Stability of the distribution network of an important African city


Background

Analysis of the electromagnetic interference between transmission line systems and buried pipelines

The scope of work concerned the distribution network of a large African urban area. The system is composed of different substations connected to power plants and/or to loads (industrial, residential and business areas). Several new equipment were added to the system since the last years.

The objective was to propose the voltage plan and the protections that ensure the ability of the system to tolerate accidental events (N+1 redundancy) which can be:

  • Disconnection of an overhead line / a buried cable.
  • Incident on a transformer,
  • Loss of a part of the production.

Realization

Powersys used for this study the software CYME and its different modules.

  • Model

The network is modelled using the different libraries of manufactured equipment provided by the software, such as the generators and their controls (exciters and governors) or the lines and the cables. A classic T-model is used for the transformer, and their tap changer system is also represented. The consumption of each areas is modelled as static loads, divided between each substation.

In order to simulate the dynamic behavior of the power system, the voltage and frequency protections of each generator are adjusted.

  • Simulations

We proceed in two steps. First, a load-flow is calculated for the normal configuration of the network and after several events which are the loss of a line, a transformer or the main generator of a power plant. These simulations allow to verify if the power balance is ensured after an event or which actions can regulate it (selective power cut). Some other parameters are watched for these simulations: current through cables and lines, voltage level at each substations…

Then, each case is simulated in time domain using the “Dynamic stability” module of CYME in order to control the stability of the system during the event (regarding the voltage and frequency protection of the generators). The objective is to avoid disconnecting generators or clients.   

Results

The simulations reveals some critical cases with overloaded lines or transformers or important voltage drops in parts of the system.

In order to fix these situations, we proposed a new voltage plan. Some selective power cuts were also necessary to ensure the stability after some specific events.

 

frequency at different point of the system

Figure 1: Frequency at different point of the system

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