Grid Management,Load scheduling and Load Balancing

In this section, we consider the aspects related to grid management, load scheduling and load balancing in a power distribution system.

Grid Management

Let us consider the following questions:

What is a grid?

What is grid management?

What does grid management involve?

Definition of a grid

Grid is defined in the Electricity Act, 2003, as : “the high voltage backbone system of inter-connected transmission lines,
substations and generating plants.”
 

There are many advantages of having a grid.

ADVANTAGES OF A GRID

RELIABILITY: The system is more reliable since we can serve the load in more than one ways. As a result, even if one generation
unit fails the rest can share its load.

STABILITY: The system becomes more stable as the chances of a fault disturbing the whole system become less.

ECONOMY: In a grid, the cost required is lesser than a dedicated system since lesser installed capacity is required as well as lesser spinning reserve is involved.

Regional and State Grid in India

In the 1960s, India was demarcated into 5 electrical regions (NR, SR, ER,WR, NER) for planning, development and operation of the power system with regional self sufficiency. As on date, we have three synchronous power systems: Northern, Central (WR-ER-NER) and Southern. Bulk power transfer is possible among the regional grids through the inter-regional links. The Northern and Southern Systems are connected to Central System through separate HVDC links and, hence, each of the three systems can operate at different frequencies. The State Grid of each State is connected to Regional Grid for inter-State power exchanges.

What is Grid Management?

Grid management, as the term implies, is managing the grid. This consists of on-line real-time operation of the grid as well as off-line operational planning.The real time operation of the grid is looked after by the Load Dispatch Centre, which is basically a round-the-clock control room manned by grid operators or load dispatchers, who operate the grid by giving instructions to the personnel of the concerned generators and substations.

Load Dispatching, as the name implies, involves dispatching of load(or power) from the generator to the load. This is done through the transmission system. Load Dispatch Centre constantly observes the grid parameters and tries to ensure good grid operation. Operational planning is the planning done in advance, in order to ensure that

♦ generation matches the load at all points of time;

♦ the voltage profile at all points of the grid remains within acceptable limits;

♦ none of the transmission lines or inter-connecting transformers get over-loaded; and

♦ the grid operates in a stable manner, i.e., there are no power swings.

Operational Planning also involves coordination of protection of the grid so that only the faulted element gets isolated and the remaining grid continues to operate in a satisfactory manner.

The Grid Management in our country is done by the Regional Load Dispatch Centre (RLDC) at the Regional Level and by the State Load Dispatch Centre(SLDC) at the State Level. Each State, Central Generating Stations and Independent Power Producers (IPPs) are treated as constituents of the Region.

What is a grid?

You know that a power system has a generating unit to generate electrical energy, which is consumed at the load. This energy cannot be stored and has to be consumed at the same instant. But since the load is not concentrated at one place and it is not possible to have a generator very close to the load centre at all times, we go for transmission lines, which facilitate transmission of power from generator to load. Thus all generation units and load centres are connected and a grid is formed (Fig.).

The grid is basically a connection of generating stations, substations and loads through transmission lines, at a voltage level above the distribution voltage. The distribution voltage, however, is not strictly defined.It is different for different areas. In some distribution systems, power is taken from the grid at 33 kV, in some it is taken at 66 kV and in some, it may even be taken at 220 kV. Therefore, the grid covers the above mentioned high voltage system down to the level of connection point of the distribution system.

Two Key Players in Grid Management

 

Let us explain further the quality parameters of electric supply.Frequency is a global phenomenon, i.e., it is the same at all points of a grid which is operating in synchronous operation. Frequency is an indication of the balance between generation and load in a grid. If the generation exactly matches the load, the frequency would be the nominal frequency, i.e., 50 Hz. If generation is more than the load in a grid as a whole, the system frequency would be greater than 50 Hz. If generation is less than the load, the system frequency would be less than 50 Hz.Voltage is a local phenomenon, i.e., it can be different at different points of the grid.

Therefore, the grid operator has to ensure that the proper voltage profile is maintained at all points of the grid. For ensuring proper voltage profile, capacitors or reactors are installed at different points in the grid. If it is observed that the voltage is low at a particular point in the grid,then capacitors are installed at that point. Similarly, if voltage is observed to be high, as per the studies, then reactors are installed at that point. The basic purpose of these elements is to ensure that the reactive power requirement of the load or transmission lines is met.

What Does Grid Management Involve?

Grid management involves

• forecasting (demand pattern);

• planning (outages, unit commitment, resource scheduling);

• coordination (between stakeholders);

• supervision (grid parameters);

• real time operation and control for optimal utilization of available resources in the grid, which involves

− scheduling,

− monitoring, and

− restoration of grid;

•off-line operational planning involving grid security issues, restoration of grid and commercial issues or billing.
 

