Technical Articles
The act of disconnecting a few non-critical loads when the electric power demand from the load exceeds the permissible electric power supply, is called Load shedding. This load is further reconnected when the demand is reduced, and the process is called reconnection.
The electrical power scenario in India is a deficit one. In the sense, the loads connected exceed the generation by a huge gap. And our fundamental concepts of Electric Power tell us that to ensure system stability, at any given point of time, the power injected into the grid (by generators or any storage elements), should equal the power consumed by the grid (Useful power plus losses). This effect is seen typically at peak load times when the load demand far exceeds the generating capacity, and the utility must either design a tariff punishing consumers exceeding demand or forcefully disconnect a few loads.
The utility, in this scenario, employs a tariff with a maximum load value that is previously agreed upon with the consumer. If the consumer exceeds this value, it imposes a penalty upon the consumer. This ensures that the consumer would rarely go beyond the maximum load value, and demand on the power system will be reduced.
In absence of load shedding techniques on the consumer level it will lead to penalties from the utility while on a power grid level, the demand may exceed the supply and that could lead to a drop in supply voltage, variations in supply frequency and can lead to system imbalance.
At this point we are forced to reduce some of the loads connected to the system. Some loads will have to be forcefully disconnected from the system to maintain balance.
This is achieved in two steps: First we will have to measure, in percentage terms, the loading on our incoming supply. Once we can measure this %Loading, we can set a limit (say 85%) as to when the system is getting overloaded and extra load needs to be shed off. The Relays can then be triggered on the %Loading crossing this limit to cut-off a few non-critical loads and thus reduce the %Loading below the permissible limit.
In the hotel industry HVAC Loads constitute a major chunk of the connected loads. These HVAC loads require a huge current at the time of starting and there is a chance of the load even exceeding the utility limits, causing penalties.
The increase in the load will be noted by the utility metering system and it will alert the user that the maximum limit has been crossed. At the same time, the user will also be issued a penalty/fine from the utility company, which he would be liable to pay during the next billing cycle.
The increase in load will trigger the alarm once the pre-set limit is hit. We can configure relay modules and employ relays to automatically trip the non-critical loads (such as parking lights or street lights) once the pre set limit is reached. Once the HVAC demand is reduced, the relays will then turn on the non-critical loads again.
So, in our example suppose if we have an incomer ACB with 4 MCCBs downstream of it as follows:
| Sr. No. | Device | Loads Connected | Priority |
|---|---|---|---|
| 1 | ACB | MCCB-1, MCCB-2, MCCB-3, MCCB-4 | Critical |
| 2 | MCCB-1 | HVAC Loads | Critical |
| 3 | MCCB-2 | Motor Loads (Elevators) | Critical |
| 4 | MCCB-3 | Room Lighting | Critical |
| 5 | MCCB-4 | Parking Lights and Street Lights | Non-critical |
We can configure a relay module that MCCB-1, MCCB-2 and MCCB-3 are critical loads while MCCB-4 is non-critical load.
So, when the load on ACB will exceed 90%, the percentage loading feature will detect it and the AL+ AL- terminals will subsequently go high. The release communicates this to the relay module using CAN Bus Communication, which will then cut off the MCCB-4 load since it is programmed as non-critical.
Now since this is remote and automated tripping, the ACB should have communication capabilities (either through UNCO* or Modbus Module). Also, the downstream connected MCCBs should also have electrical operation (motorized MCCBs).
When the demand from MCCB-1 (HVAC Loads) decrease after a few hours, the Relay module will turn on the MCCB-4 using remote electrical operation.
L&T’s Omega ACB offers an add-on relay module that can be used for load shedding on the consumer level. When the percentage loading on the ACB exceeds a specific value i.e., the load current exceeds a certain percentage of its rated current, the relay module will operate and cut off a few loads to avoid total shutdown.
This relay module can be programmed to trigger 4 configurable outputs (2NO and 2 Changeover) and is compatible with the MTX Release 3.5 and above.
Relay module helps in remote annunciation, interlocking and load management of the system. Relay module consists of four configurable O/Ps (2NO & 2 Changeover) which can be programmed through the release on any of the following protection:
Each of the above protection can be programmed for “Trip” mode or “Alarm” mode (Breaker operation can be programmed for breaker ON or breaker OFF mode).
Relay module can also be used for switching off the non-critical loads during the Overload condition so as to prevent the complete shutdown of the system.
Four relay modules can be looped together to one release at a time. Each relay module node address can be set using the dip switches on the module.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
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