network. Switching on a load or a shunt capacitor or reactor will not change the voltage materially in a high short capacity system. However, if the system is weak i.e. it has a low short circuit capacity, a large size motor switched on in such a system may stall or have difficulty in reaccelerating following fault. Motor starting will cause system voltage dip.”

• “The strength of the a.c. network at the bus of the HVDC substation can be expressed by the short circuit ratio (SCR), defined as the relation between the short circuit level in MVA at the HVDC substation bus at 1.0 per-unit a.c. voltage and the d.c. power in MW".

Usually for SCR <2, indicate large load will create voltage dip and other undesirable operational performances. The index of SCR<2 should be analyzed for the specific power system application.

 

[m3ntosan]

cuky2000,

Thanks for expanding on this, I knew the relation from HVDC power transmission and commutation failure, you need more than 3 to avoid the problem, I never thought of this for bulk AC system..

May you grow up to be righteous, may you grow up to be true...

 

[electricuwe]

To me the limit of SCR < 2 as a limit for weak does not sound reasonable. Even for a SCR in the range of 2..5 operating a power system is close to beeing impossible.

 

[cuky2000]

The concept of SCR and associated range is not universally standardized. This may change for specific application. However, the rational to characterize the strength of the system using this parameter will provide fairly good indication of the system strength.

A similar concept to the SCR is the of Effective Short Circuit Ratio (ESCR) often used to evaluate the AC system strength to interface with HVDC converter stations.

The enclosed figure provide a range to classify the system strength.