Transformer permissible loading while Motor Starting 1

[NickParker]

During the largest motor starting along with the other loads running, the transformer gets overloaded for a short period of time.

Say, if the voltage drop at the Switchgear is within the limits, still the transformer is overloaded for a short period.

Is it safe to run the transformer in this condition? What is the permissible short time overload current (percentage) & time duration as per IEC & IEE?

I would greatly appreciate if some one advice me with the code requirements and percentage short time overload allowed.

 

[RRaghunath]

Voltage drop of 20% is typically allowable at the busbars during starting of large motors.
Since the starting lasts a few seconds only, it is not a concern from transformer thermal withstand point of view (The thermal time constant of oil filled transformer is in minutes)

 

[7anoter4]

NEC Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts (as a Percentage of Transformer-Rated Current): 300% breaker rated current
IEEE C57.12.01 or IEEE Std C57.96 for dry type transformer
IEEE Standard C57.12.90 Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers
IEC 60076-12 for dry type transformer Annex B for calculating lifetime consuming
IEC 60354 for oil immersed transformer

 

[crshears]

My utility has some industrial customers supplied @ 14 kV that have very large [ 30,000 HP at full load ] motors [ driving cryogenic liquid oxygen production machines ] that require starting from time to time. The transformers supplying this yard are 75 MVA MCR each, and both must be in service for motor starts.

When a start is required, we place the transformer tapchangers on manual and raise the busbar voltage to ~14.5 kV, and leave the t/c's on manual so they will not operate during the motor start, as if the t/c contacts were to open at this time they would almost certainly fail explosively.

Once the customer advises the start has been completed, the t/c's are returned to automatic voltage control.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[rcw retired EE]

No problem for the transformer, but you should check the overcurrent settings on the secondary main breaker. The primary side protection is typically set high enough (125%-250%) to carry a motor start. The secondary protection short time function might trip with the combination of full running load current and the 600% of FLA starting current. The instantaneous protection is probably high enough or turned off.

 

[cuky2000]

If the transformer is sized to maintain a transient voltage dip during motor starting above 85% of the nominal system voltage, it is safe to run the transformer. This can be achieve with the following simplified formula kVA_SC /( kVA_LR + kVA_SC)≥ 0.85 pu
_{where : V(pu) is the actual system voltage in per unit of nominal; kVALR is the motor locked rotor kVA;
and kVASC is the system short-circuit kVA at the motor terminals.}

The transformer's thermal loading beyond nameplate in the US and ANSI marketplace follows the IEEE/ANSI Standard C57. 91 & 92 as primarily guide while Europe and other countries that follow the IEC standard 354 & 905 for the same purpose. This covers the transformer overloads typically in hours.

Transformer transient overload is not a major problem in motor starting. The main issue is excessive voltage drop due to the motor inrush that can cause interaction problems particularly sags that trip loads. A TCC plot of damage curve of a transformer, feeder and protective device superimposed in the motor stating curve could help to confirm this.
It should be noted that for a large motor it is advisable to perform a motor-starting study to include Overcurrent Coordination Setting as guidelines by the NEC in article 110.10 associated with motor starting.

 

[prc]

Standards for overloading of transformers are IEC 60076 ed2.0-2018/ IEEE C57.91-2011 for oil filled transformers IEC 60076-12-2008/ C57.96-2013 for dry type transformers. These standards give permissible continuous and short time overload limits for various types of transformers. As example IEC recommends current limits during normal cyclic overloading/ long time emergency overloading/ short time emergency overloading of 1.5/1.8/2.0 pu for small transformers and 1.3/1.3/1.5 pu for large transformers with a winding hot spot limitation of 120/140/160 C. But remember gas bubbling will start above hot spot of 140C resulting serious risk for insulation.

As Raghunath &Wilson pointed out, motor starting current duration is small (like inrush current) and hence 6-8 pu overload is not an issue ( as winding thermal constant is 6-10 minutes) Of course such overloads do put a mechanical stress on the windings.