The load dispatch centre is primarily responsible for management of the grid.Its various functions: ex-ante (a Latin term meaning before-hand), real-time and post-facto (meaning after the fact) are given in Table

Grid Management Functions

Load Scheduling and Dispatch

Load scheduling means fixing the schedules of generation of power for generating stations and the schedules for drawal of power by the States taking into account drawal schedules from shared power sector projects and schedules of power purchased from buyers to sellers. Scheduling is done for the day ahead by the Regional Load Dispatch Centre to ensure balance between load and generation in the grid with the aim of achieving an operating grid frequency of 50 Hz. Since power cannot be stored to a large extent, power generated has to be used at that instant of time. Therefore, it has to be ensured that the generation matches the load at each point of time. Schedules are prepared on a 15 minute basis, to see to it that the average generation of electrical energy over 15 minutes matches the load over those 15 minutes. Scheduling is done one day before for the day ahead, as per a time schedule specified in the Indian Electricity Grid Code (named simply as “Grid Code” as per the Electricity Act, 2003), so that the State Power Utilities plan for load management for the next day.

For example, suppose a State finds that after taking into account its own expected generation for the next day and the net drawal schedule for the next day, it would fall short of meeting its anticipated requirement for the next day by 200 MW during peak time. Then it would have to plan a load shedding of 200 MW during peak time. Schedules can also be changed on the same day due to major load variations experienced by a State due to abnormal weather conditions. For example, rains in summer could cause reduction of agricultural load and AC load; heavy rains could cause disruptions in the transmission system and hence loss of load. The rescheduling would be valid after a time gap of about one and a half hours so as to enable implementation of the new schedules. Rescheduling can also be done by the Regional Load Dispatcher in cases of transmission bottleneck and grid disturbance.

Scheduling is important because it is meant to ensure the desired operating frequency of the grid. There are financial penalties for violating these schedules if these violations burden the grid and financial incentives if the violations help the grid, through a component of the tariff, known as unscheduled interchanges.

Monitoring the parameters of the grid is the prime real time function of the Load Dispatch Centre. These parameters include operating frequency, voltage levels at all points of the grid, status of line and transformer loading throughout the grid, especially at crucial points. In order to help the grid operator monitor these parameters over the large number of points in the grid, the Load Dispatch Centre is equipped with SCADA (Supervisory Control and Data Acquisition) System.

Grid disturbance normally takes place in a matter of milliseconds and there is no time for the grid operator to react. Therefore, the operational procedures for restoration of a grid are planned well in advance and come under the scope of off-line operational planning. The grid operator just has to follow the procedure for restoring the grid.We now describe the load scheduling process as it takes place.

The Load Scheduling Process

The process starts with the Central Generating Stations (CGS) in the region declaring their expected output capability (in MW) for 96 slots of 15 minutes duration during the next day to the Regional Load Dispatch Centre (RLDC).The RLDC breaks up and tabulates these output capability declarations as per the beneficiaries’ plant-wise shares and conveys their entitlements to State Load Dispatch Centres (SLDCs). The latter then carry out an exercise to see how best they can meet the load of their consumers over the day, from their own generating stations, along with their entitlement in the Central stations.They also take into account the irrigation release requirements, distribution utilities’ load schedules for next day and load curtailment, etc. that they propose in their respective areas.

The SLDCs then convey to the RLDC their schedule of power drawal from the Central stations (limited to their entitlement for the day). The RLDC aggregates these requisitions and determines the dispatch schedules for the Central generating stations and the drawal schedules for the beneficiaries (State as a whole) duly incorporating any bilateral agreements and adjusting for transmission losses. These schedules are then issued by the RLDC to all concerned and become the operational as well as commercial datum for inter-State and CGS transactions.

However, in case of contingencies, Central stations can prospectively revise the output capability declaration, beneficiaries can prospectively revise requisitions, and the schedules are correspondingly revised by RLDC. It is for the SLDCs to further break-up these State entitlements into Discom entitlements and State Generation Schedules.

Load Shedding

During the normal operation of a grid, it is possible that the load exceeds the generation. If this happens the frequency of the system goes down. The standard frequency is 50 Hz. But the frequency can go down to about 49.0 Hz.After this value, it is not advisable to allow it to reduce it any further since it can cause the system to lose synchronism and lead to ultimate collapse of the system. As a result, we go for purposeful shedding of load, known as load shedding. The load shedding is a process of reducing load on the grid so as to save the grid as a whole.

Load shedding can be done in two ways:

1. Automatic: For this purpose automatic under-frequency relays are installed. These relays carry out automatic shedding of load if the
frequency falls below a certain level.

