Sparks Electrical News August 2019

CONTRACTORS’ CORNER

7

WORKING KNOWLEDGE WITH TERRY MACKENZIE HOY

MORE KNOWLEDGE ON SOLAR SYSTEMS I am sure many of you out there are looking at the growing so- lar power business and thinking that perhaps this is a goose that can be easily plucked, if you understand the metaphor. How hard can it be? Solar panel, charge controller, invertor, transformer and connection to the main supply. Well... here are some thoughts. Beginning with the panel: There are a number of solar panels, namely Monocrystalline Solar Panels (Mono-SI), Polycrystalline Solar Panels (Poly-SI), Thin-Film Solar Cells (TFSC), Amorphous Silicon Solar Cell (A-Si), Biohybrid Solar Cell and Cadmium Tel- luride Solar Cell (CdTe). The panel salespeople will no doubt ex- plain to you which one you should use and will, at the same time, disguise any disadvantages of the various panels they sell. I won’t write about this other than to say, in my experience Monocrystal- line Solar Panels are the best from a heat deterioration point of view and lifetime duration, and that Cadmium Telluride is toxic. The bottom line is that the panels, when in sunlight, generate a voltage which is about 17 V dc. The output voltage falls as the current increases and falls to zero at the short circuit current of the panel. But do not worry about this – your friendly panel sup- plier will guide you along. The important thing to remember is that

when you buy, for example, an 800 W solar panel, this will typically supply about 26 amps into a 12 V battery and a peak of 50 amps. Smart readers out there will work out that 26 x 12 = 312 W and that 50 x 12 = 600 W, not 800 W. But all the panels are different. The thing to bear in mind is that power output is dependent on how bright the sun is (an absolute maximum of 1000 W/m 2 ) and how much current is being drawn from the panel. Read the graphs supplied with the panel. Whether the solar system feeds into the grid (‘grid-tied’) or charges a battery makes a big difference to the power you get out of the system. Whatever anybody says, batteries are not very efcient, with the exception of Lithium Ion batteries which are ex- pensive. Thus, from the solar panel you have to allow for reduced output due to variations in sunlight and the charging/discharging inefciency of the battery. Some battery suppliers tell you that their batteries are very efcient and will last 25 years, but forget to tell you that this only applies if the battery temperature is kept below 25 °C which, in South Africa , is not very likely. Grid-tied systems are generally a better bet than battery sys- tems, but they do have a disadvantage – namely the grid. Even

a small installation may be connected to a supply which has a fault level rating of up to 10 kA. What this means is that the solar system must be connected to the main supply via HRC fuses – if not, then a short circuit in the solar system will result in some seri- ous damage. The downside with fuses is that people bypass them or unwittingly replace them with circuit breakers, which puts the system in danger. Like many things, it makes a big difference if you get somebody who is a registered professional engineer to have a look at the whole of the proposed solar system and how it will fit into an exist- ing system. One should not only take care of the prospective fault currents, but also of the sensitivity to power dips and voltage vari- ations in grid connection points due to other factors. I know of one system rated at 500 kW which was more often disconnected than connected – it supplied a 630 kVA transformer which produced a big inrush current when connected to the main supply on the medium voltage side. A lot of smart design was necessary before the solar system could get connected and actually stay connected for a whole day. It is a fact that this was in an industrial area which had quite a poor quality of supply. But it happened.

