A lot of events can disturb Power Systems. These events will jeopardize its stability by engendering frequency and voltage variations.

These variations result from the interaction of a wide number of elements. Therefore, studying these interactions need to use offline simulations to take all effects into account.

TSOs want to control these frequency and voltage variations and, therefore, enforces in their grid code limits electrical parameters must not cross longer than a fix amount of time. In particular, it is necessary to check whether some events provoke the disconnection of some devices or a loss of stability.

A wise power system management consists in anticipating difficult situation to make the power system as robust as possible. A good way to achieve this goal is to estimate what are the margins that exist between current situation and instability situation and verify that the power system would endure events even if a piece of equipment were lacking (Contingency analysis).

When an instability is anticipated, several solutions exist to mitigate this situation. It can consist in increasing the transmission lines number, increasing power capacity by starting more generators or connecting dynamic equipment as FACT (SVC or Statcom).

In this context, the POWERSYS consulting team can provide the following services:

• Simulating an event on Power system (generator connection/loss, a load increase/decrease, line loss)
• Determining stability margins (module margins, phase margins)
• Simulating Power System reaction to a voltage step or a voltage gap
• Verifying stability on short-circuit
• Determining the minimal short-circuit time that provokes a loss of stability
• Determining the impact of a mitigation solution
• Determining whether the impact of the offloading of some load is required and its impact on Power System

More and more devices that are connected to the electrical network use switch-mode power supplies. Each switch-mode power supplies provokes a reduction of Power Quality on the power system, in particular with the creation of current harmonics.

For several years, renewable power production has been growing. Renewable power plants supply Power Systems through inverters and use numerous power converters. So, it has become necessary to check the quality of the power that the installation will inject into the Power system.

Power Systems contain a large number of capacitive elements (capacitors banks, cables, etc.), inductive elements (lines, transformers, etc.) and resistive elements. Interactions between these elements can engender a resonance with some harmonics that circulate on the power system. Besides, harmonics are sources of losses and degradations of connected equipment.

Another hazardous phenomenon that must be controlled is the “flicker” effect which consists in an erratic voltage fluctuation, which can sometimes be perceived by individuals (lights brightness variation).

TSOs and DSOs therefore integrate Power Quality requirements into their grid cod. Predicting the impact of the connection of new pieces of equipment in terms of harmonics will determine its compatibility with this "grid cod".

In order to limit these harmonics, filtering solutions can be considered. These filtering solutions must be sized during the upstream part of the project.

In this context, the POWERSYS consulting team can provide the following services:

• Evaluating current and voltage harmonics according to standards IEEE 519, IEEE 399 et IEC 61000:
  • THD (Total Harmonic Distorsion),
  • TDD (Total Demand Distorsion),
  • IHD (Individual Harmonic Distorsion).
• Realizing a frequency analysis :
  • Calculating the frequency scan of the impedance at several bus to identify Power system weaknesses,
  • Determine possible resonances
• Quantifying the flicker effect cause by a new piece of equipment
• Sizing and validating a filtering solution in case of excessive harmonics or flicker effect
• Validating the compatibility with “grid code” of a new installation

For several year, wind and solar plants power production has been growing and political / economic context will continue and intensify this movement. It represents a huge challenge for power system management.

The first one of these challenges concerns intermittency. Renewable power plants production depends on varying environmental factors, as insulation and wind speed. Their active power production is adapted in real time to be as high as possible thanks to an MPPT (Maximum Power Point Tracking) algorithm. Therefore, they harden frequency regulation.

Besides, renewable power plants deliver the power they produce through inverters. The use of these inverters engenders the emission of a huge number of harmonics. These harmonics reduces power quality and provokes an additional use of equipment.

A usually solution to mitigate this power production variability is the use of Batteries Energy Storage System (BESS). This make global power production more stable. These pieces of equipment are expensive. Therefore, it is important to estimate accurately if a BESS is large enough to maintain stability.

In this context, Powersys Engineering Team propose to realize the following studies:

• Simulate Power System evolution with a long-term analysis based on a time curve of irradiance or wind speed,
• Determining the voltage and current harmonics produced by renewable power plants at multiple buses in accordance with standards,
• Sizing required BESS to stabilize active power supply

Power systems require maintenance and operations to ensure the continuity of service without any failure. Those operations are realized by technical people in the vicinity of installations in service.

