I wish to present here the benefits of utilization and operation of MV/LV parallel transformers in electrical distribution. Having such experience, i will be interested to get feedbacks from other engineers.
2 MV/LV parallel transformers distribution network
For all my experience, I’ve never met a working electrical installation, where operation of parallel transformers was used. Once, we used this solution in the design of welding shop, mainly to reduce the voltage drop in the network and maintain a stable level of short-circuit current. But the project was stopped by the Employer, and the idea was gone. Below is the list of benefits of such solution, as it is seen. It would be great if interested engineers commented and shared experiences on this issue.
Generally, application of parallel transformers allows achieving the following benefits:
1) Reducing the total capacity of electrical transformers (as compared to separate their work). The decrease of total installed capacity is reached:
- by lowering the overall demand load to the diversity of loads connected to different transformers
- by using a higher load rate of parallel transformers
- less required backup in case of electrical transformer failure
2) Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded transformers
3) Improving the power quality due to the stable level of short circuit current throughout the network
4) Increasing the reliability of operation of protective devices in the case of phase-to-earth short circuits in the network.
5) Possibility of placing electrical transformers in operation phase-by-phase
Parallel transformers are allowed, provided that none of the windings will be loaded by current exceeding allowable current for that winding.
Of course, there are limitations for using electrical transformers connected in parallel. For instance, Russian Federation “Rules for technical maintenance of electrical consumers” allows operation of parallel transformers under the following conditions:
- windings connections of transformers have the same vector groups
- the ratio of transformers capacities of less than 1.3
- rates of transformation differ by no more than +/-0.5%
- short circuit voltages do not vary more than +/-10%
- transformers have same phase polarity
Artem Kropachev
Question regarding possible elimination/reduction of the disadvantiges of circulatig currents, load sharing etc.:
Will the adding of inductors on the secondary side of each transformer help. It’s obvious this cause higher cost but in spite this disadvantage, will it help?
Kind regards,
Thomas Rooth
Disadvantage of circulating currents is increasing of losses. Load sharing is connected with different short circuit impedance (mainly reactance because R<<X) characteristics of transformers. In most cases transformer impedance has the biggest value of short circuit impedance at lowest position and the smallest at highest postiton. Characteristic of short circuit impedance is not straight line but curve which is mostly close to straight line. It should be noted that in some OLTC and winding construction there could be large steps between two position while other has smooth step.
Parallel operation methods for parallel operation of transformers imply circulating currents minimization so circulating currents (which could not be fully removed due to different factors) are at low values and thus circulating currents increase losses at very low level.
Of course increasing overal inductance (reactance) using inductors (transformer reactance+inductor reactance) will effect in circulating current reduction. Load sharing could be equalized if added inductors equalize overall impedance (reactance) among transformers.
Inductors built in secondary side of transformers could help to decrease short circuit currents in places where it is necessary.
My opinion is that there is no need to add inductors on secondary side of transformers regarding to parallel operation of similar transformers (circulating currents and load sharing) because of inductors costs, increasing maintenance cost, decreasing reliability (more devices), need for new parametrisation of protection devices and automatic voltage regulators, etc).
Best regards, Sinisa Spremic