Generators: Load Shedding vs. Load Sharing

Hello rizwan,

I do presume you are referring to AC Alternators (NOT DC Generators)

If you are referring to DC Generators, please advise further.

AC Alternators

Alternators connected on a network do all run at the same speed, the "synchronous speed" which is related to the desired mains frequency = 50Hz in UK and their original market, 60Hz in US and their original market.

Load Sharing occurs when all alternators run at the same output = the total load, divided equally between all alternators, according to their actual output ratings.

Load Shedding occurs when one or more alternators "disconnect from supplying power to the network".

This disconnection can be because of lack of fuel in engine driven plant; water in hydro plant; shortage of steam in coal-fired plant and so on.

The disconnection, or load shedding may also be done manually.

Disconnection is often automatic, so that System Operators, who control geographically widely spaced Powerhouses, can shut down one or more alternators or even whole Powerhouses for maintenance, to conserve fuel or water etc.

It is often not realised that when an alternator on a network is not being supplied with sufficient motive power (fuel in engine, water in turbine, steam etc) the alternator just "coasts along" at the same speed = synchronous speed, like a car in free-wheel mode.

For that "coasting" alternator to actually supply useful power to the network, extra energy input is required = Water pressure in hydro; steam in High Pressure Turbine; diesel or fuel in engine generation equipment. (Car equivalent is step on accelerator/throttle)

When the extra energy is supplied - say in diesel powered engine mechanically coupled to alternator, the actual shaft speed of the assembly does not increase, because of the way system synchronisation occurs.

In a large Power Network, System frequency Control = via the synchronisation "Master" Alternator" is generally done via the largest alternator on the System at the time. Because coal-fired or Nuclear Power Stations generate steam which does take time to make, and the turbines need constant temperature to avoid damage due to violent expansion/contraction, Steam Turbine powered alternators are generally kept on-line 24/7 and run at full power. This is for greatest efficiency and least damage.

So...what happens is the alternator which has more energy put in, delivers a greater share of the power in the network - for example: if it is a diesel powered alternator, smoke pours from the diesel engine exhaust, the turbocharger starts to scream, and the diesel engine makes much more noise. Hydro plant turbine wicket gates open, allowing more water to pass through, increasing the energy input to the Alternator)

System Control Engineers have to take special care with load balancing, and power generation. They are assisted in this by many special control systems, which use very expensive and sensitive relays etc, to help them do this, and many large Power Networks can actually run 24/7 without any human intervention.

In general, for safety reasons, and the fact of Murphy's Law - Which states that "if something can go wrong, it eventually will", almost every Power Station and Network has 24/7 people, who do keep watch, just in case of some malfunction.

Should you need further information, please reply here, thank you....

Load shedding does not refer to disconnecting the alternator from the grid. Rather, it it is the disconnection of load circuits (feeders, major customers, etc.) from the grid when the connected load is greater than the capacity of connected generation. Some customers are sacrificed to prevent total failure of the grid.

In a utility, load shedding is usually accomplished automatically using underfrequency relays.If the generators are operating in overload condition, the drivers (turbine or engine) are also overloaded, and will slow down. On the electrical side, the slower speed is seen as a lower frequency. Distribution feeders and smaller transmission circuits have UF relays with staggered setpoints (recently enacted US regulations require a minimum of 3 steps of load shedding, with setpoints between 59.5 & 58.5 Hz). As the frequency drops, more load is disconnected until the remaining load is within the capacity of available generation. The UF relays will not reset automatically, but require a manual reset from an operator. This prevents a cyclic loading/unloading scenario.

Load shedding can also be initiated manually, to prevent an expected overload, using what the media calls "rolling blackouts". Feeders are divided into "Load Groups. Each load group is disconnected for a period of time. When the time is up, the next group is deenergized, and the first group restored. This pattern continues until the balance between generation & full load can be maintained.

Hello pwr2thepeople,

Thank you for your clarification, re Load Shedding.

Please accept this fine

Kind Regards from far away.....

