Power System Planning For Reliable Power Supply

POWER SYSTEM PLANNING

The motivation for this post came as a result of continuous power rationing prevalent in my area. We get power supply here based on rationing which entails having 8 hours of light once every two days. The level of load shedding adopted bt the utility control office is a thing of sorrow.We can trace this intermittent power supply to lack of power system planning.People build houses and connect to the power grid without even notifying the utility company. This lack of planning and knowledge of the maximum demand of the consumer overstresses generating and network facilities leading to regular and persistent power outages.

Pixabay: Transmission Line

Power system planning is a tool for evaluating the consequences of different proposals for the provision of economical, reliable and safe electric energy to consumers. And the primary objective of distribution system planning is to ensure that the growing demand for electricity can be satisfied in an optimum way by additional distribution systems, from the secondary conductors through the bulk power substations. These additions must be technically adequate and reasonably economical.

In implementing these objectives, sub-transmission network is subject to upgrading or modification as determined by the proposed plan on future growth/extension. It also defines the locations and sizes of sub-stations, service areas of sub-stations, locations of breakers and switches, sizes of feeders and distributors, voltage levels and voltage drops in the system, the position of capacitors and voltage regulators, and the loading of transformers and feeders.
Some of the factors that influences the system planning include:
Demand for ever-increasing power capacity and higher distribution voltage, ecological considerations, scarcity of available in urban areas, limitations on fuel choices and the need to minimise production charges.

LOAD FORECASTING

The load growth of a geographical area is a very critical factor that dictates the expansion of distribution system. Thus, there is need to forecast the load increase, and the manner in which the system will react or adjust to such growth is critical in the planning process.

The time frame for such forecast can be subdivided into two:
• Long-term: this is for period covering seven years and beyond
• Medium term: this for period spanning one to seven years
• Short term: this is a period covering one day ahead.
The short-term is necessary for planning the level and mix of generating capacity that will take care of the actual demand coming to the power grid, and the sequence in which generating stations are brought into operation.

We use short to medium forecasts to prepare operating plans, financial planning and setting of the tariff structure.
The long-term forecast of electricity demand and consumption are used in planning the investment in generating capacity and making sure that the fuel for the power station will be available at a reasonably low price.

The essence of accurate forecasts in planning is that ensures the optimal use of the available capacity while at the same time providing a reliable supply of electricity. Optimizing the available capacity leads to better economics, and therefore the cost per kilowatt of the power generated will be affordable by the consumers.

LONG-TERM FORECASTING TECHNIQUES

There are about five forecasting techniques which include: subjective, univariant, multivariant, and end user.More than one method could be combined to achieve a level of accuracy no obtainable by using one only.

In the subjective approach, forecasts are made relying on judgement, intuition, hunches, general knowledge and any other relevant information available. In this method, the use of historical data may or may not be taken into consideration.

Univariant forecast relies so much on the history of the behaviour of the system involved in a given time series. Many forecasting methods fall into this category such as extrapolation of trend curves, Box-Jenkins, exponential smoothing and Holt-Winters.

Multivariant technique strives to establish a casual or explanatory link and as such relies on the method of creating a relationship between variables in a defined way. This process tries to relate system loads to various demographic and economic factors, like trying to connect electricity sales to variables like the price of the electricity and customer’s income.

Essentially, such factors as population, employment, building permits, weather data etc. are often used in this approach. The regression model and econometric model fall under this category.
In the end-user approach, the forecast can be made by identifying those activities that result in sales or consumption of electricity demand.

PLANNING OF GENERATION

The essence of generation planning is to adopt the most economic expansion scheme which will also maintain high reliability with regards to the perceived demand growth in a certain period.

Firstly, we establish the appropriate location of the generating station, next is it to erect and commission it and lastly, take a decision as to when the power station can come into operation (connected to the grid).

It is crucial to consider the following factors to achieve optimal generation expansion scheme:

We should examine the feasibility of the generation structure. There are different types of generators and care must be taken while making a choice, to put into consideration the cost and availability of the fuel, distance from the load centres, the effect on the immediate community hosting the plant and government policies, disposal of waste from the operation of the plant.

The reliability indices of electricity supply

The sensitivity of the scheme to variation in fuel price and generating unit investment.

The effect of slowing down specific vital projects.

The points mentioned above make generation planning a rigorous and a non-linear exercise. There are numerous uncertainties involved with regards to data required for generation planning such load forecasting, fuel and equipment cost etc.
This problem and uncertainty could be reduced by making a reasonable assumption that will simplify the mathematical model of the optimal generation expansion scheme. This assumption is in order because the mathematically optimal solution may not be optimal from the engineering perspective.

Network Planning

Network planning is a central and essential aspect of power system planning. Network planning aims to achieve optimal network configuration (taking cognisance of load growth and generation planning scheme of the period) that can successfully meet the requirement of economic, safe and reliable supply of electricity to the consumers.

Network planning and Generation planning cannot be isolated or planned independently of one over the other. In some scenario, network planning determines the generation capacity to be erected. The fundamental principle of network planning is to use minimal materials and resources in the building of the network while still ensuring safe delivery of electricity to the load centres.

Network cost encompasses the investment in transmission and distribution equipment and accessories and their operating cost. Network planning has many decision variables and, thus it is difficult to obtain a complete mathematical model. The way out of it is to divide network planning into two steps: scheme formation and scheme evaluation.

Scheme formation deals with the different alternatives available to finance the cost requirement of erecting the power transmission line according to the transmission line capacity. Small power system planners can rely on their experience to actualise the network configuration, but for a large system, we will enlist the help of computer loaded with suitable programmes to actualise the network configuration.

The second stage which is scheme evaluation entails making the entire economic and technical assessment of the formed network schemes. This assessment involves power flow studies, stability analysis, short circuit current capacity, reliability and economic calculations, and at the end, a final decision is reached factoring in the above studies/analysis.

The good thing about scheme evaluation is that additional information can be gotten from the resulting calculations which will further reveal the best operation of the network configuration and as such, we can use it to improve on the prior recommendation.

Network planning should be able to cater to the following questions
• Location of the line
• The time it should come into operation
• The type of transmission line the intending line would be.

Network planning methods can be classified into two: Intuition approach- which allows the engineer to rely on his personal experience for decision making, although this approach has been criticised for its inability to yield a mathematically optimal solution.

The second approach is mathematical optimisation which formulates the design requirements of network planning as an operational mathematical planning model which is solved by an optimisation algorithm to yield an optimal planning scheme which satisfies all constraints.

It is subject to the argument that combining the intuitive approach and mathematical optimisation approach gives the best result.
It is important to remember that network configuration starts at the customer level because the demand type and the load characteristics influence the type of distribution system required.

CONCLUSION

No one forecasting method is effective in all situations; forecasting techniques must be adopted as tools to assist the planner, sound judgement, analysis and experience are irreplaceable. Also, no particular method or approach will work for all utilities. This post is not exhaustive, and the intention is to introduce some ideas currently employed in forecasting system load requirement.

Forecasting in its entirety is not enough as it is somewhat dependent on some factors which include government policy, the accuracy of the said forecast, the behavioural pattern of consumers etc.Therefore, the power system engineer should shift the emphasis so much from a single forecast or projection and focus more on the adaptability of the power system to changes in load pattern.

REFERENCES

1. Electric Load Forecasting:Advantages and Disadvantages
2. A Methodology For Electric Power Load Forecasting
3. Power System Planning