transformer in a consumer-type
substation
In certain national standards*, the choice is made according to two current values: c the reference current Ib, the value of which will be:
v in the case of metering at low voltage: the nominal rated current of the transformer, v in the case of metering at high voltage: the sum of the nominal rated currents of transformers and other HV plant (e.g. motors, etc.),
c the minimum value of HV 3-phase short- circuit current at the installation.
* There is no equivalent IEC standard.
N 3 2 1 1 2 3 HV LV E/F relay
When the reference current is less than 45 A and there is only one transformer, the protection may be by fuses or by a circuit breaker.
When the reference current is equal to or greater than 45 A, or when there is more than one transformer, the protection will be by a circuit breaker.
The maximum IEC standard kVA ratings of transformers corresponding to a HV full-load current not exceeding 45 A are given in the table C18.
c protection by fuses
The relationships between the reference current Ib, as defined above, the rated current In of the fuse, and the short-circuit current Ic at the primary terminals of the transformer, are determined according to the national standards previously referred to, as follows: v when the substation consists of a single HV/LV transformer, the rated current In of the fuse must satisfy the following relationships: In > 1.4 Ib and In < Ic/6
Where:
In = rated current of the fuse,
Ib = rated primary current of the transformer, Ic = the minimum current at the primary side of the transformer when the secondary- winding terminals are short-circuited, v when the substation is supplied from an overlead line, or when the installation is sensitive to unbalanced-voltage conditions (for example three-phase motor loads), it is recommended that the failure of a fuse causes all three phases to be cut off, by automatic tripping of the HV load-break switch (i.e. a combined switch-fuse). Standard current ratings for fuses according to IEC 282-1 are listed in table C19.
table C18: power limits of transformers with a maximum primary current not exceeding 45 A.
primary maximum IEC voltage standard ratings (kV) for transformers rated nominal (kVA)
3.6 3 250 3.3 7.2 4.16 5.5 500 6 6.6 12 10 800 11 17.5 13.8 1,250 15 24 20 1,600 22 36 33 2,500 40.5 36.5 3,150
3.2 electrical protection
(continued)supply voltages nominal transformer ratings (kV) (kVA) rated nominal 25 50 100 125 160 200 250 315 400 500 630 800 1,000 1,250 1,600 2,000 2,500 3.6 3 16 25 40 50 50 63 80 80 100 125 160 200 250 3.3 16 25 40 50 50 63 80 80 100 125 160 200 250 7.2 4.16 10 25 31.5 40 50 50 63 80 80 100 125 160 200 250 5.5 10 16 25 31.5 40 40 50 63 63 80 100 125 160 200 250 6 10 16 25 31.5 31.5 40 50 50 63 80 80 100 125 160 200 250 6.6 10 16 25 25 31.5 40 40 50 63 80 80 100 125 160 200 250 12 10 6.3 10 16 25 25 31.5 31.5 40 50 50 63 80 80 100 125 160 200 11 6.3 10 16 25 25 25 31.5 31.5 40 50 63 63 80 100 125 160 200 17.5 13.8 6.3 6.3 10 16 25 25 25 31.5 31.5 40 50 63 63 80 100 160 160 15 6.3 6.3 10 16 16 25 25 31.5 31.5 40 50 50 63 80 80 100 160 24 20 6.3 6.3 10 10 16 16 25 25 31.5 31.5 40 50 50 63 80 80 160 22 6.3 6.3 10 10 10 16 25 25 25 31.5 31.5 40 50 50 63 80 160 36 33 6.3 6.3 6.3 6.3 6.3 6.3 16 16 16 16 16 31.5 31.5 40 50 63 80 40.5 36.5 6.3 6.3 6.3 6.3 6.3 6.3 16 16 16 16 16 25 25 31.5 40 50 63
table C19: rated current (A) of HV fuses for transformer protection according to IEC 282-1.
It is strongly recommended that, following the operation of a fuse (or fuses) to clear a fault or overload condition, all three fuses be replaced, since it is possible that the fuse (or fuses) that had not operated may have deteriorated, due to the passage of excessive current during the disturbance.
c protection by circuit breaker
When the substation is supplied through a HV circuit breaker, it will be a contractual condition that no disturbance occurring within the installation shall cause the operation of any protective relaying in the power-supply network.
To ensure that this condition can be complied with, the supply authority must specify the longest times permissible for clearing the following faults on the installation: c short-circuit fault between all 3 phases, c short-circuit fault between any 2 phases, c short-circuit fault of one phase to earth, c short-circuit fault of any 2 phases to earth. The maximum level of a 3-phase short-circuit at the installation was known at the outset of the project, in order to purchase adequately- rated equipment. To ensure correct operation of the protective devices, the minimum value of 3-phase short-circuit current must also be stated by the supply authority.
When planning the protection scheme for the installation, the general principle of co- ordination is that the circuit breaker closest to the power source will have the longest tripping time. In the present case, this is the HV circuit breaker.
This longest tripping time must not, however, exceed that given by the supply authority; a constraint which generally is satisfied only by protective relays at the HV circuit breaker, to supplement the transformer-mounted protective devices previously mentioned. As far as earth faults are concerned, there is no co-ordination problem, provided that the transformer HV winding is a delta- or unearthed-star connection, since, as already mentioned, earth faults on the LV system will then appear as phase-to-phase faults on the HV system. HV earth faults occurring in the substation can therefore be cleared instantaneously by a REF scheme. Instantaneous tripping for phase-to-phase short-circuit faults occurring on the HV side of
the transformer can also be achieved very simply by devices which are sometimes referred to as “high-set” relays.
The “high-set” principle depends on the fact that, if the current is sufficiently high to operate the relay, then the short-circuit must be on the HV side of the transformer, because a short-circuit on the LV terminals or windings of the transformer will not produce sufficient current on the HV side to cause the relay to operate.
The high-set relays (2 or 3 as noted in Sub- clause "protection against short-circuits") will each be connected in series with one of the inverse-time/overcurrent relays, shown dotted in figure C17, and for distribution-type transformers are generally set to operate at 25 times the full-load current of the transformer. By these simple means therefore, instantaneous clearance of short- circuit faults on the HV side of a transformer can be achieved, without affecting the co- ordination scheme for downstream protection. At periods of the lowest levels of short-circuit fault current, however, the high-set scheme may not be sufficiently sensitive, i.e. the current may not be high enough to operate the relay (there is no similar problem with earth faults, the REF scheme being very sensitive).
In extreme cases, where the difference between maximum and minimum fault levels is very large, it may be necessary to provide a differential-protection scheme for the transformer.
Differential-protection schemes compare the currents entering the primary windings with those leaving the secondary windings (after correction for current-level and phase changes) and any significant difference will operate the relay, which trips the circuit breakers controlling the transformer. Such protection will provide adequate sensitivity with high-speed tripping, and will not affect co-ordination of downstream protection.
It may be noted that the high-speed relays used for the REF, high-set and differential protection schemes are stabilized against false operation due to CT saturation (for example, when energizing the transformer). Overcurrent, REF, and high-set relays are commonly contained in a single relay case.