Sparks Electrical News August 2021

MCC AND MOTOR PROTECTION

10

WEG soft starter upgrades mill operation at South African plant

The need for type tested distribution panels SANS 10142, as well as the mainstream international wir- ing codes, call for similar requirements. During the train- ing of many groups of young, upcoming engineers, this fact was not fully known. This article tries to explore the reasons for this requirement.

are widely published but probably not well understood. In addition, copper conductors contribute to the temper- ature rise. Selecting their application also requires careful consideration. The current carrying capacity considers the safe working temperature to be achieved. For bare cop- per, it may be 120/125°C; for rubber or nitrile insulation, 95/105°C and, for PVC conductors, typically 70°C. Traditional circuit breaker tests will have been done with PVC sleeving and, if done at the higher temperatures, will be damaged or seriously derated. The over tempera- tures are generally offset by the fact the breaker rarely carries currents above 80% in any case. Tripping circuit breakers under high load on hot days is a fairly common occurrence. Lack of care in terms of temperature control can be seen in many, many panels where PVC tape or PVC shrink-sleeving has been used to provide the insulation for bus-bars and is showing local burning or splitting. The reasons outlined here are typically the issue e.g., 70°C PVC at bus bar 120°C rated loading. Temperature issues can be controlled by: • Correctly rating conductors, considering over-rating for better temperature reduction. • Having enough inner volume in the panel. • Natural vented panels. • Forced panel ventilation with fans. • Forced cooling e.g., AC. • Anticipate reduced lifespan for lack of proper controls . IP rating for the panel Tests will be configured to measure the claimed level of compliance. It does not make much sense to over-specify a DB intended for indoor use and sit with potential tem- perature rise problems as described in this article. The fit- ting of rubber door seals will effectively prevent even the basic natural vent condition. It is not possible to have a fully sealed panel without issues with reducing the tem- peratures. Raised temperatures will also have long-term effects on all insulation and will reduce performance over time, posing a raised short-circuit flash risk. Feeder cubicle tests Typically, each size/variation of the circuit breakers will be required to pass a short-circuit trip test. Interestingly, this is the first time any circuit breaker will be tested in this manner. The circuit breaker is tested in the open/non- panel condition. The main aim is to check that there is no inter-phase secondary flashover or insulation breakdown after testing. The door and cubicle are to remain closed. Cheap door hinges and locks will not generally contain the tripping blast conditions. Do not change after testing! Also, are all inter-phase barriers fitted? Simple modifications such as a different breaker and orientation will negate the certification. Never forget that an operator is most likely going to be standing just out- side the DB door. The door could be blown out at him/her, possibly followed by an electrical flash with enough heat to cause serious burns. For certain sized panels, it has be- come the norm to add remote switching, the so-called ‘chicken switch’, and to do the actual switching from a safe distance away. Future proofing Believe your type-tested panel supplier when he suggests spare compartments for future growth as this is possibly the easiest way to comply with future additions. The certi- fication process is arduous and costly and results in a full procedure on how to configure any future panel in detail. All users of these type-tested panels need to fully comprehend how to handle future maintenance and re- pairs, as well as any additions. Typically, any replacement circuit breaker must be of the same manufacturer model and type. There are many overseas panel manufacturers and a few local manufacturers who can assist with these select panels. I once lost a fairly high kA panel to a small panel shop who had saved thousands by placing all the MCBs in a cramped fashion, all together. Experience tells me that that style of panel has absolutely zero chance of survival should any feeder to trip at the designed site short-circuit level.

It is also believed that many DB panelboards are not built to requirement or that, when adding to type tested panels in use, the addition compromises the original type test’s integrity. Whilst individual items may have been cer- tified, this is the first time the panel is tested as a complete system. The reason for this requirement is safety, safety, and safety! Any person who has been in the situation where a nearby DB panel erupts into a massive arc-flash during switching, will attest to the massive burn risk to personnel and the total destruction of the panel itself. Unfortunately, in reality, few survive unscathed. Industrial plants that en- sure that personnel are removed from harm’s way and insist on a full anti-flash suit for the operator are doing so precisely because of previous, often-deadly events! During certification tests to claim compliance to the levels of the board manufacturer, a complete panel will be subjected to a battery of tests in a lab to simulate the claimed incident. This is the most practical method to prove the manufacturer’s claims. It takes considerable time and energy, which translates to cost. To the un- schooled, this is where an apparent cost saving is seen in using an alternative self-made panel. The following are some of the tests carried out to determine the DB’s integrity: Bus-bars Short-circuit withstand on the bus-bars as installed, e.g., 30 kA for, say, one or three seconds, typically. The thick- ness of the copper, the method of holding (e.g., spacing) and the kA all play their part here. There are great me- chanical forces built up during these tests on the copper work and these need to be contained in order not to break free and result in the copper conductors touching. Should they break free and create a short, the panel would indeed be history. Unless you are in the industry and know these forces, it is very difficult to visualise the forces per se. That is why, once these tests are concluded, there can be no changes to the supports by way of number, type or spacing. It would be extremely unlikely that there would be total success from a new design of 30 kA and above during these tests, thus making this one of the major test con- siderations. Added support on the bus-bars would also be on the droppers to, say, the air circuit breaker (ACB). Any long run of cable or bus-bars would require support at high levels of short circuit. Temperature rise tests Thermocouples are placed at various points on the con- ductors within the pane setup and the rated current/s sent via the panel sections are recorded. The current could be low voltage -- all that matters is the correct current level. In the author’s experience, this is by far the main failure during panel testing, even by manufacturers with many years’ experience! Good old Ohm’s Law and basic physics catch many out. All conductors, be they the main bus- bars or dropper feeder cables, and even the circuit break- er itself, offer some impedance/resistance to the current flow, the result which is a slow temperature rise. Physics tells us that heat rises so, if the heat is gener- ated in the lower region, it will migrate up to the upper region. There are watt loss figures for all circuit breakers, but this is typically rarely common knowledge. Natural ventilation with slots at the top and bottom of the panel can be very effective. Mid-door types seen by the author are useless and their users often did not attend that all-important physics lecture! It is very important to note why this test is called for. Circuit breakers and indeed much of the equipment to be housed within the panel may be specified for a maximum ambient temperature, for example, 40°C. So, we have a contribution from the internal loading as well as the ex- ternal climate conditions. The test is done until a thermal equilibrium is obtained and reaches a maximum in line with the daylight trace, some time just after 12pm. Thermally sensitive circuit breakers would be required to be derated under ambient temperatures exceeding 40°C and may be slower by 15% at 60°C. These curves

