In this article I‘ll cover basic working principles of automatic transfer scheme (ATS). I‘ll describe ATS for industrial power network.
Power supplying scheme is shown in picture:

Here we have two 6 kV busbars. To each busbars are connected two asynchronous motors (M1,M2), two synchronous motors (M3, M4) and two power trasformers 6/0,4 kV (F1, F2). Each busbar have their own voltage transformers (VT1, VT2) and two incoming feeders (IC1, IC2: incoming cubicles). Via sectional cubicle SC busbar 1 can be connected to busbar 2 and vice versa. Here SCD is sectional disconnector.

I‘m going to install PLCs REF54x on cubicles in the manner as shown in picture:

On voltage transformers 1, 2 cubicles are installed PLCs REF541, on incoming and sectional cubicles – REF543. What is diference between REF541 and REF543? These PLCs are absolutelly identical equipment, both have same functions, the only one difference – the number of binary inputs and outputs. PLC RE543 has more binary inputs and outputs than PLC REF541.

More about PLCs REF54x you can read in manufacturer‘s ABB manual Feeder Terminal REF54_ Technical Reference Manual General 1MRS750527-MUM.

Principles schemes of incoming and sectional cubicles are bigger: there are connected more external circuits (to binary inputs and outputs) to PLC, so this schemes need PLCs with more binary inputs and outputs.

ATS algorithm
On busbar 1 drops voltage U1 (below 0,5xUn) or frequency f1 (below 48 Hz). This dropping measures PLC REF541 of voltage transformer VT1. At the same instant PLC REF541 of voltage tranformer VT1 receives data from PLC REF541 of voltage transformer VT2 (from busbar2) about voltage U2 and frequency f2 of busbar 2. If voltage U2 and frequency f2 of busbar 2 are normal then PLC REF541 of voltage transformer VT1 send signal „ATS start“ to PLC REF543 of incoming cubicle IC1. PLC REF543 checks if circuit breaker of incoming cubicle IC1 is closed and protections of IC1 aren‘t active. If these conditions are fulfilled then PLC REF543 of IC1 checks signal „ATS ready“. This signal receives PLC REF543 of IC1 from PLC REF543 of sectional cubicle SC. „ATS ready“ signal is active when: 1.Sectional disconnector SCD is closed. 2. Truck of circuit breaker SC is in work position. 3. Circuit breaker of sectional cubicle SC is opened. 4. Spring of circuit breaker of sectional cubile SC is charged. 5. ATS switch is closed. If „ATS ready“ signal is active, PLC REF543 of incoming cubicle IC1 is checking the position of circuit breaker of synchronous motor M1. If circuit breaker of synchronous motor M1 connected to busbar 1 is opened, then PLC REF543 of IC1 sends command to open circuit breaker of incoming cubicle IC1. This signal calls „opening from ATS“. When circuit breaker of IC1 will be opened, PLC REF543 of IC1 sends another command to close circuit breaker of sectional cubicle SC.

The same algorithm is for busbar 2, when there will be dropping of voltage U2 or frequency f2.

Under Frequency Load Shedding (UFLS) and undervoltage protections are using for opening synchronous motors.
Setting of UFLS:
f<<=48 Hz, t=0,5 s

Undervoltage protection is using for opening asynchronous motors.

Setting of U<:
U<=0,5xUn, t=0,5 s

Setting of frequency ATS:
f< =49 Hz, t=0,2 s, kai Umin=0,3xUn

Setting of voltage ATS:
Umin=0,5xUn, t=1s

Structure scheme of ATS algorithm:

You can easier understand ATS working principle by exploring this structure scheme of ATS algorithm.

Connections between PLCs REF54x: when there are dropping voltage or frequency on busbar 1:

Connections between PLCs REF54x: when there are dropping voltage or frequency on busbar 2:

ATS basic principles are covered.
In the next article we are going to configure PLC REF541 of voltage transformer VT1.