Load Flow Study

Load flow study is considered as an essential electrical power system study and it serves as a basis for all other analyses.  It is performed to ensure that all electrical power system elements operate properly and securely during normal (“steady state”) system conditions. Load flow studies determine parameters such as voltage profile / loading of network elements/ power factors / power and current flows at various locations in the power system and suggest application of corrective actions.

Even though fundamental approach to load flow studies is same, they can significantly differ based on the:

Studied time horizon:

• Short term or operational studies that assess existing system during particular time (week, month or calendar season)
• Mid term planning studies that assess electrical power system which will be developed in several years (typically 3-5)
• Long term planning studies that assess electrical power system which will be developed in more distant future (typically more than 5)

Study boundaries:

• Single change (commissioning, decommissioning or modification of existing elements) in the network can be observed
• Multiple changes in the network that typically refer to the area or even the whole country electrical power system
• Interconnection studies which observe power flows across multiple countries or even whole continents

Number of details:

• Voltage levels which are being observed which is related to load aggregation
• Power plant modelling (aggregated plan model vs. detailed plan model)
• Investigation of particular network equipment or larger part of the electrical network

Electrical Network type:

• High voltage network (typically with voltages higher than 110 kV)
• Medium voltage networks (typically voltages in the 11 kV – 110 kV range)
• Low voltage network (voltages lower than 1 kV)

Therefore, approach to load flow studies may vary based on the set objectives.

Recommendations for completing load flow studies can be found in “3002.2-2018 – IEEE Recommended Practice for Conducting Load-Flow Studies and Analysis of Industrial and Commercial Power Systems”.

Also, many electrical companies have their own sets of procedures, manuals and instructions that may be binned together in a form of a grid code, so it is necessary to ensure that their recommendations are followed and adhered to.

There are no standards that require load flow studies be completed. Load flow studies just like many other calculations are risk reduction actions, and not mandated by official standards but instead are the option of the owner as part of risk management. However, many electrical utilities prescribe set of studies that need to be completed as part of their established procedures for risk mitigation. In the case load flow studies are done for a company with already established study requirements, it is be necessary to follow the same.

Since there are so many types of load flow studies, many different organizations request studies. Large transmission and distribution utilities with large number of substations and transmission lines are typical study users. Similarly, a number of industry facilities (oil refineries, smelters, mines, etc.) with dedicated electrical networks or embedded generation are common users and requesters of studies and these are being done to improve their reliability and increase uptime. Also, new developments that are planned for the interconnection to the main electrical (transmission or distribution) power system typically need to demonstrate compliance with the utility requirements by completing studies set in the grid code.

If your organization is responsible for development, upgrade or operation of the existing of future electrical network, then you are likely to be interested in the load flow study

In all types of studies, the basic steps are similar.

1. Clearly define the objective of the study. Integration of the new project to the electrical network, improving the reliability of the existing network or planning the long-term network development clearly involve different objectives, hence approach may not be exactly the same.
2. Gather all relative input data. That may include existing software models, reports, reference documents, data sheets, test reports, etc.
3. Model the system in software such as DIgSILENT PowerFactory, etc.
4. If possible, validate the model against the actual system by cross comparing software outputs with real time measurements
5. Plan and develop potential study cases based on the objectives that have been set and based on the actual requirements that are prescribed in local regulations.
6. Rerun cases with mitigation options if that is part of the purpose.
7. Summarize results including voltage profile, network loading, power factor, active and reactive power flow in a graphical and tabular form.
8. Clearly list study conclusions.
9. Make recommendations and if necessary, propose future steps.

The data collected for a load flow study is dependent on the purpose of the study, but the following are examples of required data.

1. Collect single line diagram of the electrical network. This may also include conceptual scheme of the future developments.
2. Collect information about existing and future load and generation. It is important to know their rating and connection points.
3. Collect or determine electrical characteristics (resistances, inductances, nominal current and voltage levels, etc.) of network equipment (overhead lines, transformers, underground cables, capacitor banks, shunt reactors, busbars, etc.).
4. Preferred running arrangement (topology) of the electrical network and operation philosophy.

Load flow studies are done for all geographic locations. However, based on the location a number of scenarios that need to be investigated can significantly differ. Weather seasons are different in different parts of the work which affects load and generation levels, hence power flow in electrical network will be different. Similarly, depending on the generation portfolio, there could be substantial time shifts in load peak demand. Also, high ambient temperatures may limit electrical network transfer capacities which need to be accounted for when evaluating calculation results and suggesting mitigation measures.

Very simple load flow studies that include limited number of study scenarios and that refer to small electrical systems can take as little as a few days to complete, if all relevant data is available. Most of the load flow studies require one or two weeks to complete. However, studies of large and complex electrical systems that involve many multidisciplinary variables may take months to complete.

Load flow study can help of selection or confirmation of the electrical equipment (transformer, underground cable, overhead line, etc.) before it is introduced into the power system. Also, it can provide suggestions on optimal operation of the existing power system by suggesting the optimal network topology that may decrease network losses and operational costs. Finally, it can provide recommendations on strategic long term expansion plans. All of this is achieved by ensuring that essential system parameters such as voltage profile, power factor and network loading are within tolerable limits.

There is a number of definitions that are used in load flow studies and they depend on the actual electrical system. Some of the most commonly used are:

• Load flow convergence – obtaining the solution for the load flow calculation
• Slack bus – A slack bus is generally defined as a bus with generating units that balances the reactive power (Q) and real power (P) of the system while doing load flow analysis. A slack bus is also known to many as a “Reference Bus” or a ” Swing Bus “.
• Lagging power factor – network load is inductive in nature. Also, power plant operating with lagging power factor injects reactive power into the electrical network.
• Leading power factor – network load is capacitive in nature. Power plant operating with leading power factor absorbs reactive power form the electrical network.
• POI – Point of Interconnection is a place at which plant or subsystem is interconnected to the main network
• SVC – Static Voltage Compensator is a shunt-connected static VAR generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage).
• Voltage and reactive power regulation – set of measures used to control voltages and reactive power flow in the electrical network which include but are not limited to adjustments of transformer TAPs and shunts and additional reactive power compensation (capacitor banks, shunts, SVC).

Absolutely, below are links to three studies that can be read and downloaded.

• Example of load flow study for industrial system
• Example of load flow study for transmission system
• Example of load flow study for integration of the Photovoltaic (PV) and Battery Energy Storage system

Contact us for more information or to make arrangements for a quote.