Control System Design Helps Minimize Overflows, Maximize Reliability
Many sewer system overflows and distribution systems outages are the result of problems with pump station controls and electrical systems.
By Grant Van Hemert
Many sewer system overflows and distribution systems outages are the result of problems with pump station controls and electrical systems. Causes of control system anomalies are varied, but some can be addressed in the design phase, including such issues as harmonic and surge, panel temperature, performance during an electrical fault and code compliance.
Harmonic and surge protection
Electrical devices, such as computers and variable frequency drives, can create a continuous electrical noise that is embedded on the main 60 hertz frequency. If the noise is a multiple of the 60 hertz frequency, then it is a harmonic. Harmonics shorten equipment lifetime by creating excessive heat and stress.
High intensity but non-continuous noise is called surge. Common causes of surge include lightening and contact closures. Damage is caused if the surge exceeds insulation ratings or overloads a device's capacity.
Surge devices can divert the high intensity energy to ground before it can reach the device. To help protect the equipment, surge protection should be placed on incoming power lines, as well as phone and cable connections.
Surge protection should be placed on any other conductors that are not inside a grounded metal conduit. This includes float switches, level transmitter cables, cables inside flexible conduit, and submersed motors power cables. Finally, surge protection needs to be installed on radio antennas to help protect against lightning.
Harmonics are a greater concern in large power systems and not a concern in many smaller lift stations. Harmonics are typically mitigated in three ways: passive filters, active filters and variable frequency drives with 18 pulse technology.
Panel temperature has an impact on component life and performance. Let's assume a pump panel with three 20 hp full-voltage, non-reversing starters, a PLC, 480 vAC to 120 vAC transformer and circuit breakers is placed outside. This panel can reach more than 140 degrees when the ambient temperature is 85 degrees.
PLCs and RTUs are often required to have a 60 degrees Celsius (140 degrees Fahrenheit) rating. However, most PLCs have a battery backing up the memory. Some manufacturers' data indicates that batteries should not be exposed to more than 140F for 30 days. In such an example, to help prevent an accidental memory loss the battery needs to be replaced every 30 days. As an alternative, the utility can use a controller that does not need a battery. This will minimize the heat challenge and eliminate the costs associated with battery maintenance.
Another area where heat is a concern is the motor starter overload. Sometimes called a heater, this device protects the motor coils from damage. A eutectic heater is a solder filled pin set to melt at a certain temperature. The solder is heated to its melting point by higher than normal current. The assumption with a heater is that the motor and heater will be at the same ambient temperature.
For many applications, the heat around the heater and motor is not the same. In these applications the heater selection must be modified. To complicate matters, the heat load on the heater can change with the seasons, requiring the heaters to be changed throughout the year to assure adequate protection.
Instead of a eutectic heater, a solid state overload might be a better solution. This device directly measures current and determines if an overload exists. It does not rely on amperage to temperature correlation so the temperature issues disappear.
Motor starters are the heart of water and wastewater pumping applications. During an electrical fault, such as two-phase wires touching downstream of the starter, the starter must stay intact even at the risk of sacrificing its ability to operate. This means that replacement of the starter, or its parts, may be necessary before the pump station can be placed back into service. During the down time, the wet well is filling or the towers are depleting their capacity – all while the clock is ticking.
To minimize a CSO/SSO or lack-of-water event, it is now possible to put the starter back in service without an immediate repair. This is possible if you choose a UL 508 Type E starter. (One point of clarification, there is a document called UL508E, this is not the same as UL508 Type E.)
A UL 508 Type E starter is a combination starter and includes overcurrent protection, motor overload and contacts in one device. This grouping of devices allows for a faster overcurrent response time then if the devices were separate. This combination limits the total energy passed through the contacts and minimizes damage. To better understand this, see a video that shows a fault event comparison between a UL508 Type E NEMA starter and a traditional NEMA rated starter at http://www.senamicro.com/nspp/presenter-tesysU/index.htm.
With a UL 508 Type E starter, you can put the unit immediately back into service. The unit can remain intact through a second fault, but may not be able to be put back into service. Thus a UL 508 Type E starter allows you to repair the device at a more leisurely pace, while minimizing the chance of an overflow or lack of water.
The National Electric Codes (NFPA 70) dictates minimum acceptable standards in design. However, with the amount of sections in the book, it is easy to miss information. One of the sections often overlooked affects panels using storage batteries.
NEC 480 defines a storage battery as any type of rechargeable battery (NEC 480.2). NEC 480.9(A) states "Provisions shall be made for sufficient diffusion and ventilation of the gases from the battery to prevent the accumulation of an explosive mixture." However, NEC 480 does not provide a formula or method to determine when sufficient diffusion has occurred. This leaves one to wonder how much space is required to allow sufficient diffusion and whether the typical NEMA 4X control panel allows for "sufficient diffusion."
To avoid the issue, a redundant power supply with battery can be located remotely. As an example, the Schneider Electric Phaseo® redundant power system converts 120 vAC to 24 vDC and uses a 24 vDC battery to provide power during a power outage. This battery can be placed in a ventilated NEMA 3 enclosure with bug screens and connected to the main panel through a conduit – the panel and its controls can be NEMA 4X rated.
In the bid nature of water and wastewater projects, control panels in a facility may have been manufactured hundreds of miles away. The designers may not understand the area's logistics. For example, one facility may be in a big city with plenty of local resources while another is in a rural town where resource may not be as plentiful.
When it comes to the components inside the control panel, it is important to understand the component manufacturers' supplier and service network. Some manufacturers have an extensive network that penetrates into remote areas, and others have suppliers and service capabilities predominantly in machine building regions. These manufacturers' may not be as strong in rural areas. Since many wastewater plants are located in small cities and towns, selecting a component manufacturer with a strong rural distributor network may be the best option. For a utility, having a part close can dramatically impact how reliable the system is, and shorten repair time. For panel builders, systems integrators and OEMs, having parts close can keep a start-up on schedule.
The reliability and proper operation of the collection and distribution system is a key criterion for a utility. With as many as 16 percent of SSOs being caused by control outages, it is clear that proper design of an electrical control panel is crucial. During design there are several factors that need to be examined. These are harmonic and surge protection, effects of panel temperature, performance during an electrical fault, code compliance, and impact of the manufacturers supply network on repair. When properly addressed, these factors can help prevent your collection and distribution system from becoming a statistic. WW
About the Author:
Grant Van Hemert, P.E., is an automation and control applications engineer at the Schneider Electric Water Wastewater Competency Center. He may be contacted at Grant.VanHemert@us.schneider-electric.com.