Load shedding

[Mayur Bhavsar]

For load shedding protection on basis of df/dt or under frequency, frequency should be checked of line or bus?

 

[cranky108]

What be the difference between the bus and the line frequency?

 

[beyond86]

A relay has to check bus voltage. In case when your section supply from a tie-breaker, using line voltage is not correct.

 

[stevenal]

In regards to cranky's question, there is no difference when the breaker is closed. If load shedding causes the line to open, the bus VT will continue to provide system frequency data while the line VT won't.

 

[che12345]

Dear Mr. Mayur Bhavsar
"... For load shedding protection on basis of df/dt or under frequency, frequency should be checked of line or bus? "
1. I take it a case of an island generation station or a marine installation on board. Assuming that there are numerous gens connected to a common busbar.
2. Assuming there are three gens on board, but one is [down]. Two gens are fully loaded. But the load demand is increasing exceeding the two gen's capacity. The BUSBAR frequency is going to [drop]. The load shedding df/dt protection relay comes into action (may be with some time delay) to shed (switch off) some of the [non-essential] loads. The shedding may be in stages with the least essential loads to be the first stage and followed by the second stage, etc.
2.1 The (frequency) should be checked on the common [BUSBAR].
3. Caution: If shedding is not done in time, one of the breakers is going to trip on over-current and followed by the second; resulting to total blackout!
Che Kuan Yau (Singapore)

 

[sushilksk]

Frequency should be checked for Bus. This protections is not feeder protection, it is to protect power system from situation when demand is more than supply. Frequency is monitored even in CB open condition.


Considering case of UFR connected to line. It may got reset by "under voltage" after trip. Attempt can be taken to close breaker in under frequency condition.

 

[Mayur Bhavsar]

Actually in my case, there is single incomer & 4 outgoings. There is no bus pt…so for load shedding, frequency shall be checked through incomer line pt or bus pt is required

 

[beyond86]

In this case, you can use line voltages for load-shedding.

 

[Mayur Bhavsar]

Then when we need to go for bus voltage

 

[beyond86]

In cases when you have another incomer to a bus.

 

[waross]

Your bus may not be a physical set of bars.
A point of common connection between the load side of the incomer and the line side of the out-goers may be considered the bus for metering and protection.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mayur Bhavsar]

What will be the case when there are two incomers & one buscoupler in between them…no paralleling between two incomers…if buscoupler closed with one incomer ON, whole bus will be treated as single bus…in such case can i go for line voltage of incomer for frequency checking

 

[che12345]

Dear Mr. Mayur Bhavsar 17 Oct 21 07:10
1. [highlight #3465A4]"...Actually in my case, there is single incomer & 4 outgoings. There is no bus pt…so for load shedding, frequency shall be checked through incomer line pt or bus pt is required ? "[/highlight]
Since there is only [one source], with breaker closed, the frequency at the incomer = bus. The intention of load shedding is to [switch off] some non-essential loads when the BUS frequency goes down, when the gen is over-loaded. See 2 and 3. below.
2. [highlight #CC0000]"...What will be the case when there are two incomers & one buscoupler in between them…no paralleling between two incomers…if buscoupler closed with one incomer ON, whole bus will be treated as single bus…in such case can i go for line voltage of incomer for frequency checking..."[/highlight]
NO. For load shedding, monitor the frequency of the BUS, irrespective of which of the two breakers is on.
3. BTW for info only: in cases [there are numerous gens connected to a common bus], [each gen can be running at a different speed (frequency) for kW sharing]. But this is NOT evident on the frequency shown on the BUS. Therefore, for load shedding purpose, monitor only the common BUS.
Che Kuan Yau (Singapore)

 

[Mayur Bhavsar]

If gen is overloaded, line frequency will also go down, then why cant we check for line frequency instead of bus

 

[mgtrp]

3. BTW for info only: in cases [there are numerous gens connected to a common bus], [each gen can be running at a different speed (frequency) for kW sharing]. But this is NOT evident on the frequency shown on the BUS. Therefore, for load shedding purpose, monitor only the common BUS.

This is a little misleading.

If the generators are connected to a common busbar, the frequency measured by VTs on the busbar or the on the feeder will for all practical purposes be identical. Generators might have different speed setpoints as part of a droop load-sharing scheme, but this does not mean that the generator itself runs at a different speed. You could argue that during a system disturbance the shaft speeds of each generator might end up at slightly different frequencies transiently, but again I think that for all practical purposes this difference is going to be negligible, especially if the generators are electrically near each other.

