Power System Studies
There are three over all time periods that a power system study will look at, they are:
- Transient
- Dynamic
- Static
Transient studies look at the over all power system response to an event over a very short period of time. This time period can be from a portion of a cycle to 1 or 2 seconds. An example of a transient study would be analyzing the system response of a capacitor bank being switched on to the system, and verifying that it won't introduce an instability.
Dynamic studies look at the power system response over seconds to a minute until the system has reached a new steady state. An example of a dynamic study is starting a large motor, line an ID Fan. Motors have high starting currents and can have major impacts on the system voltage.
Static studies review the system in a steady state and is used to report the values. Load Flows, also referred to as Power Flow studies are good examples of this. They are used to determine the voltages, currents, and power factor at all the buses and cables within the power system.
The majority of the studies that we will focus on here are static type. They look at the system using steady state assumptions to see if the system will be stable, during a single event. These include:
- Short Circuit
- Load Flow
- Arc Flash Analysis
- Protection Coordination
- Harmonic Analysis, and
- Ground Grid Studies
The first four in that list I think every system should have current and as-built. I make my case for why here - Power System Studies - The Critical Four.
What are Power System Studies
Short Circuit Analysis
A short circuit study is used to determine the worst-case current scenario during a 3 phase and single phase bolted fault. This is critical design information when selecting equipment for a new design, and replacing existing equipment during upgrades. Two factors contributing to the fault current is the size - and impedance - of the transformer supplying the circuit, and the available fault current that can supplied by the utility, and on-site generation.
A short circuit study is used to determine the worst-case magnitudes of these currents. These currents will be used as design inputs when purchasing new equipment, and verify that system changes haven't affected the maximum design currents of the equipment. The equipment must be capable of withstanding the maximum short circuit available. When a short circuit occurs, these high currents create strong magnetic forces and if the equipment design hasn't been tested, or braced adequately, for these forces there may be catastrophic failure causing additional damage. Under normal circumstances, panels, switchboards, etc are designed to contain the fault; but this is only to a specific current rating.
When should I have a short circuit study completed?
The short circuit study will be completed, and updated during the design process, but should be reviewed periodically to ensure that there have been no changes that affect the system. I recommend that the short circuit study be reviewed at least every 5 years, this is based on the the requirement in CSAZ462 4.3.5.1(b) regarding arc flash risk assessments.
A short circuit study is one of the inputs of this risk assessment.
If there have been changes to the facility, it is important to update the design short circuit study with the updated information. If the changes include installation of a new utility transformer, new generation on site, or significant load modifications there is a danger that the available short circuit will increase.
DC Short Circuit Analysis
A DC Short Circuit Analysis is has the exact same purpose as the "typical" AC study, its just with DC voltage. These studies were typically completed by hand as the DC systems were relatively simple. With the higher voltages in chemical storage (batteries) and PV Solar installations, and the growing complexity of these systems, modelling software such as ETAP is critical to ensure that the results are valid.
When the string voltages were in the 48-125V range and the relatively high immpedance of the system resulted in relatively low short circuit, there wasn't a lot of concern. That's not to say it wasn't hazardous, but the equipment could be easily designed to account for these levels.
As the voltages are now routinely 1000V-1500V and the system immedances are lower, the short circuit current needs to be accounted for earlier in the design.
These higher voltages and currents also means that DC Arc Flash is a real concern. DC arc flash behaves differently that AC arc flash, and understanding the system dynamics is critical to selecting the proper method.
Load Flow Analysis
A load flow analysis takes a snapshot of the power system and calculates the currents through each branch of the system, and voltage at every node. They are typically completed for various operating scenarios to determine what the likely operating load for the system is and if there is a chance of certain areas of being overloaded.
They are also useful for determining if there is any power factor concerns with a large inductive load on the system, from there you can size powr factor correction equipment, including STATCOM, power factor capacitors, etc.
Arc Flash Analysis
An arc flash analysis is something that every facility should have, without it your electrical safety program is incomplete.
The output of every arc flash analysis includes, at the minimum, the arc flash boundary and incident energy (at the working distance). From this information hazard/caution labels can be produced to inform workers of the arc flash risk of a particular bus and the PPE required to reduce the risk to an acceptable level.
For most installations an arc flash analysis is not complex or costly, especially when they are completed close to either the design or the project, or construction when contractor shop drawing submittals and the commissioning reports are available as the input information.
When in person site visits are needed, and circuits need to be walked down because the drawings aren't current, this is when the studies start to be expensive.
DC Arc Flash
DC arc flash is in a category of its own, and something that there isn't nearly enough understanding of arcing currents in high voltage (250-2000V) arcs.
There has been some testing completed that has found that this is overly conservative [1], however there hasn't been industry consensus on how to address this. More testing and better DC arc modelling is needed to apply the latest understanding of arc dynamics within enclosures, like what has been developed for AC arcs in IEEE 1584-2018 [2].
The software I use is capable of using three different methods of estimating the arc flash risk: maximum power [3], Paukert [4] and Stokes & Oppenlander [5].
