Medium Voltage Motor Starter

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Abstract
This paper purposes to examine general considerations in the design of 4160V motor starters into the switchgear. Particular focus is on the safety, maintenance, protection coordination, and operation impacts of including the 4160VMedium Voltage Motor Starter, into switchgears of important systems. In light of the above, this paper will present an analysis of some of the best practices and techniques used to design Medium Voltage (MV) motors. This encompasses the critical considerations made for medium voltage switchgear commonly found in generation power plants and safe class facilities such as nuclear plant. The primary focus is on power system facilities that are intolerable to any form of power interruptions. Such facilities in areas of utility, industrial, commercial, and residential applications require power systems that have the capacity to provide safe and reliable energy for the utilization of equipment. Since the other earlier designs specifications of MV motor starter already exist, this paper essentially emphasizes the advantages in designing 4160V MV motors starters into MV switchgears with protective relays, switch and circuit breaker control circuits, and solid-state power. Discussions made include the advantages of integrating MV starters in the 4160V switchgear, with regard to fault clearance, maintenance, down time, and operations. The paper summary includes recommendations of the best practices necessary for safe reliable system, and control circuits maintenance to guarantee that protective equipment operates as intended.

Medium Voltage Motor Starter
Power system utilities in industrial plants, commercial and residential establishments are all subject to occasional power outages. Common causes of power system failures that cause outages range from; line faults, insulation failures, improper terminations, disoperation, and failures due to corrosion, or heat build-up [1]. Bearing this in mind, the principle aims in power-system design focuses all attention on creating safe, efficient, and reliable systems to meet the utilization of a system’s equipment. Safety, reliability, and efficiency are particularly important for power systems in generation facilities and Safe Class facilities, where interruptions to normal system operations are intolerable [2]. In order to guarantee ultimate power system security in such facilities, protection system circuits are integrated after considering proper design, installation, and maintenance requirements. With regard to Medium Voltage motor operations, protection systems such as fused contactors, circuit breakers, and protective relays are integrated into the switchgear to offer fault protection and permit coordination of protective devices. Overall, the most significant factors considered in the design of electric protection system are safety of life and preservation of property [3] [4]. However, the design of MV switchgear does not integrate significant features like the 4.16kV motor starter necessary for safe and reliable operation to vital plant systems such as air compressors and chillers [6]. Most designs for the 4160V Medium Voltage Switchgear lack dedicated ground fault protection for the feeder circuits to the motor starter. As a result, it becomes hazardous to work with systems once energized [10]. Additionally, power fuses used require constant replacement following fault occurrence as indicated in [2], and further make it difficult to manage the protective devices [7]. This may consequently result in the loss an entire bus during a fault. Protection circuits combine several power electronics components together in in order to ensure safe operation. Consequently, the American National Standard IEEE sets regulations and recommended practice for Electrical Power Distribution for Industrial Plants under the standards stated in [5] and [8].
The focus studied in this thesis aims at presenting a detailed design analysis of 4160V Medium Voltage motor starter in Power Plant or Safe Class facilities; why it should be designed into the MV switchgear rather instead of being a standalone starter. Medium Voltage synchronous machines exist in almost all industrial and commercial application at varied load condition. Thus, basic design considerations of safety and reliability should be implemented in the design of the 4160V switchgear [1], [3]. Designing 4160V switchgear using MV starters could provide: protection from ground faults and faults due to loss of excitation; recovery from transient disturbances resulting caused by switching operations and load changes; and enabled coordination of protective devices [7]. Maintaining steady state conditions after a transient disturbance ensures the MV motor to operate reliable under stated loading conditions.
Research Problem
Medium Voltage (MV) motors are highly sensitive and susceptible to overvoltage which can compromises the motor insulation [9]. Recurrent overvoltage result into weakened points along motor insulation, and overtime damage occurs [2], [7]. According to McIntyre, principal causes of the overvoltage include switching transients arising from the supply network, power electronics devices, capacitor discharges, or lightning strikes [3]. In 4160V motors, the overvoltage may occur because of high voltage spikes during their starting. In general, the resulting outcome is motor insulation damage, resulting into short circuits or ground faults [4]. In the case of grounded supply systems, the resultant ground currents may rapidly develop and reach very critical values. difficult starting conditions normally encountered in MV motors greatly influence operational availability, safety, and fault occurrence of such systems [10], which affect the standalone motor starter solely relying on disconnect fuse for protection. However, the correct selection and integration of 4160V MV motor starter into switchgear can provide necessary protective devices that will monitor motor data for functions such as, starting time or starting cycle. This thesis addresses the need to integrate the 4.16kV motor starters in the switchgear since this design using starters could provide ground fault protection and permit coordination of protective devices.