2. Manual: Special guidelines have been provided by RLDCs/SLDCs for the load shedding at different frequency levels. These guidelines
depend upon the grid parameters at the particular instance as well as

some fixed guidelines for frequency falling below a particular limit or area-wise/consumer category-wise shedding.

Off-Line Operational Planning
• grid security issues;

• restoration of grid; and

• commercial issues or billing.

We discuss these briefly.

Grid security issues involve

• load generation balance planning in respect of active power for the next year, which is reviewed on a quarterly and then monthly basis, in order to ensure that the frequency stays at the nominal level;

• installation of capacitors or reactors to obtain a proper voltage profile in the grid;

• line and transformer loading;

• protection coordination;

• monitoring of the grid; and

• proper analysis of tripping of lines as well as of grid disturbance and taking corrective measures thereof.

Under-frequency and rate-of-change of frequency relays are installed as security measures to cut off load in case of gradual or sudden drop in generation, respectively, to ensure nominal frequency in the grid. Islanding schemes of important generators and loads are also planned as a last resort to isolate or island them in case of a blackout so that the important power stations and loads keep functional. Therefore, these islands are made in such a way that the generation and load in these islands approximately match.

Under operational planning, procedures are also formed for restoration of grid in case of tripping of some or more elements of the grid or for total blackout. This is done by the Regional Load Dispatch Centre responsible for real time operation of the regional grids, in consultation with all the players involved in grid operation. One of the points involved in grid restoration is the “black start”, which means starting of a generating unit after a blackout.Since hydro generators require the least power for starting, they are normally started first or, in other words, used for black start.

Commercial billing by the various generators is done in accordance with the Availability Based Tariff approved by the Central Electricity Regulatory Commission. Under the Availability Based Tariff (ABT), the beneficiaries are required to pay charges in three components, viz., annual fixed charges,energy charges and Unsheduled interchanges (UI) charges.Annual fixed charges are required to be paid by the beneficiaries irrespective of actual drawals or schedules. The implemented schedules, as described earlier, are used for determination of the amounts payable as energy charges.Deviations from schedules are determined in 15-minute time blocks through special metering, and these deviations are priced depending on frequency.These deviations are called unscheduled interchanges (UI).

The pricing for UI is linked to system frequency such that the constituent causing the grid frequency to improve/worsen in worst conditions gets rewarded/penalised at higher price and vice versa. Further, the UI pool account is zero sum account, i.e., the amounts received from constituents are distributed amongst the other constituents. As long as the actual generation/ drawal is equal to the given schedule, UI is zero and the payment on account of the third component of Availability Tariff is zero. In case of under-drawal, a
beneficiary is paid back to that extent according to the frequency dependent rate specified for deviations from schedule.

Load Balancing

Load Balancing is the process of achieving and maintaining equal load on each phase of a distribution transformer. The loadings on primary and secondary side of a DTR are shown in Fig. if load on each phase of the distribution transformer is not equal, it is called unbalanced loading of transformer. Practically speaking, balanced load cannot be maintained on the transformer due to the inherently varying nature of load. Each transformer supplies power to resistive loads (bulbs, heaters, etc.) and inductive loads (motors, etc.).
 

These loads can be either single phase, distributed separately on the three phases, or three-phase in nature. If the distribution transformer is supplying power to only three phase loads, then achieving and maintaining balanced load on transformer could be an easier task. But in practice, this happens very rarely, because each installation possesses either three phase or single phase or both the loads,which keep changing at different points of time.

Balancing of Load in a DTR

Some Tips to Operate Transformers Near Balanced Loading

• Connect single phase load on each phase of distribution transformer, so that at the end, current in each phase of transformer will be almost equal. It has been seen that, linemen or wiremen connect the single phase load to the lower phase of a pole. It may be due to illiteracy and/or hesitation to connect the single phase load on the top phase of pole. The distribution utility must ensure supervision of the job at the time of connecting new single phase load to avoid such practices.

• In some distribution utilities, transformers do not have current measuring instruments and, hence, continuous surveillance cannot be done to check whether distribution transformer is equally loaded (Balanced). In this case, the current must be measured for all phases by using Clamp-on-Meters, at least once during peak hours and a record should be kept. By analysing the past trend, the average current can be calculated. But due consideration must be given to changing weather conditions and/or extra loads due to festivals, etc.

• Providing Solid Earthing to the neutral of transformer.

• Using proper size of Blow-out-Fuses.

On this note, we bring the discussion in this unit to an end. In this unit, you have learnt about the Power Distribution System, and the general goals and practices for its maintenance. We now summarise the contents of the unit.