In order to protect the operators and prevent any electrical issue, the installations have to be adapted: they have to take into account protections, grounding systems or alert systems. These elements are generally described by standard (IEEE, IEC) and should be considered during the design phase. Such installations are required in different parts of a power system:

• In a substation, a grounding system can be important to drain the short-circuit current and avoid any risk of localized high potential point.
• Warning labels in compliance with standard are required in the electrical room to alert operators on the recommended equipment or the safe distances.

In this context, the POWERSYS consulting team can provide the following services:

• Design, validate or adapt any grounding system for a substation or a facility. It will take into account detailed soil model and can comply the common standards.
• Study your grounding system and provide the step and touch voltage map.
• Analyze your facilities and perform the arc flash hazard analysis. The warning labels and other safety recommendations for the electrical equipment in the vicinity of the operators will be provided.

The determination of the short-circuit currents is essential for the design and the operation of a power system. Indeed, in fault conditions, the circulation of those currents in any parts of the system can damage the equipment and present a risk for the persons. Basically, a short-circuit analysis consists in determining the different short-circuit characteristics (peak value, DC current, RMS current, …) at any place of a system. It has to take into account the different sources of current, including the motors.

The knowing of the short-circuit currents will allow to size the equipment and to define the appropriate protection strategy. It is possible to get them using computing simulations that can consider the different conditions and influent factors (location and level of the production, specific loads that can reinject the current). Some standards among which IEC 60909, IEC 61363 and ANSI C37 define the values and the process for the calculation of the short-circuit currents.

In this context, the POWERSYS consulting team can provide the following services:

• The calculation of the short-circuit current by simulating the involved power system in compliance with the standards IEC 60909 and IEC 61363.
• The calculation of the short-circuit current by simulating the involved power system in compliance with the standards ANSI C37.5 / C37.010 / C37.13.
• The analysis of the shot-circuit in specific fault conditions (simultaneous faults, unbalanced fault impedances, …).

An efficient power system management depends on knowledge of the transmitted powers and bus voltages. Moreover, thanks to good information on power flows, choosing accurately the equipment becomes possible.

Each power system must comply with limits (voltage and frequency) enforced by grid code. When the voltage of some buses does not respect these limits, a compensation system, consisting of capacitors/reactors banks or FACTS, can be installed.

Operators always seeks to increase cost effectiveness of their system. Therefore, the rules the power system must be achieved without oversizing power system installations, which would lead to higher cost. The more we know the constraints equipment will have endure, the less the equipment will have to be oversized.

This also implies it is important to find the power system configuration that optimizes its operation. For example, this can consist in choosing the best power rating of each generator or determining which line should be open or closed.

A wise power system management consists in checking the power system is as robust as possible. To achieve this requirement, operators needs to know what the load flow would be if one or more devices were disconnected.

In this context, the POWERSYS consulting team can provide the following services:

• Determine power flows and buses voltage
• Verify the compliance of the operating conditions with equipment requirements
• Size the equipments
• Verify conformity to grid code
• Size and locating reactive power compensations
• Determine (U, Q) characteristics of generators
• Optimize Power System configuration
• Realize a static contingency (N-p) analysis

Power systems can be impacted be a large variety of phenomena. A part of them concerns brief but sudden variations of the system variables (voltage, current, frequency, power, …). Those variations are called “Electromagnetic Transients” and can be due to very different causes.

Lightning strokes, switching events of equipment like transformers, capacitor banks, or the integration of new production are some examples of possible causes for any EM transients. In most of the cases, those transients cannot be neglectable and offline simulation are the best way to anticipate the phenomenon and take the appropriate decision.

Simulate an EM transient is a challenge that requires the proper level of detail in the modelling of the system: high frequencies events are impacted by very specific characteristics and very sensitive to the system parameters. In that context, considering the non-linear behaviour of a transformer, the variable switching time of a breaker or the stray capacitance of the buses become essential.

In this context, the POWERSYS consulting team can provide the following services:

• The modelling of your power system considering the analysis to perform
• The analysis of the electromagnetic transients of your power system. Many different analyses are possible:
  • Lightning effects (direct and indirect strokes),
  • Switching transients - inrush currents for transformer, capacitor and inductor bank
  • Transient recovery voltage (TRV analysis)
  • Ferroresonance
• The research of the origin of a misoperation/incident in your system: we can investigate the incident conditions through simulations to identify the most likely causes.
• The validation of the conformity of the system regarding specific standards/references/specifications
• The proposal of mitigation solution to keep the EM transients under the acceptable/referenced limits.