Load sharing is explained correctly by other posts. 'Pwr' explained load shedding perfectly, and I only can add that load shedding can also be done on a priority basis. So if you had a 1000 kW and 800 kW paralleled, they will both load share equally if you have load sharing governors, but, if you loose the 1000 kW (trips on low oil pressure, for example), the load shed priority panel 'knows' you lost the big unit, and selects 800 kW of remaining loads to still be connected to the bus by shedding all others, but, on a priory basis. Not just random. So you can have 400 kW loads selected as your priority one loads (stair well lighting, fire safety things, elevators, phones), and inside the priority ones, you have sub priorities 1-A, 1-B, 1-C etc., then select the priority 2 loads to be 600 kW and sub priorities, 2-A (100 kW), 2-B (300 kW), 2-C (200 kW) with each sub priority fed by a transfer switch to a MCC panel group for further distribution. So in the above scenario of 800 kW capacity on line, all of the priority ones will remain (400 kW), but only 400 kW of the priority 2's can remain such as 2-A and 2-B.

Load sharing - when more than one power source are in parallel, equally sharing the load between them

Load shedding - when the load on the power source is at/over it's rated capacity, circuits that are non-essential/non-vital automatically trip off

I really enjoyed reading this post. I just wanted to add that your load shedding scheme is only as good as the next scheme down the line in a grid type transmission/distribution system if the downstream scheme even exists. I think some of the posts include the case where a large generator unexpectedly drops offline. There should be a scheme in place to limit the essential loads from the offline genset but there may not always be a load shedding scheme in effect for their loads. Then the grid must pick up the slack of the unshedded loads. This may overload the other gensets and they can start to dropout on underfreq and overcurrents. Then that can cause serious problems on the entire system as it did in the northeast US a few years ago. I guess what I'm trying to say is there should be a total system plan in place that includes load shedding.

Hello RidetheWave,

"serious problems on the entire system as it did in the northeast US a few years ago"

Yes, the entire Power Network needs to be very carefully planned.

Each alteration to a Power Network affects the entire Power Network, which may need alterations to allow for the change/s.

Power Network Systems are controlled mostly automatically, via expensive and complex "Protection Relay Systems", which control Circuit breakers, Switchgear, and Alternators etc.

The designing of a Power Network takes considerably more effort as the number of variables and Nodes increases.

The Northeast US Power failure a few years ago, was caused by a single relay contact which was slightly oxidized, thus no proper contact being made, a train of catastrophic disconnecting collapse was set in motion.

The System Operators saw what was happening, but the speed of the automatic system, set up to protect lives and expensive hardware (transformers, switchgear, lines, insulators, circuit breakers etc), was faster than the ability of the Network System Operators to regain control ahead of the automated shutdown, and stop the collapse.

This high Power Network System complexity problem has caused other major Power outages, in various countries.

All automatic systems are designed as best as possible, but it is impossible to allow for all possible events, because if that were done, we would not be living even in draughty caves.

So...we do the best we can, and now and then, some event occurs to show that our designs and systems are not 100 % for 24/7/365+.

As far as the Northeast US situation, the single Mega-Network that existed before that major outage, has been returned to several areas of more local control,so the "cascading collapse effect" of a series of automatic shutdowns can not occur.

It was realised some 50+ years ago, that if two large Power Networks were to be joined, particularly if the Control Centres were physically far apart, the best way to avoid most of the problems was to connect them via a DC (Direct Current) Power transfer link - This has been done in several situations. (We have one here in New Zealand between the North and South islands)

The advantage of using DC Power linking, is that AC transmission has reactance problems over long distances, which do interfere with the speedy control of the Power Network.

Well, that's what the experts say, let us just hope that in this situation, they are correct....

Excellent! I have heard that the state of TX in the US interfaces with other power grid systems via a DC interconnect. I now have a much better picture of why. I guess it's one of those Electric Reliability Council of TX (ERCOT) decisions that everyone scratches their heads about but is a good thing for the system as a whole. It's a great thing to "talk" via this forum.

Dear Mr.rizwan,

In simple Terms

LOAD SHARING is DEVIDING OF LOAD between 2 or 3 Generators - main requirement, COMPATIBILITY for Synchronisation, Matching of Droop Charater of Governor for Speed, and Droop Character of VOLTAGE of the Alternator.

LOAD SHEDDING is CUTTING of the FEEDERS, by rotation to reduce the Load in order to AVOID TRIPPING of the Generator, on account of Over-Load, on account of EXCESS DEMAND than the GENERATION CAPACITY.

DHAYANANDHAN.S