2.18 Switch Gear and Control Panels
2.18.7 Type of starter
The designer should consider the following points when choosing the starter type to be used.
Motor size
The motor size (kW) will determine if a standard starter can be used (direct on line DOL or start delta starter Y/D), or if a more advanced type of starter such as a soft starter is required. The main issue to consider is the starting current. The greater the (kW) rating, the greater the starting current required. A high starting current has an overall effect on the system stability and other equipment installed. The following ratings can be considered as general guidelines only. The designer should apply knowledge and experience to justify the starter method to be used.
Table 2.18.2 – Guideline Starter Methods for Motor Ratings (kW)
Motor rating KW Starting method
≤ 5kw Direct online (DOL) 5 ≥ kW ≤25 Star delta (Y/D) >25kw Soft starter ( solid
state drive) (S/S)
Motor duty and application
The motor duty will vary according to its application. The following table gives examples of such duties. Table 2.18.3 – Example Motor Duties and
Applications
Duty type Application example
Continuous run at constant load and speed
Potable water
Short run at constant load and
speed
Continuous run at variable load and speed Irrigation network Intermittent periodic duty Injection system Motor Application
The type of motor starter can also be selected according to the motor application as mentioned in Table 2.18.3, as a high number of starts per hour will cause even a small motor to overheat. An example of a suitable starter for each application is presented in Table 2.18.4.
Table 2.18.4 - Example Starter Methods for Duty Types
Duty type Starter
Continuous run at constant load and speed
DOL, Y/D, S/S
Short run at constant load and speed
DOL, Y/D
S/S if sufficient cooling time between operations Continuous run at
variable load and speed
VSD
Intermittent periodic duty
D.C starter, DOL
Notes: DOL: direct online, Y/D: star/delta , s/S: soft starter, VSD: variable speed drive
Voltage level
Starter type can be varied according to the voltage level. In the medium voltage range (e.g. 3.3kv) the starting current will be very low when compared with a lower voltage (e.g. 415v). In this case, the use of a direct contact starter would be acceptable.
Cost considerations
The cost of the starter should also be considered when compared to the motor size and application. As an example, a soft starter could be used to reduce the starting current for a 10kW motor. Taking into account the cost of the soft starter and
comparing it to the cost of the motor, the starter could cost more than the motor however.
Star delta starters can for most applications be considered more economically viable than a soft starter, therefore balance the motor cost against soft starter cost.
2.18.8
Protection device
The designer should categorise all loads connected to the switchgear according to critical status in the process and effect on operator safety. Table 2.18.5 provides examples.
Table 2.18.5 – Examples of Protection Required for Load Types
Load type
Type of
protection
Protective device
Main incomer feeder (local authority/ generator set) Overload, short circuit, restricted earth fault, phase losses, phase reveres. - main MCCB or ACB Pump, grinder Overload, short circuit, earth leakage, phase losses, phase reveres, under voltage, motor stall, winding temperature. 1- conventional protection device (OLR), MCCB 2- Electronic protection devices 3- motor manager protection unit Valve actuator Overload, short circuit, earth leakage. Conventional protection device (OLR), ELCB Instrument (level/ flow/ pressure) Overload, short circuit, earth leakage Conventional protection device (OLR), ELCB Building services (lighting/ sockets) Overload, short circuit, earth leakage, phase losses , phase reverses. Conventional protection device MCB, ELCB, Fuses
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Note: ELCB = Earth leakage circuit breaker OLR = Over load relay
MCCB = Moulded case circuit breaker ACB = Air circuit breaker
Type of protection 1. Short circuit protection:
This type of protection is required to protect the equipment against short circuit (with three phase, two phase or single phase), which can occur due to: insulation failure or damage, or by an incorrect switching operation. Short circuits are associated with electrical arcs and can therefore pose a fire risk.
2. Overload protection:
This type of protection is required to protect the equipment against overload current which is due to operational over current present for an excessive period of time. This over current will raise the motor winding or cable temperature above the permissible level and shorten the service life of the insulation. The task of overload protection is to allow normal operational overload current to flow, but to interrupt these currents before the permissible loading period is exceeded.