WEARCHECK ACQUIRES RELIABILITY TESTING COMPANY, BOOSTS SERVICES

AN EFFECTIVE WAY TO REDUCE CABLE FAILURES

F aults on underground power cables are almost always dis- ruptive and costly to fix. So there’s a very big incentive to minimise the risk of such faults occurring. One of the most effective ways of doing this is to use partial discharge (PD) test- ing, which looks for the characteristic electromagnetic signature produced by small electrical discharges in air gaps, voids and cavities in cable insulation, and in joints and splices. These dis- charges typically start long before the problems develop into a fault, so detecting them early gives time for action to be taken to avert outright failure. A large percentage of failures on new cables are attribut- able to poor installation work. PD testing before a new cable is energised will reveal many of the issues, such as improper or incomplete stripping of insulation, improper shrinking of in- sulating sleeves, or nicks and cuts in the insulation. The faults can then be rectified, almost always at the installation contrac- tor’s expense, before the cable enters service. PD testing is also an invaluable way of revealing similar issues after repairs have been carried out on a cable. Regular PD testing on in-service cables provides useful data that can be trended or compared with data from similar cables to flag changes that need further investigation. It is also an ex- cellent tool for evaluating suspect cables. Megger’s new PD detector PDS 60 combines many years of field experience, customer demands and innovations in one device. That makes it possible to carry out a VLF test along with a PD diagnosis. This not only increases the reliability of the withstand-test, since partial discharges are simultaneously recorded, but is also cost-effective. If a PD-free cable is detected during the withstand-test, a PD diagnosis for quality control is

no longer required. While no test method can reveal every incipient fault, PD testing is undoubtedly the best form of ‘insurance’ against cable faults that’s currently available. And, considered in the light of the financial impact of a typical cable fault, it's insurance well worth buying.

D urban-based condition moni- toring specialist, WearCheck, has bought Anglo Field Services (AFS), which adds three new established divisions to the company’s comprehensive reliabil- ity solutions portfolio, including non- destructive testing (NDT), technical compliance (TC) and rope condition assessment (RCA).

Enquiries: info@megger.com

The fourth AFS division – asset maintenance management (AMM) – has been absorbed into Wear- Check’s existing Reliability Solutions team, boosting the company’s man- power in this section as well as add- ing technical expertise and additional instrumentation.

Neil Robinson, managing director of WearCheck

Branching into Non-destructive Testing means that WearCheck now conducts a range of analysis techniques through which the properties and condition of a component or system are evaluated without causing any damage to it. NDT methods are gaining popularity because they do not permanently alter the item undergoing inspection, making NDT a valuable tool that can save money and time in condition monitoring and inspections – an ap- proach which dovetails perfectly with WearCheck’s ethos. WearCheck’s new NDT division offers a range of techniques, including eddy-current-, magnetic-particle-, liquid penetrant-, radiographic-, ultra- sonic-, and visual testing. The new rope testing division provides specialist cable strength assessment and is manned by seven of only 12 people in South Africa who are qualified to conduct these tests. The technical com- pliance division provides expert guidance to assist companies to comply with regulatory requirements, and to rectify violations or problems high- lighted during audits. Prior to merging with WearCheck, AFS operated as part of Anglo’s Technical Division under the Anglo Research section, where they provided specialist NDT and materials consultancy services to Anglo Group com- panies and other clients on a global basis. WearCheck is now positioned to offer customers access to an aug- mented portfolio of reliability solutions in a comprehensive one-stop- shop, leading to cost savings and boosted convenience for customers. WearCheck MD Neil Robinson believes that the addition of new services enhances the availability of assets operated by WearCheck customers by expanding the choice of condition monitoring options. “For example,” says Robinson, “the development and implementation of NDT procedures will have a profound impact on keeping operational maintenance costs down for our customers. The incorporation of AFS into WearCheck has been a seamless process so far, mainly because the two companies operate with a parallel devotion to data integrity and a shared dedication to customer service excellence,” said Robinson. “All 30 AFS staff members have been absorbed into WearCheck’s staff complement, adding their invaluable expertise and knowledge to the WearCheck team. From our side, WearCheck extends a heartfelt welcome to all current AFS customers, and we look forward to bringing new field services clients on board as well as giving our existing clients the benefit of our increased reliability optimisation service offering,” he said.

Enquiries: +27 (0)31 700 5460

SPARKS ELECTRICAL NEWS

AUGUST 2019