Protection Coordination Study
A protection coordination study is used to ensure that the power system will act in a predictive manner if there is an abnormal event on the system, whether it is an overload, short circuit or arc flash event.
Sample of a simple Time-Current-Curve
The goal of the protection coordination study is to remove as little of the system from service when there is a fault. To do this we make sure that the protection settings of every device in a chain on the system, from the 15A breaker feeding a 3-phase compressor to the 69kV breaker in the substation have some space between them.
The secondary goal of the protection coordination study is to minimize arc flash incident energy.
If there are situations where the protection settings needed to properly coordinate lead to large incident energies, then the site may prefer to sacrifice coordination to have a lower level. Another option is to install relays that have two sets of settings, one which the relay/breaker has proper coordination and a "maintenance mode" that will trip the device quickly if there is a fault.
Harmonic Analysis
The electrical systems built today are alternating current, typically 60Hz or 50Hz. However, more of the load today that is connected at the user end is DC. That includes VFDs as the recitfy the AC current to DC before inverting it back to a variable AC frequency to the motor.
This recification introduces harmonics on the system that can cause excessive heating and loads on equipment. IEEE 519 is the industry standard when it comes to what level of THD and TDD are acceptable.
Ground Grid Studies
The outputs of a ground grid study include:
- Maximum touch potential
- Maximum step potential
- Ground Potential Rise
- Resistance to Ground
To get to these values the system short circuit current, power system protection settings, ground resistance (at a couple of depths ideally) and site layout are needed.
From this information the ground grid is designed and/or modelled to calculate the potential step and touch shock hazards for that facility.
This is typically a study that is completed for outdoor stations greater than 5kV.
Why you should have current power system studies
These power system studies are not just for engineers and should be current for every facility.
The four that you should have always current are mentioned above, studies like Harmonic Analysis or Ground Grid study should only be needed during design of a new system, when there are changes to the system, or when something isn't working correctly and you need to figure out what and why.
Every facility, including yours, has the following roles. If you work at a smaller company some of these roles will be covered by thte same person. They are:
- Management
- Operations and Maintenance
- Capital Projects
How these fit in your org chart will be different, maybe its operations and maintenance, etc; but that't not critical here.
Management
For management these power system studies represent the known information of how the facility operates. The Safety Committee will use the information in the Arc Flash Anaysis and Incident Energy Study to determine the risk associated with shock and arc flash for various work orders.
They will be able to complete well informed risk analysis and include it directly on the work order to ensure that the person performing the task will be working in a low risk environment.
When management are looking at improvements to the facility, and before they hand a project over to the Capital Projects group, a current load flow will allow them to understand, at a glance what parts of the electrical system can be added to easily, and what areas will require more intrusive upgrades.
This allows for better business decisions very early in project ideation.
Operations and Maintenance
Down time is a killer for an operation, especially if it is not planned. Power system studies allow for a better designed power system, one that limits nuance tripping, voltage sags, etc.
With current short circuit, protection coordination and load flow studies, the Operations and Maintenance groups will be able to analyze the current power system and determine the best place for preventative maintenance, etc.
Capital Projects
When Management determines a project and passes it to Capital Projects for execution, having a current model and/or study that they can use to determine the current state of the power system allows the project managers to better define projects, scope of work, etc.
They will know what areas of the system will be impacted and run different scenarios to see how the project will affect the existing system. They will be able to determine what areas will need outages and how long those outages may be.
They will also have a powerful tool for detailed design, regardless if they are using an EPC, or completing the project in-house.
Performing A Power System Study
We offer consulting services to execute any of the studies detailed above.
However, outsourcing the project to a contractor like us is not the only way to have a study completed. There are three models that we like to think about, they all have their pros and cons, and depending on the size and complexity of your company and system the method that is chosen may be obvious. Below is a brief description of each method, including a couple pros and cons of each. They are:
- Completely In-House
- Completely contracted out
- Hybrid of both
In most cases we recommend some type of hybrid model, but the split will always be different and dependent on the resources of the company that we have engaged with to complete the analysis.
By using a hybrid model you will be able to leverage our strengths in power system modelling and analysis to produce a great product, while using your internal resources to gather the necessary input data to keep the costs low (and value high) without sacrificing quality.
Let's Talk
Not sure what you need, schedule a 30 min with Jeff to discuss your project and lets see if we can help.
Schedule Today| [1] | C S Weimann, R J Kerertes and B. M. Grainger, "Comparative Analysis of Experimental DC Arc Flash Results to Industry Estimation Methods," IEEE Open Journal of Industry Applications, vol. 1, pp. 181-193, 30 October 2020. |
| [2] | IEEE, IEEE Std 1584 Guide for Performing Arc-Flash Hazard Calculations, IEEE-SA, 2018. |
| [3] | National Fire Protection Association, NFPA 70E: standard for electrilcal safety in the workplace, 2015. |
| [4] | J Paukert, "The Arc Voltage and Arc Resistance of LV Fault Arcs," in Proceedings of the 7th International Symposium on Switching Arc Phenomena, 1993. |
| [5] | A D Stokes and W T Oppenlander, "Electric Arcs in Open Air," Journal of Physics D: Applied Physics, pp. 26-35, 1991. |