3. Under/over voltage protection:
This type of protection is required to protect the equipment against over/under voltage which is present due to main power supply instability (e.g. transformer tap changing/load fluctuating) or unstable supply from a standby generator (due to large load connected, faulty governor or voltage regulator). Operation with an under- voltage condition will draw more current from the supply, this over current will raise the motor winding or cable temperatures above the permissible level and shorten the service life of the insulation. The same will be the case with over-voltage which will effect the insulation of the motor or cable leading to insulation failure. This type of protection can be applied at the main incomers of the switchgear by a special relay to sense the voltage supply and trip the main incomers if the set limits are exceeded. 4. Phase losses/phase reversal protection:
This type of protection is required to protect the equipment against phase loss from the main supply, or phase reversal which can happen in the event of main supply reconnection or reconnection of the motor after maintenance. Operation with phase loss will raise the motor winding temperature due to an unbalanced current in the motor winding. In the case of phase reversal, the motor direction will be reversed, which will result in equipment damage or faulty operation (pump vibration, high sound levels etc). This type of protection can be applied at the main incomers of the switchgear or motor feeder by a special relay to sense the phase status (direction/availability) and trip the main incomers/feeder when a fault occurs.
5. Earth leakage protection:
This type of protection is required to: protect the equipment and personnel in the event of indirect contact; give additional protection in the event of single phase direct contact; earth fault protection; and protection against fires resulting from earth fault leakage current.
This type of protection can be applied at the switchgear outgoing feeders (motor / distribution board) by a special relay which senses the earth leakage current through a summation current transformer, the unbalanced current from the transformer will release a mechanism that will trip the breaker when a fault occurs.
6. Motor protection relay (electronic relay): This type of protection is used to protect the motor against many faults that can affect the motor operation and safety. The actual protection type can be varied according to the motor application (critical/normal) and size (cost). The following types of protection can be achieved by a motor protection relay:
• Over / under current;
• Phase loss/ unbalance/reversal; • Ground fault;
• Motor stall.
This type of protection can be applied at the motor terminals. The fault signal from the relay will release a mechanism that will trip the breaker when a fault occurs. Fault indication will usually be displayed on a LCD screen or by indication LED’s.
2.18.9
Interlocking facility
An interlocking facility is required where more than one incomer is used in the switchgear required. Some examples are as follows:
• Supply from two transformers/local authority supply;
• Supply from two incomers - one from transformer/local authority supply, and one from standby generator(s) panel;
• Supply from three incomers - two from transformers/local authority supply, and one from standby generator(s) panel.
The interlock facility should guarantee the safety of operation by not allowing under any condition the connection of two different incomers to the same bus bar section (transformer/transformer) or (transformer /generator) or main bus bars with the bus coupler closed.
2.18.10 Accessibility
The panel access for cable termination and maintenance can be arranged in the following format:
• Front access (suitable for installation area with limited space at the back of the MCC); • Back access (suitable for installation area with
available space at the back of the MCC, minimum one metre);
• Front/back access.
2.18.11 Cable entry
Cable entry to the MCC can be arranged in the following format:
• Bottom entry (suitable for MCC fixed at the top of cable/MCC trench);
• Top entry (suitable for MCC with cables such as feeders and incomers installed at ground level or above the MCC top level). Top entry panels are not preferred and should only be used in special circumstances.
Cables should be sized and installed in accordance with the IEE (Electrical Wiring) Regulations and QGEWC Regulations, and de-rated in accordance with the Electrical Research Association Report No. 69-30xlv.
Instrument, alarm, and control cables should be segregated from power cables.
The designer should consider the following when selecting cable routes:
• Number, size and function of cables; • Access for installation and maintenance; • Interface with other equipment, e.g. cable
routes should not prevent other equipment being removed for maintenance;
• Risk of mechanical damage ; • Means of support;
• Effect of installation method on de-rating factors;
• Hazardous area classification.