 

[che12345]

Dear Mr. Mayur Bhavsar (Electrical)(OP)18 Oct 21 04:50
"...If gen is overloaded, line frequency will also go down, then why cant we check for line frequency instead of bus "
1. This is true for single gen case, where the (line frequency = bus) when the breaker is [closed]. Monitoring the (line frequency for load shedding purposes) is meaningless; when the breaker is [open].
2. Attention! It is possible for the gen to be running at a [lower speed (frequency) and current] i.e. NOT over-loaded. In this case, load shedding is NOT required as the current is lower that the gen rating. The breaker is not going to trip on [over-current] but perhaps on (under-frequency) protection; which is always set at a much low value than the commencement of the load shedding setting.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. mgtrp
1. My experience on board of ship where the main gens which may be of different or equal sizes; located at close proximity. When we are going to rotate the gens, we [lower] the out-going gen speed (frequency) (A also kW). The other gens pick up the load (A +kW). The in-coming gen adjusted to "slightly" higher speed (frequency)than the bus frequency and syn in. Note: syn scope reads + rotation and close the breaker at 12 o'clock. After syned in, increase the speed to pick up load (A+kW) according to the size.
2. If the running gens are [sufficient for the load], the out-going gen can [lower the speed (frequency) also (A, kW)]; but the bus frequency remains the same.
Che Kuan Yau (Singapore)












when we

 

[waross]

I have run sets in parallel both on land and sea.
When dropping out one of a paralleled set of generators, it is impossible to run one generator at a lower frequency than the others.
It is common to drop the load from a set before opening the breaker to minimize system upsets.
Yes, you use the frequency control, but the frequency does not change directly.
As the frequency is set lower on one set, the set sheds load in accordance with the droop characteristics.
This puts more load on the remaining sets and they drop their frequencies slightly in accordance with the droop characteristics.
Example: Three 1000 KW sets running in parallel, each with 667KW load for a total load of 2000 KW.
Droop is 3%
One set is to be taken off line and the load is reduced to almost zero.
The load on the remaining sets will increase to almost 1000 KW each.
The load on the loaded sets will be increased by 33%, and the frequency of all three sets will drop by 1%, or .5 Hz or .6 Hz.
Good practice is to never go below zero load. In practice 10% load is a good limit.
Dropping off line at 10% load will cause little system upset.

Under-frequency at a substation:
When the breakers are closed, the frequency will be the same on all lines and on the bus.
As long as the incoming breaker remains closed, the frequency will remain the same on both sides of the breaker.
Load may be shed by opening one of more out-going breakers.
What you must consider is the condition when the incoming breaker is opened.
If the PT is on the bus, then you will have no indication of the incoming line frequency when the incoming breaker is open.
If the PT is on the incoming line, then you will have no indication of the bus frequency when the incoming breaker is open,
BUT, with the incoming breaker open, the bus frequency will be zero.
With two incomers: You may want to compare frequency and phase relationships before closing the second incoming breaker.
Again, You may want to see the frequency ahead of the incoming breakers before closing the last incoming breaker.
Or, put the PT on the bus and hope that both incomers are at the same frequency before closing the last incoming breaker.
Your choice.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mayur Bhavsar]

In my case, incomers are fed from transformer. As i already stated, there will be no paralleling between transformer, individual incomer feeding respective section…here my line freq = bus freq… so shall i go with line freq check for load shedding in such scenario

 

[che12345]

Dear Mr. Mayur Bhavsar
1. I take it that the primary side of each transformer is fed by a gen; which is [within your control]. Each gen is having basic V,A,W,Hz,pf metering, with engine speed raise/lower toggle switch and voltage trimer. These metering and control may be on the secondary, which is your incomer; which controls the gen.
2. The Hz meter is always connected on the incomer LINE side. When the incomer is:
2.1 open, the Hz reads the gen speed (frequency), toggle up/down as required. The bus is dead; V=0, Hz=0 . Load shedding does NOT come into the picture.
2.2 closed, the line Hz=bus Hz. Load shedding comes into the picture.
3. Proposal: In principle, load shedding monitors the bus, NOT the line.
3.1 However, if there is only one set of PT on the Line side, the load-shedding which is connected on the Line side would function equally well.
Che Kuan Yau (Singapore)