Fritz Hoogenboezem, branch manager at Zest WEG Middelburg.

Umagezana technician terminating a 6,6 kV cable.

WEG SSW7000 medium voltage soft starter for the rod mill installed on site.

W hen a process plant in South Af- rica’s Mpumalanga province needed an improved starting method for its rod mill, Zest WEG was able to supply its WEG SSW7000 medium-voltage soft starter. The plant had recently upgraded to a 450 kW, 8-pole, 6600 V WEG W50 motor, which was started by a direct online starter. According to Fritz Hoogenboezem, branch manager at Zest WEGMiddelburg, the soft start- er was imported into South Africa from WEG’s Brazil factory in time for the plant’s scheduled shutdown, and was installed by local systems integrator Umagezana Engineering. “By avoiding the sudden in-rush of current when the motor is switched on, the soft starter reduces both the mechanical stress on equip- ment and the peak load drawn from the national power grid,” says Hoogenboezem. There is a direct financial benefit to be de- rived by the customer if their peak consumption reduces to a lower peak demand value. This is because exceeding the maximum demand – as can occur when a large motor starts up – leads to costly penalties being applied by the power utility. The in-rush current to start a heavy-duty load can, in those first few milliseconds, be sev- en to ten times higher than the level at which the motor is specified. “By exceeding the maximum demand, the plant could pay much more than necessary for its power consumption, not just for that month but for 12 months afterwards,” he says. The installation of the soft starter was a com- plex task, according to Umagezana Engineer- ing director Ernest Bean. Once the direct online starter was removed, it was clear that the cable lengths would not fit directly onto the new soft starter. “This required changes to be made with the motor control centre (MCC) on the high voltage side,” says Bean. “The different cable lengths meant that cables had to be re-terminated to ensure safe and compliant operation; nine cores or phases were terminated.” WEG SSW7000 inner body with the fibre optic cables.

Another challenge was that the door to move the equipment in and out was not wide enough. This required specialised rigging to be planned and conducted carefully, as the WEG SSW7000 soft starter weighs almost a tonne. Once the new equipment was installed and connected, Zest WEG’s drives and automation technician Reiner Gundlach assisted with the cold and hot commissioning. Extensive tests were run in accordance with WEG’s detailed OEM specifications, laying the technical founda- tion for the optimal performance and lifespan of the equipment. “As a sophisticated medium-voltage item, there are numerous aspects that need checking and testing,” says Hoogenboezem. “The results are then communicated with WEG as part of our rigorous quality management procedures.” Bean highlights that, as a technician in the field, he was impressed by the smart use of fibre optics between the control section and the high voltage operational side. In particular, the func- tionality test in the soft starter’s software can test each individual fibre optic cable using a control voltage of 230 V before the system is opened to the full 6600 V used for operations. “Among the other benefits of the soft starter is its monitoring functionality, which allows the plant management to closely track the interac- tion between the motor, the soft starter and the plant PLC,” he says. “Management can at any time log into the PLC and use that interface – together with the soft starter – to view important information such as tonnages processed, and the number and duration of stoppages.” Umagezana Engineering was appointed by Zest WEG as a distributor for the WEG low-volt- age switchgear range in January 2021. Its whole team has also recently completed the required MV training for high voltage terminations and cable joints, which was done through a specialist firm accredited by the Energy and Water Sector Education Training Authority (EWSETA).

By Kevin Flack. Email: kevin.flack@outlook.com

Enquiries: www.zestweg.com

SPARKS ELECTRICAL NEWS

AUGUST 2021