Starting of Large Compressor Motors on a Weak Grid – Case Study Adrian Aparaschivei Global Operations - Engineering Air Products BV Rotterdam, The Netherlands apax@ieee.org Gabriel Chiriac, Dorin D. Lucache Faculty of Electrical Engineering "Gheorghe Asachi" Technical University of Iasi Iasi, Romania gchiriac@tuiasi.ro, ddlucache@gmail.com Abstract—This paper presents case studies of different starting methods of large compressor motors on remote locations where the grid is not strong enough to be able to start the motors Direct On-Line (DOL). Some criteria based on common practice have been presented. Following these criteria and making some rough calculations it is easy to anticipate how the installation will look like, what the grid influence will be and how that will affect the cost of electrical installation of a large driven compressor. Keywords-large compressor motors; weak grid; motor starts; autotransformer; synchronizer I. INTRODUCTION Large power high-speed squirrel-cage induction motors are used as drives for compressors, pumps, fans, or blowers, where the starting load torque is around 15-30% and 60-80% at rated speed [1]. Starting and accelerating of these machines bring some challenges when it comes to the power grid hosting capacity, as defined in [2]. The power system strength, expressed in short circuit MVA at the connecting system bus from which the motor is to be supplied, must be known prior to choose a starting solution. In a weak grid each load switching can have an effect on the grid performance. As classified in [2] an ideal strong grid has a flat voltage profile with constant rated frequency, quasi independent of the connected loads and drawn currents. As a general rule, a grid connection is strong when the short-circuit capacity is several magnitudes larger than the load size. In weak grids however there is a strong interaction between load and system parameters. The term "weak grid" is used to refer to a grid connection with a small short-circuit capacity compared to the size of the load. A limit for the total load connected to the grid, called the “hosting capacity”, can be identified. When this hosting capacity is exceeded, the grid performance becomes unacceptable. During the motor starting, the inrush current flowing to the motor causes a momentary voltage drop on the adjacent buses of the distributions system, which results in a reduction of the voltage supplied to the motor and to the other loads of the distribution system [3]. Usually a starting current of between 5 and 5.5 time the motor full load current is achievable on large induction motors [4]. A limit of 6.5 time the rated full load current must be set in order to obtain a standard torque of 60% of rated during starting and 175% break-down during running conditions [1]. The magnitude of the voltage drop during motor start-up is a function of the motor inrush current, the impedance of the circuit between the utility power system and the motor, and the available short circuit fault capacity of the power system. The starting voltage drop must be known and the impact to both the incoming power supply authority system and plant internal distribution system must be established and agreed. Power system analysis studies should be reasonably well advanced with worst case scenarios demonstrating that starting voltage drop will remain within acceptable limits and that the driven equipment is capable of being started [4]. If the distribution system is stiff, then direct across the line (DOL) starting is usually the preferred motor starting method [3]. Although significant developments have been made in the last years in the area of variable-speed drives, they are only used in the higher ratings, if the additional costs can be justified by advantages in the industrial process. For standard constant speed duty, well-proven motors with squirrel-cage rotors which are simple and rugged or synchronous motors are used [5, 6]. This paper will present a large industrial gases company’s challenges on starting large motors-driven compressors in a relatively weak power system in Netherlands and UK while not violating the Utility company’s standards. Compressors are used to bring gasses up to a higher pressure. In this application compressed gas is air and therefore MAC is the main air compressor, and MP GAN is the medium pressure gaseous nitrogen. These are usually large machines driven by large squirrel cage induction motors, supplied from a weak grid (grid subsystems connected to the main grid via a weak interconnection, e.g. loads at a remote location). II. STARTING PRINCIPLES OF HIGH POWER MOTORS The aim of each starting method is to minimize the voltage drop at the motor terminals, the power distribution or utility bus, or to provide a soft start of the driven equipment while still keeping adequate accelerating torque [7]. Depending on the applicable requirements and the characteristics of the utility grid, criteria for selecting a starting method are set. 693 2014 International Conference and Exposition on Electrical and Power Engineering (EPE 2014), 16-18 October, Iasi, Romania 978-1-4799-5849-8/14/$31.00 ©2014 IEEE Authorized licensed use limited to: Gheorghe Asachi Technical University of Ia¿i. Downloaded on March 22,2023 at 17:34:05 UTC from IEEE Xplore. Restrictions apply.