Power System Analysis

NESIS-R 2015 (as amended) and the Grid Code stipulate a range of power system studies that must be carried out prior to the connection of generators to the Nigerian power system. The level of studies required depend on the generator rating and connection voltage. At present there are four main categories – Type A Generators (<1MW) do not require any special analysis; Type B Generators (>1MW and <10MW) which require a Fault Ride Through
(FRT) study, Limited Frequency Sensitive – Over (LFSM-O) study, and a basic reactive power flow study. Type C Generators (>10MW and <50MW), that require a full spread of studies including Reactive Power Flow, Reactive Power Stability, FRT, LFSM-O, LFSM-U and FSM-O. Type D Generators (>50MW and/or connected at 110kV and above), are
identical to Type C generators.  

We are experienced in these complex studies and can accurately simulate the dynamic response of the system to upstream faults. Studies are carried out using PSSE, and DIgSILENT Powerfactory.

Within the NESI, the National Independent System Operator (NISO) publishes and manages the Grid Code. This document has a lot more requirements in it. The Grid Code requires that power system models are provided in an open format (unencrypted) and do not use any external DLL reference files.

NESO has also required that developers provide a full Electromagnetic Transient (EMT) model of the system in applicable formats, that represents all main items of equipment, and associated protection and control elements.

We are experienced in these complex studies and can accurately simulate the dynamic response of the system to upstream faults. Studies are carried out using DIgSILENT Powerfactory or PSCAD.

The application of correct protection settings to relays are of vital importance to any electrical system, as it ensures that should a fault occur, only the faulted item of equipment is removed form service, while the remaining part of the system in healthy condition, remains in service. If the protection settings have not been coordinated correctly and there is insufficient grading margin, a simple LV fault can trip a whole HV substation. iPEC has significant experience in undertaking a wide range of protection grading studies from simple over current & earth fault coordination studies for a Ring Main Unit, up to configuring complex differential protection schemes and investigating nuisance protection trips. We specialise in undertaking difficult studies, and using PSSE, PSS SINCAL, IPSA, ETAP, NEPLAN, or DIgSILENT software package, we can simulate faults on any part of the network and confirm the exact operating sequence and times for the protective devices. iPEC can also help specify and set-up differential protection schemes for transformers, switchgear, cables etc. where appropriate. We also have AMTECH Protect HV for carrying out simple studies when this approach is requested by a client.

As more and more power systems contain harmonic polluting sources, such as inverters and variable speed drives, the need for harmonic analysis has been dramatically increasing. iPEC has experience of a wide range of harmonic analysis techniques and are familiar with IEC 61000-3 and IEEE-519.  As part of iPEC studies approach, we will examine the individual levels of harmonic distortion and the Total Harmonic Distortion (THD) on the system and key busbars, as well as identify any resonance points in the system using frequency dependent profiles or harmonic impedance loci. Where necessary we can suggest suitable modifications to the power system design or specify suitable mitigation measures such as harmonic filters and power factor correction equipment. Studies can be carried out using either DIgSILENT or ETAP as required.

Voltage disturbance studies cover a wide range of phenomena including, transformer energisation, battery storage power swings and ramps, motor starting, site trips and voltage flicker. Where necessary these studies can be used to accurately size any pre-insertion resistors (PIRs) to reduce the magnitude of the volt dip for transformer energisation or specification of Soft Starters or a VSD for motors. iPEC can undertake these power system analysis studies by creating a detailed electromagnetic model using either DIgSILENT or ETAP, and for more complex cases, studies can be carried out in PSCAD or EMTP-ATP.

Load flow studies are steady state analysis calculations, that are used to validate how the system performs during normal, outages and standby conditions. As part of the load flow analysis, the system is analysed for active (MW) and reactive (MVAr) power flows, to identify the need for any shunt MVAr compensation equipment or power factor correction. Studies are used to identify equipment loadings, the network voltage profile, transformer tap changer requirements, system losses, generator ratings and loadings, and for more complex DisCo network, outage and contingency cases are analysed in detail. Studies can be carried out in either PSSE, PSS SINCAL, IPSA, or ETAP.

iPEC has experience of a wide range of short circuit study approaches and are familiar with IEC 60909, NESIS R 2015, and the Generator Circuit Breaker standard IEC 62271-37-13. As part of any short circuit study, we will determine the fault levels and ensure that the selected equipment can safely interrupt any short circuit that may occur on the system. iPEC also has a detailed knowledge of more complex short circuit scenarios and the issues relating to asymmetrical duty of circuit breakers due to high X/R ratios. Using either the PSSE, PSS SINCAL, NEPLAN, SKM Power Tools for Windows, HV Amtech Protect, DIgSILENT or ETAP package we can also calculate the asymmetrical, peak and DC component experienced by circuit breakers and identify increased fault duty any delayed current zeros.

Arc flash studies are becoming increasingly important, as they can help ensure that operational personnel are adequately protected, and provided with suitable PPE, for any switching duties. These studies are typically carried out to IEEE 1584 and look at the typical busbar clearances, working distance, fault levels and protection operating time to identify the total incident energy in Cal/cm², to ensure it is within safe operating limits, and what level of PPE is required. Studies can be carried out using either ETAP or DIgSILENT as required.

Transient and Dynamic stability studies are most frequently associated with island power systems and embedded generators, and are used to measure voltage stability, frequency stability in relation to faults, loss of main supplies, generation and load acceptance and rejection. These studies are useful for identifying any the need for load shedding systems, fast differential protection schemes or other form of power system compensation. Studies can be carried out using either ETAP or DIgSILENT as required.

Electromagnetic Transient (EMT) studies are becoming increasingly important in modern power systems, due to the increasing presence of inverter-based generation and shortfalls in accuracy of traditional simulation methods. EMT studies are typically carried out either in DIgSILENT or PSCAD and can cover a wide variety of events such as circuit breaker Transient Recovery Voltage (TRV) calculation, insulation coordination studies considering lightning impulses, switching over-voltages and temporary over-voltages, sub-synchronous resonance (SSR) and detailed operation of inverter-based generation Phase Locked Loop (PLL) and Fault Ride Through (FRT) capability.