This article consists of 94 pages and 15980 words. In order to have full access to this article, email us at thedocumentco@hotmail.co.uk
Ref No: 2164
Key words: transient stability, power quality, voltage dips, synchronous motors, protection/control/excitation and motor starting.
ETAP stands for Electrical Transient Analyser Program which is software by OTI.
1.1 Introduction and rationale
Synchronous motors with ever increasing ratings from a few megawatts up to 30MW and higher are being utilised more frequently on ‘mega’ refinery and petrochemical projects, mainly to drive compressors [1].
Whilst synchronous motors have the side benefit of improving the power factor of the overall power system by running them at unity or leading power factor, they are more susceptible to power disturbances than induction motors, as the separately excited stator is prone to oscillations e.g. after a voltage dip.
The use of synchronous motors can avoid the use of large capacitors for power factor improvement. However, if a main compressor motor trips regularly this can result in the shut-down of the process with consequential revenue loss or possible damage to the motor/compressor due to mechanical forces on the rotor [1]. The obvious focus of the protection system is to protect the motor.
The separate excitation and control mechanism of synchronous motors is more complex than the simpler robust induction motor. As mentioned above the synchronous motor is more susceptible to voltage dips than the more commonly used induction motor and it’s capability to ‘ride-through’ the volt dips needs to be determined.
The synchronous motor’s sensitivity is on the stator voltages where the disturbances could result in the motor losing synchronism from the supply voltage. This phenomenon is an electro-mechanical one relating to a large disturbance of rotor angle and is influenced by the relationship between power (P) and rotor angle (δ) [1].
The most common scenarios that cause voltage dips are 3 phase faults especially phase to phase ones which result in almost zero bus volts or the starting of large loads or groups of loads e.g. group restarting which give rise to less severe volt dips.
There are various recent research papers on the subject e.g. [1], [2] & [3] but none found on specific aspects of balancing transient stability, power factor improvement, motor starting and voltage dip mitigation of synchronous motors connected to the same bus which is the configuration of this project and dissertation.
The uniqueness of this configuration needs the use of special fast acting protective devices between the bus sections to open during high faults. The rapid action may clear faults but there still may be oscillations in voltage to be stabilised by the control systems. There were also no previous Electrical Power System MSc dissertations on this specific subject in the library.
The relevant research papers found on this subject [1], [2] stated that voltage dips affecting directly connected large synchronous motors have not been researched or investigated thoroughly, hence this dissertation is timely and a subject of interest in industry.
There are reports and feedback from operators of refineries relating to the unplanned downtime of the synchronous motor driven compressors due to spurious and unwanted trips. Some historical or statistical voltage power quality data, from an operating refinery is also available for the study which may assist in the research [4].
To enable higher rated synchronous motors to start effectively, they are connected closer to the grid network or closer to power generation at higher voltages e.g. they’re fed at 132kV via a step down transformer.Transient stability and voltage control study of synchronous motors in a refinery.
As a result, they are more susceptible to voltage dips or sags as they are closer to the source of voltage dips or sags.
These voltage dips cause oscillations in machine current, voltage, torque and speed. The voltage dip increases the load angle or power angle, which could increase it beyond π/2 rad, thus losing synchronism [2].
For large disturbances such as a 3 phase fault on the incoming feeder closed to the motor, the overcurrent protection would trip the motor before the critical clearance time.Transient stability and voltage control study of synchronous motors in a refinery.
For less severe voltage dips, the research papers in [1] and [2] both considered the voltage dip % and duration that the synchronous motors can ride through. This dissertation has investigated this to an extent.
The appropriate protection systems and settings to achieve this ride through have also been investigated.
As mentioned above the synchronous motors can generate reactive power into the power system if they are run at leading power factor.
The 6 compressors to be studied in this dissertation may be run at unity up to 0.9 leading power factor to maximise power factor compensation. The sensitivity to the voltage dips at various power factors have investigated at these operational cases.
The protection and control system (AVR / Exciters) need to consider all these sometimes conflicting conditions. This study’s aims were to rigorously assess various operational and abnormal cases/scenarios to determine the optimum settings….
Recent Comments