Justification
Most alternating current MV motors currently used in industrial applications fall in the voltage range of 2 kV to 14 kV (100 kW to 40 MW power ranges) [1] [3]. According to Kissell [2], this includes the three families of motor: asynchronous, synchronous, and synchronized asynchronous motors broadly used in numerous industrial applications. Considering the aforementioned transient disturbances in MV motors caused by switching operations, it is imperative to have prompt detection and protective devices for ground faults [4]. In this regard, improving basic design considerations for MV motor stators into 4160V switchgears can provide safe and reliable operation for vital systems like plant air compressors, and chillers. Furthermore, having motor starters in switchgear will limit the extent of the subsequent damage resulting from overvoltage attributed to starting transients, thereby reduce outages, and repair costs [10].
Secondly, present motor starting and control devices rely on outdated techniques such as bimetallic overload relays, and fuses that require constant replacement according to [2], [4]. Designs the 4160V switchgear eliminates the need for constant replacements, since protective relays and contactors -that only require regular resetting, also provide ground fault protection thereby enabling coordination of protective devices. Implementing a feasible design improvement in MV motor starter and switchgear system, may extensively reduce maintenance costs and downtime power systems in generation facilities and Safe Class industrial applications [10].
Objectives
The main objective intended in this thesis is to explore and increase the design knowledge base towards the successful application of Medium Voltage motor starters in 4160V switchgear systems in power plant and safe class facility industries. The specific objectives of the paper is to specifically address the need to incorporate the motor starters in the 4.16kv switchgear, in place of standalone MV motor starter that is fed by a fused disconnect switch. For this purpose, the paper explores best design approach for safety, design, maintenance, protection coordination and operation impacts on MV motor systems. The thesis also highlights the advantages of incorporating the starter in the switchgear with reference to fault cause by ground, short circuit (SC) and open circuit (OC) faults, maintenance, down time, and operations. Finally, this study intends to recommend some of the best industrial practices required to have a safe reliable system.
Literature Review Numerous publications and studies exist on the subject of Medium Voltage motor starting, MV motor control, and MV motor switchgears systems. Most of these literatures focus their discussion on the topologies, harmonic output content, and internal controls. Nevertheless, limited deep research presently exists particularizing the need to incorporate MV motor starters into MV switchgear, -particularly the 4160V switchgear [11], [13]. Consequently, this literature survey investigates the design of a motor starter, best practice in MV motor protection, and the relative advantages of an integrated MV motor starter, with the 4160V switchgear
Switchgear Assemblies. The IEC60050 descriptive definition of MV switchgear is an enclosed combination of busbars, circuit breakers, power contactors, fuses, relays, and indicating electrical devices [13]. The 4160V MV switchgear comprises serial connections of protective device components that protect power system installations. The upstream device (relative to the flow of power) in the series-connection, serves to provide short circuit protection [12]. Going by Kiank and Fruth, due to the high marking and breaking capability of ‘molded-case’ circuit breakers, present MV switchgear assembly such as the 4160V system can serve as motor starter combinations [12]. In the context of motor starting and control, MV switchgear can have circuits that serve circuit breakers, and contactor cubicles that distribute power to a motor control center (MCC) [13]. Instead of having an upstream motor starter connected in series to the switchgear, the two can merge up into a switchgear-motor control center cubicle for 4160V motor [13]. By comparison, the operation of 4.16kV switchgear controls tends to be infrequent compared to MV motor starters, which operate frequently as demanded by motor processes. Therefore, a combinational approach of integrating the two will demand the design of the motor starter and control with fuses and contactors, in order to clear fault currents in a cycle fraction of the AC current [12].
Design assembly of 4160V switchgear with MV Motor Starter. According to Kiank and Fruth in [12], the integration of MV switchgear with motor-starter combination must be either fused or non-fused. In the case of 4160V switchgear assemblies with fused MV motor starter, the design should comprise of: a contactor -for starting and stopping motor (switching on and off); short circuit protection fuses with ‘gL/gG utilization category; and a thermal/electronic overload relay with inverse-timer-delay (protection against motor overloads). However, for this design to provide ground fault protection, and coordinated interaction of protective components in the 4.16kV switchgear-motor starter combination, particular conditions must be met [12]. These include: * Fuses should interrupt overcurrent 10 times the rated contactor load current * Fuses and overload time-current characteristics must allow motor to start and run to speed * Fuses must interrupt overcurrent exceeding 10 times overload relay current rating[].
The rated current is calculated as Ir (A) = P in kW3 • U •cosϕ•η ; where U = phase to phase voltage in kV Cos ϕ = motor power factor η = motor output
In cases where the MV motor (2.3kV, 4.16kV or13.8kV) experience high switching frequencies, limited cooling, high ambient temperatures, and asymmetrical intermittent duty, the 4160V switchgear assembly can include thermistor protective devices [6]. Applications where the MV motor has a thermally critical rotor or stator, deploying additional overload relays or releases may function to protect the motor starter in the 4160Vswitchgear.