2.19
PLC’s
SCADA/Telemetry
2.19.1
PLC
PLC stands for Programmable Logic Controller. The PLC is a microprocessor-based device which is programmed to perform certain controlling tasks. The PLC is the brain of the overall process. It can receive analogue and digital signals from the process devices, analyse them and send digital and analogue signals to control these devices or activate certain alarms.
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PLCs were originally used for controlling purposes. Almost all PLCs are now equipped with signal transmitters (i.e. include some RTU features) that are capable of transmitting data to the network operation centre.
A redundant PLC system with hot standby configuration is highly recommended for critical applications where uninterrupted control is required. The power supply for the PLC system is usually 24Vdc or 110Vac. In case of power failure, the equipment should be backed up by a UPS system, which can supply the PLC with up to eight hours of power depending on the importance of the process. The modular type CPU (Central Processing Unit) in the PLC is capable of: solving application logic; storing the application program; storing numerical values related to the application processes and logic; and interfacing to the I/O systems.
The PLC carries out PID control, which is a significant task. PID (Proportional-Integral- Derivative) control action allows the process control to accurately maintain a setpoint by adjusting the control outputs. For example, pump flowrate setpoint is maintained by the following:
• Proportioning Band: is the area around the setpoint where the controller is actually controlling the process. The output is at some level other than 100% or 0%. The band is generally centred around the setpoint (on single output controls), causing the output to be at 50% when the setpoint and the flow rate are equal;
• Automatic Reset (Integral): corrects for any offset (between setpoint and process variable) automatically over time by shifting the proportioning band. Reset redefines the output requirements at the setpoint until the process variable (flowrate) and the setpoint are equal; • Rate (Derivative): shifts the proportioning band
on a slope change of the process variable. Rate, in effect applies the ‘brakes’ in an attempt to prevent overshoot (or undershoot) on process upsets or start-up. Unlike Reset, Rate operates anywhere within the range of the instrument. Rate usually has an adjustable time constant and should be set much shorter than
reset. The larger the time constant, the more effect Rate will have;
• Modulated Simplex I/O system: is the preferred solution for safe process since the duplex (redundant) I/O system is usually expensive, and the modulated simplex I/O configuration guarantees that any failure of a single I/O card will not cause the relevant I/O rack to fail. For instance, if a rack contains three I/O cards, which controls three pumps (two duty, one standby), the failure of one card will cause the whole pumping process to fail. In Modulated Simplex I/O systems however, it will cause the failure of one pump, which will be classed as the standby pump, and the other two pumps will continue run normally.
2.19.2
RTU
RTU stands for Remote Telemetry Unit. This unit delivers remote information back to network operation centres. Operations staff can access remote sites that have RTUs, via a web browser, SNMP (Simple Network Management Protocol) Manager, and XML (Extensible Markup Language). If an ethernet connection is not available, then the RTU's may be accessed via PSTN (Public Switched Telephone Network), normal dialup and even SMS (Short Message Service) messaging.
Earlier generation RTUs were hardwired and supported limited functionality’s such as data transfer and alarming. The new generation RTUs are equipped with powerful processors that allow the RTU to control certain instruments and devices, and to receive/transmit analogue and digital I/O (input/output) signals.
The microprocessor based RTU have a proven track record within the water and wastewater industry, a robust modular construction, and are constructed for ease of maintenance and repair. These are intelligent devices, capable of handling data collection, logging, report by exception, data retrieval and pump sequence control programs. RTU’s equipped with RS232/485 links are recommended for interconnection to standalone control systems, standard equipment packages and PLCs (Programmable Logic Controller). A dedicated
serial port should be provided for connecting a hand-held programming unit or PC.
The RTU software enables the RTU to process local input equipment information, before transmitting it to the master station to reduce transmission overheads. A report by exception operation is necessary for cost effective communication. The report is triggered by change of state of digital values, analogues reaching threshold values or varying by specified amounts. The RTU also reports when polled, and when the memory buffer is full.