Distribution and Industrial System Consulting

Background

The scope of work concerned the power system of an offshore gas platform. The system is composed of a 100kV substation, connected from the upstream network through subsea cables and to the system through two main power transformers. Different loads (gas compressors, pumps) are fed.

The objective was to analyze the energization of the transformers and simulate the dynamic of different configurations (motor starting, emergency state) of the system.

Realization

Powersys used for this study the software EMTP and its capabilities to simulate accurately the switching transients and the behavior of machines.

The aim was to verify the stability of the platform power system is ensured when transformers are connected and motors are started. A specific configuration is also studied: emergency generators are providing the power to the platform system (which is insulated).
Generators and their controls (exciters, governors) are modelled in EMTP using control devices. The torque/speed characteristic of motors (pumps, compressors) is represented too.

• Transformer Energization

  The substation, the transformers and the other equipment are modelled in EMTP. An ideal voltage source with short-circuit impedances represents the transmission network at the PCC, which is connected through a constant parameter cable model. The T model is used to model the transformers. A particular attention is given to the modelling of the saturation (from the B-H curve data), which is creating the inrush currents. The procedure explained in the CIGRE C4.307 Brochure 568 (“Transformer Energization in Power Systems: A Study Guide”) has been used to model the air core reactance.

  Different cases are simulated in EMTP:

  • Switching of transformer at the zero voltage (flux is maximal) or when the voltage is maximal (flux is null).
  • No residual Flux / 80% residual flux (when the transformer is connected).
  • Minimal and maximal possible values for the short-circuit power at the PCC.

  Then the transformer energization is simulated with and without other already-connected transformers in order to estimate the worst inrush currents and the voltage drop.

• Dynamic stability

  The aim was to verify the stability of the platform power system is ensured when transformers are connected and motors are started. A specific configuration is also studied: emergency generators are providing the power to the platform system (which is insulated).
  Generators and their controls (exciters, governors) are modelled in EMTP using control devices. The torque/speed characteristic of motors (pumps, compressors) is represented too.

Results

The results of the study allow Powersys to determine the minimal time between the switching of two transformers that ensures the stability and proposed authorized sequences for the energization of all transformers.

Moreover, the stability has been controlled for various events (starting of a motor, loss of a generator).

Figure 1 : Voltage drop due to the switching of 2 transformers with a delay of 200ms