Best Practice in design of 4160V switchgear with MV motor starter. Best practice in the construction of the 4160V switchgear incorporated with the MV motor starter should customarily house the motor starter and protection devices in a single enclosed metallic module [13]. According to the switchgear-planning guide in [6], the best design approach is to construct vertical sections covering vertical and horizontal incoming and outgoing buses, and compartmented MV motor starters. This is particularly significant in industrial applications such as generation plants where most 4.16kv motors may be subject to server weather and environmental conditions. Typical construction of the metal clad enclosure demands the grounding of all live components in the mild-steel enclosure [8]. International standards on safety and protection of power system equipment such as IEC in the UK and Europe, or NEMA and NEC in USA, dictate the degree of personnel and access protection for such assemblies [5], and [7]. Thus, it is necessary to sub-divide the enclosures to enable safe accessibility by personnel without danger of electric shock. Applications to such design as recommended in [6], [11], and [12] include medium voltage motor starter systems rated 2400, 4160 up to 7200 volts, due to the design’s a high-interrupting capacity, and high-voltage control [12].
Advantages of MV motor starter in the 4160V switchgear. The 4.16kV switchgear comprises metal clad indoor circuit breakers [6], [11] that have an interrupt rating at 4.16kV equal to, or higher than the peak short-circuit current. According to the IEC, this value is 2.5Ik [11]. By integrating switchgear and motor starter, upstream protective devices placed in series before the motor starter, helps in rapid fault clearance at 10 cycles of the fault closing [12]. State of the art construction of motor starters in MV switchgear like the 4.16kV, have the advantage of inbuilt motor protection [6], thus preventing the risks attributed to ground fault, phase loss/imbalance/ reversal, and electronic overload. Switchgear component such the thermal and electronic overload relays detect and protect the motor from overcurrent, under current, and loss of load, as indicated in [6], [10], and [11].
Additional advantages regard to operation costs. As mentioned earlier, motor starters operate rather frequently compared to the infrequently operated switchgear switching devices like circuit breakers and contactors [13]. However, contactors are not able to endure high fault currents [11], accordingly, power fuses placed in series to incoming bus help interrupt fault currents and sustain overcurrent [6],[12]. This makes the device physically more compact than a circuit breaker and hence less expensive since replacement and maintenance of MV motor starter fuses will not be frequent. Thirdly, integrating MV motor starter to 4.16kV switchgear provides a simplified and reduced-cost means of attaining reduced starting voltage. Kiank et al [12] also point out that integrating these two MV systems presents an array of added functionality not attainable with a standalone fused fed by a fused disconnect switch. These functional utilities include: remote switching, automated pole-braking, system indication facilities, adjustable start times in motor field, interlocking faculties, and is available for reclosing after a short-circuit clearing/overload trip [12: 354].
With regard to maintenance, combining the MV motor starter with the 4160V switchgear greatly reduces the cost factors. Geyer, and Schroder are of the opinion that accurate and timely detection of fault events in motor systems helps prevent fault progression and any catastrophic consequences [15]. The MV motor starter in series with the fuse, and contactors, overload relays and additional circuit breakers in the 4160V switchgear simplifies management of phase-to-phase faults, SC, and OC ground faults, which are most common to MV machines [14], thereby extending fault-tolerant starting, and operation by reconfiguring motor control strategies [15]. MV motor starter in 4160V switchgear are held and closed by a single magnet [6], thereby creating simplicity in the contactor mechanical design and an overall increase in the contactor mechanical life [6][11]. Going by the information in [6][ 14], typical contactors used in the motor starter-switchgear combination may not require modification or mechanical repair for several years, owing to their mechanical simplicity and robustness. Nonetheless, Geyer, and Schroder recommend regular preventive maintenance checks conducted no less than once per year, in [14].
By implementing a 4160V switchgear design incorporating MV, motor starters could provide rapid fault correction and protection, and consequently permit coordination of protective devices [12], [13]. According to General Electric [6], the dependable performance of vacuum circuit breakers used in 4160V switchgear provides the necessary motor protection utilities, thus offering reliable overcurrent protection against the destructive properties of overloads, SCs and OCs. This reduces the overall system downtime. Furthermore, the “quick-make quick-break non load-break disconnect” switch of the motor starter in 4160V switchgear permits coordination, thus enhancing the system control integrity [11]. The system achieves this by eliminating the necessity to pull out the contactor, in order to isolate the motor load from the incoming power bus. By extension, the meantime to repair (MTTR) of the system reduces to few seconds, and increases the system availability by minimizing the annual downtime per year.
In summary, this study present the practical need to integrate the 4.16kv motor starters in the switchgear rather than having it as a standalone starter that is fed by a fused disconnect switch. The switchgear assembly houses circuit breakers, power contactors, fuses, overload relays, and programmable devices that provide improved fault protection, and coordination of protective devices. With the above-mentioned advantages attributed to incorporating 4160V motor starter into the switchgear, make such an application feasible in the Power plant and Safe Class facilities. Reliability and availability are both highly significant in the Power plant and Safe class facilities for instance, a nuclear power plant where interruptions to operations are intolerable. Nonetheless, there is still a need to perform in-depth practical evaluations on the reliability of 4.16kv motor starters in the switchgears, for overall project feasibility.

References
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