difference between SVC & shunt capacitor ? 1

[shalhoob109]

long time ago, I have learnd that shunt capacitor bank is
used to support system voltage and produces VAR's. and
now i started reading about Static VAR compensator
which almost have the same purpose of use in application
side.
kindly please explain breivly the difference between
them from application point of view ?



Shalhoob

 

[rbulsara]

I believe Static VAR compensator still uses capacitor banks (as well as reactors and controls)and hence "static" compared to rotary compensator or synchronous condensers.

Rafiq Bulsara

http://www.srengineersct.com

 

[bacon4life]

Typical capacitor bank switching devices can only be switched a few times a day. They generally would not be activated during short voltage excursions, but rather would have a response time measured in minutes. Capacitor banks usually have just a couple of large steps, so they don't provide particularly precise control.

SVCs use power electronics to turn on and off reactors and capacitors on a sub-cycle time frame. Thus they have can supply the exact amount of VARs needed, even during transients such as motor starting.

 

[shalhoob109]

thanks rbulsara , bacon4life for helpful info you post.
for me I need some more time to cover this topic which
is now applied in so many places in my country.


regards

 

[bilalk]

hi,
I used to work with adoptive var compensators in the past.

Capacitor banks are mainly used for Power Quality applications, such as voltage sags, voltage flickers, and power interruptions. Sags are short term reduction in voltage and power interruptions are zero-voltage events. Both can be helped by using UPSes. But flickers are rapidly occurring voltage sags which can be smoothed by using capacitor banks for voltage control. These capacitor banks can be STATIC or DYNAMIC.
Static capacitor banks are good in price and have their applications on utility loads (just Google static var compensators utility) or in motors. The value of kvar is once calculated and the capacitors are deployed in the field....they sit there on the line.

On the other side, dynamic var compensators usually comprise electronic controls to add or reduce the amount of capacitors that will be switched up or down as per the demand for vars on the line.
A higher level of VAR requirement will require fast additions of capacitors versus low demand of VAR which requires lesser capacitors to be disconnected from the shunt. Usually, there are also reactor cabinets involved into dynamic var compensators, in order to absorb switching harmonics.

for dynamic var compensators there are many companies out there such as S&C electric, ABB, and I believe SIEMENS is active in that area of Power Quality as well. The websites of these companies provide great information from application point of view on your question.

I hope that helps.

Bilal

K-Bill

 

[waross]

There is a mid range application that doesn't seem to have been mentioned.
That is power factor correction.
In an industrial plant there will a power factor controller that will switch in small banks of capacitors to maintain the plant power factor between set limits. This will be much more precise than the typical large banks used by utilities but contactors are often used, so forget about sub-cycle effects.
The newer models where the contactors have been replaced by solid state switching devices may accurately be described as "Static Var Compensators".

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

 

[shalhoob109]

Bilal . I really appreciate your answer to the question
DVC is a new thing to me which I will start searching to read more in this field.


waross thanks. when voltage supported and needed VAR's provided to system, then automatically power factor will be
corrected. they are related somehow but the new thing you
mentioned that svc use switches to be more precise in
handling capacitor units with system demand instead
of using one circuit breaker as it appear in Capacitor bank

thanks again

 

[waross]

The type of correction that I referred to is most often found in industrial plants. Typically these plants must pay a penalty for poor power factor. Bulk correction may be used in some plants, but may cause overvoltages under light load conditions, hence controllers and small switching steps. In North America penalties often start below 90% lagging.
In some countries, penalties start below 99% lagging or leading. These conditions call for a power factor controller with very many steps to hold the plant power factor between the narrow limits imposed by the utilities.

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

 

[SMB1]

Shalhoop,

Shunt Capacitor has no effect to the system during dynamics,while SVC does. Specially, voltage stability or slow recovery.

SVC has fast control system via thyristors which makes it very helpfull to the system during transient. While Shunt capacitor is controlled by breaker.

SVC can provide reactive power capacitive or inductive depends on system requirements with smooth moving avioding CB switching in case of shunt capacitor of shunt reactor.

the disadvantage of SVC is the harmonics which are produced by the TCR, this is why usually you need fillters. Another disadvantage is the colling system which shall be designed carfully so as to avoid tripping of SVC at the time you need it.

Saleh

 

[shalhoob109]

Great valuable information that you added eng.saleh.
It's true that static VAR systems with thyrristor-controlled reactors (TCR)have some effects produced by injection of harmonics into the connected network.
therefor , filters are required to supress them.


shalhoob

 

[bilalk]

Shaloob,

you welcome.

You can read more on DVC for example at the website of the company

http://www.sandc.com

under Power Quality and then AVC (or Adaptive Var Compensators).

As Eng. Saleh was saying, the switching of capacitors produces harmonics, which require filters. Therefore, the AVC has a cabinet compartment dedicated to filters (also called reactors).

In general, the control cabinet detects voltage flickers on the line, and communicate with the trigger electronics of each capacitor bank....to fire up the VAR energy into the line exactly at the zero crossing of the line voltage to minimize harmonics.



Khashfe

 

[bilalk]

I wonder who can help me in answering this question that relates to the SVC from Shalhoob:

The dynamic var compensators have capacitor banks which are equipped with discharge circuits that requires a maximum of 10minutes discharge time for the capacitor and/or the bank.

Are discharge circuits similarly designed for SVCs?
Do SVCs require such cirucits?

 

[SMB1]

Hi Bilal,

In the SVC that we have, capacitor discharge is done through the SVC main transformer Delta winding. There is no discharge circuit for the filter.

 

[bacon4life]

SMB1

The individual capacitor units in the SVC don't have an internal discharge resistor??? Do SVCs not use IEEE standard 18 capacitors that must discharge to <50V in 5 minutes? Perhaps IEC doesn't require the internal resistor?