Load Dispatch Center

India is a very large country with 173GW generating capacity. The most important point to ponder on is how such huge amount of energy is transferred to such a huge number and variety of customers. Surely the network needs to be too much efficient and rules and regulation laid down concrete.
The power is generated at 21kV in the power plants. These are stepped Up to 400kV and fed to the National Grid (transmission line). Substations of 400kV and 200kV are connected to the appropriate transmission lines and step down the voltage to a value required by the various customers. Also we have 765kV HV lines to which they can be stepped up to.

As an electrical engineer an important point to note is: The stability of the power system decreases with decrease in size. To explain it a bit further let me define a term Power Number as the Power output per unit frequency. A 500MW plant producing at 50Hz has a power number of 10MW/Hz. Consider an isolated power grid or system with 10 generating plants and capacity 1GW at 50 Hz. In a generating station of 200MW trips, a fall of 10 Hz would be observed. This number will go on getting small as we increase the total capacity ie. As we go on increasing the size of power system the stability will improve. Thus for a large country like India producing at more than 100GW, tripping of a 1000MW unit would produce a minute change in of .5Hz would be observed.

The need for widening the power system was recognized in way back in 1950s and by 1990s we were able to prepare a national grid synchronized at a common frequency and various regions combined together through hvdc and ac links. This has helped to increase the stability of our system to a very great extent.

Our power grid is divided into 5 regions:

1.    Northern Power Grid
2.    Eastern Power Grid
3.    Western Power Grid
4.    Southern Power Grid
5.    North-Eastern Power Grid
 

Interconnection between various regions:

International Interconnection:

Now the important question is why do we require load dispatchers in this system?  

Consider a case in which a power plant trips, or some auxiliaries or components in the system needs to be repaired or replaces. If every such unit was free to take related to these issues on their own, there would be lot of instability in the system. Power cuts are very disastrous for industries, hospitals, and various emergency services. It also brings down the economy. Apart from maintaining the stability of the system, the load dispatchers act as the link between the generation side and the customer side. They ensure that all the rules regulations are being followed. They maintain the operating frequency within desired limits thus making our national grid more reliable.
Hierarchy followed among Load Dispatch Centre:

 

• National Load Dispatch Center
• Regional Load Dispatch Center
• State Load Dispatch Center

NLDC is the apex body in nation which looks after enhancing and optimizing national grid. It is responsible for keeping the grid synchronized and grid security. It has the authority to schedule and dispatch electricity over inter regional links. The RLDC’s have to comply with the orders of NLDC failing which they are heavily fined.

RLDC looks after the scheduling and dispatching of electricity to the states within its region. They have the authority to give directions to the SLDC’s. SLDC’s are heavily fined if they don’t comply with it without a proper reason. They keep an account of electricity transmitted between various states in their region.

SLDC is the apex body in a state. They keep account of electricity transmitted from licensees, the generating companies, substations and the consumers. It is responsible for optimum scheduling and dispatch of electricity within state.

Details of regular schedule of load dispatch centre:

Pre Dispatch function

Work in LDC start with a day ahead scheduling. Load forecasting is done based on the previous year’s data available for the same date. Also national holidays, festivals and other important events are taken into account. Load dispatchers collect the information from various generating stations about how much they will be providing the next day. Then a schedule is prepared with 96 time blocks of 15minutes each by 3PM. Schedule preparation takes into outage planning, electricity from central pool, cost of electricity from various generating companies and the forecasted demand. The schedule is sent to the generating companies at 10PM and given time for any revision up to 11PM and then freeze. This schedule becomes applicable from 12AM. The various licensees, withdrawing companies are given the schedule how much they will be withdrawing and at what time.

Few lines above I used the term outage planning. Let me explain what outage planning is. There are various auxiliaries and components in substation, or generating plant etc. which would require maintenance. They cannot be disconnected without prior permission of the area LDC. This would seriously affect the scheduling. The substation or the responsible authority have to apply for undertaking maintenance to the LDC. After looking at various aspects the LDC give them a time slot to undertake the repairing. This is called Outage Planning.

Various appliances that are used in homes or factories are designed to operate at 50Hz ideally. Large deviating from operating frequency would cause large damage to the appliance. The losses will be huge in case of large machines and equipment. The turbines at the generating plants are set to trip at around 5percent deviation (<47.5 and >52.4 or 51.5) after generating a series of alarms. It is the task of LDC to keep the frequency in the operating range. Because if the a power plant trips, frequency will fall leading to tripping of other plants and finally the grid would collapse.

Another important thing is the active and reactive voltage availability. A minimum amount of reactive voltage is required by induction motors and few other appliances, but large amount of reactive power lead to lowering of voltage which is undesirable. To cope with all these capacitors banks are installed at various power stations and substations to absorb reactive power. Also the LDC can direct the power plants to absorb reactive power which is very rare. It causes loss to the power plant so they are very reluctant to do so.

As for the frequency, the LDC can ask the power plants to boost their generation in case of low frequency or reduce their generation in case of high frequency. If all these do not help to make the scene better, they go for load shedding option.

Load shedding is done in a much planned manner. First of all the areas are classified into various groups depending upon the total cost the distribution company incurs and the revenue they collect.
The load shedding is maximum in those area where this ratio is high in other words the region where there is too much of electricity theft.

A lot of relief came in the year 2005 with the introduction of self-correcting tariff system named ABT or Availability based tariff. The ABT system divides the total cost into 3 parts:

1. Fixed Charges
2. Energy Charges
3. UI Charges

Fixed charge is levied if the OLC declared is greater than 85% availability. Energy charge is the charge per unit. Most interesting is the UI or the unscheduled charges. These charges vary from 0 to Rs 16 per unit. If we overdraw power at frequency higher than ideal frequency, the UI charges are null. However if we overdraw the power at lower frequency UI charges are levied depending upon how large is the deviation. This helps to discourage the consumers to overdraw the power at lower frequency thus helping to maintain the optimum frequency. An important question that might arise is who will be paying for the overdrawn power at high frequency if we are not paying. Well this is paid as a penalty by those who are under drawing from what they have been scheduled to draw.
One more case can arise is that the generating plant fails to deliver what it had been scheduled to. The customer is drawing power according to the schedule. So extra charges can’t be levied on him. But the frequency has fallen down. Who will be paying for this? Here penalty would be imposed on the generating plant. One way to escape so much loss due to all this is to ask for revision of schedule to the LDC which help them to reduce the losses incurred.
Also it can happen that the power generating companies in a lure of earning more give the LDC lower capacity, and when the frequency goes down, they would inject more power to the grid to become wealthier. To prevent this sort of exploitation, rules have been laid that they can’t inject more than 2 to 3% to the scheduled amount. If they do so, they won’t be given the extra UI charges they are expecting.

Hydro Power plant are a great relieve to the LDC. One important property of the hydro power plant is that they can boost or reduce their generation in very less time. So the SLDC use them to optimize and enhance the grid in case of emergencies. Area LDC of Nagpur uses Koyna Project frequently for this purpose.

Now let us come to the most disastrous part that can happen ie. Failure of a grid. How do the LDC cope and bring the things to normal. They ask for power from the neighboring regional grids and power its own generating stations first and high importance areas like hospitals. As the generating power stations start they ease the load from the neighboring link. Thus after all the generating stations have started, the normal power is restored.