Someday, somewhere, someone will cook up an effortless, instantaneous, unattended, free magic index tool for warehouses of old paper records.
Craig— You have presented an interesting but somewhat agonizing scenario. In an older A.S. Gill book on electrical testing, he points out {likely from a CYA standpoint} that one need always have spare parts available should the stresses of field testing* initiate unforeseen and undesirable circumstances, chiefly with implied and significant contributions of age and duty in the equipment and cables under test. In many cases this advice is probably ignored, but is a valid perspective none the less. Loss testing implies a degree of insulation stress, as do transformer short-circuit tests such as described in ANSI C57** standards. I would be gravely concerned of failure in one component, much less potentially multiple failures that could occur under field conditions and close scrutiny. You could probably use some less-invasive rating or prioritization methods like oil-DGA sampling if the transformer sizes warrant the expense, and accept published calculation/modeling of cable characteristics, based on published consensus literature and plausibly tempered for their age.
Another approach may be to get an initial reference point by readings at the site’s main service entrance, with instruments connected in the secondaries of existing current and potential transformers. Evidence of this capability may be switchboard volt/ammeters and phase-selecting tap switches; or possibly protective relays at the same location.
It is crucial that the user be appropriately cognizant of the overvoltage hazards connection errors or splitting existed wiring may induce. There are portable meters intended for 3ø ‘snapshots’ that have fairly decent very-low-range amplitude and phase-angle/power-factor accuracy/resolution measurements. Also, note that portable split/probe-type current transformers often have hideous ratio- and phase-angle errors at lower currents.
Two-element 3-wire tests would be acceptable if that’s what the utility uses. Zero-sequence currents should be of no significant concern, and would require three wye-connected PTs and three CTs.
{Typically} utility-owned 2-element metering installed for three-wire delta service, especially if older. Utility metering may indeed only be fed from two phase-to-phase voltage connections and two current transformers. Id est, if you’re not getting billed for it, the measurement becomes academic.
It’s not clear how the 57kW average was determined, but is the peak value known or anticipated to be significantly different? Some units, eg, “75,000 annually” could use clarification. No doubt you are aware of fundamental differences of AC versus DC losses and their corresponding measurements. The term ‘ohms tester’ is not apparent if of AC or DC excitation. Low system loading does not guarantee no overloading on a particular component, but you are likely aware that logger data may be of lesser value if loading is consistently as you have described.
The 57 versus sum-totaled 1000 comparison suggests fairly light use, although insulation deterioration can result from mere energization. Generally diverse loading does not completely dismiss possible small “bottlenecks” or “hot spots.” Has any of the equipment or cables, switchgear or transformers had heavier legacy loading periods? Your close observations, and those of others having regular contact with the system are most critcal for making others understand the relative significance that you and your immediates place on the gear, or merely the “service” it provides. Realize that each part has its own mode of aging. Particularly in your situation, it would be effort well spent soliciting RFQs from as many consulting firms as time permits, weighed against the opinion of the system’s value to upper tiers.
If you decide to tackle this, it can be a big job trying to prioritize a long list of observations and comments, but do not forget to include input by people who have and may continue to have daily contact with the system components. Their understanding of present and future system iterations have a big effect on service continuity and basic operational safety. Because cutover from old to new will probably not occur at an immediate point in time, scheduling will be increasingly important, as will your sense of local recipients’ perception of electrical service.
There will be some obvious resistance to unbolt so much as a cabinet cover, so a mandatory, documented walkaround of potential vendors to demonstrate as fully and honestly as possible what they are up against [and listening for feedback on several levels.] It may be blatantly obvious, but extensive communication between you, your crew, managers and vendors will be necessary for this to work. This may be invaluable to take to your procurement and maintenance managers, to best appeal to their concepts of common-sense with deference to the immediate and, if applicable, historic worth of the system [and their budget projections.]
If the inspection tour or preparations made by your crew exposes any otherwise enclosed live surfaces, include PPE as carefully suitable for the application. Absolutely acute and unquestioned knowledge of broad and specific operational hazards by your personnel encroaching or nearing legally-defined minimum approach distances becomes literally a life-or-death matter. [Be cautions of remarks stemming from the sometimes deep-seated ‘done-it-that-way-for-forty-years’ rationalization.]
Written descriptions and photographs made available to potential participants in the project are usually of great value. If you have not already discovered slate, cotton or maybe some asbestos, it will probably turn up on closer inspection.
For some served facilities, short-tem procurement, nursing and rotation of portable, isolated gensets may be desirable during modifications where disruption could be costly or compromising.
Be well aware that in the US, regardless or your affiliation, OSHA practices are unwise to skirt. Critically, practices for >600V systems—known to be energized or deenergized—are extremely different and most unforgiving if the associated personnel are not thoroughly and respectfully aware of and rehearsed in accepted, established procedures. Ungrounded-delta distribution adds still further need for solid understanding of how theory is manifested in operational procedures. Use of modified or experimentally conceived electrical assemblies should be avoided if at all possible. Load-monitoring/analyzing equipment is often rentable/loanable, eg, contact Arbiter Systems or Reliable Power Meter in NorCal or Sensorlink in Acme, Washington.
You are translating your understanding of the importance of electric service to your facilities, and the significance of doing that with the present gear, or that of radically overhauled version.
Consider in the rework approach that a new, higher operating voltage may have economies in the form of standardized equipment applicability and availability. As ad rem, a grounding zig-zag or grounded-wye/uncommitted-delta transformer bank usually limits overvoltage extremes and permits more effective use of surge protection. This typically provides more-sensitive, faster and associated increasingly effective ground-fault protection. It is remotely possible that a 4160Y/2400 high side could ease systematic-swapover burden, if parts of existing are not found in immediate peril.
If no one is able to present typical transformer numbers here, you may want to call 541.826.2113 in Medford and explain your situation—alternately Tom Steeber 541.826.8840 Alstomtransformers.com formerly Balteau-Standard {Casually hint that you may need quotes on assorted replacement distribution transformers soon.}
Note that there are immediate and long-term consequences of electrical-distribution failure, and how that fits into your expectations. Also, if removal, repair or modification of any existing components is needed, as you see fit take time to become oriented—however unavoidably—to environmental, not-always-rational restrictions that may surface with operation or modification of the old system. Expensive accommodations that may be needed can become a budgetary landmine.
It may seem inappropriate to say, and discouragement is not intended, but be very careful and best of luck in your efforts. At the upshot, it could be a very valuable electrical-operations episode. If you suspect so, I hope that if some record of the existing system would be of historical benefit to a larger community, that you will be allowed to adequately record its present and past significance for others.
*Inanimate anthropic bias/quantum theory: “observation affects reality”
**Besides insulation quality, there are basic no-load {‘core,’ iron, low-pf, generally voltage-influenced} and full-load {‘winding,’ copper, higher-pf, short-circuit, mostly current-influenced} loss tests. Make the determination that instrumentation for core-loss checks is suitable at very-low current range and low PF.
