VA burden

The VA burden and the corresponding p.f. of all such components are provided by their manufacturers. VA COS0 is the content of watt loss. For more details see Section I5.6.l(iii). 

These VA burdens are for moving iron instruments. For electronic meters these values would be of the order of 0.1 to 0.5 VA. 

At voltages lower than 5%, the limits of error may increase. They become up to twice the specified errors at about 2% of the rated voltage, the limits of VA burden and p.f. remaining ihe same. 

The total VA level of a control or an auxiliary circuit is the phasor sum of the VA burdens of each individual component and device connected in the circuit, and... 

The control transformer to be selected may have a rating nearest to the maximum hold-on VA burden so calculated and must be suitable to feed the required inrush current at the p.f. so calculated without affecting its regulation.. So long as these two points fall below the inrush curve of the control transformer, its regulation... 

Approximate VA burden and cos 0 for each component, as available from the manufacturers catalogues ... 

Where three CTs for unrestricted or four CTs for restricted ground fault or combined O/C and G/F protections are employed in the protective circuit, the VA burden of the relay is shared by all the CTs in parallel and a normal VA CT may generally suffice. Such is the case in most of the protective schemes discussed in Sections 21.6 and 15.6.6(1), except for those employing only one CT to detect a ground fault condition, such as for a generator protection with a solidly grounded neutral (Figure 21.12). 

These errors are valid only when the CTs are loaded by a minimum 25% of the rated VA burden, for CTs of class 1 and 50% for CTs of classes 3 and 5 and a primary currenl of not less than 5% or more than 120% of the rated current. The measuring CTs may not transform correcllv unless the above conditions are met. 

These are employed to detect a fault, rather than measuring the current of a power sysletn or the connected equipment. There is a fundamental difference in the requirement of a measuring and a protective transformer in terms of accuraev. saturation level and VA burden. Unlike a... 

Circuit whose current is being summed Current rating Main CT ratio VA burden shored by the main CTs... 

Consider a protective scheme having a total VA burden of 15 and requiring an accuracy limit factor (ALF) of 20 ... 

In most applications, a bar primary CT is generally used and a normal CT may be suitable. But for too small ratings, where the use of a wound primary CT is imperative, short-circuit effects must be considered, except the CTs for an LT system, where the fault level for such small ratings may be very low and may not matter (Section 13.4.1(5)). For applications on an HT system, where a wound primary CT is imperative, choice of a CT from standard wound primary CTs may still be possible, meeting the minimum requirements of class of accuracy, VA burden and short-time rating. lEC 60044-1 indicates for measuring and protection CTs the maximum short-time factors (STF) that can be obtained economically for a normal wound primary CT where... 

Similarly, for a protection CT from Table 15.11 choose an accuracy class of I OPS with a VA burden of 5 for a second short-time current. If this does not meet the need, the protection CT may also have to be selected with a higher-rated primary current. 

One should select a lower secondary current, say, 1 A CT, for installations requiring long lengths of connecting leads, such as for remote measurement of current or other quantities. It is advisable to limit the extra VA burden on the CTs, on account of such leads. 

To ensure proper sensing of the incoming current and its phase displacement by the relay it i.s es.senlial that the CTs ratio and their VA burden chosen for the required duty are close to the actual requirement as noted in Table I. S.8.. Sometimes this fact is overlooked and CTs with a much higher VA burden or ratio or both are chosen while the secondary circuit may not be adequately loaded. In this case the CTs may not accurately transform the primary parameters to the secondary and. in turn, the relay may not send accurate signals. Moreover, the relay itself may operate only at minimum 19c or more of its rated current (I or 5 A), depending upon its design and type (lEC 60051-1). 

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