Technical Articles
Ensuring continuity of supply is critical in certain installations like Hospitals, Data Centres, Process industries, etc. One way of ensuring this is with Transformer Redundancy along with back-up Diesel Generator sets. Transformer Redundancy ensures power supply is not disturbed in case of any one Transformer failure / planned shutdown. In this method, multiple transformers are inter-connected in parallel with Bus Couplers. Loads are distributed among all transformers and if there is any failure/planned shutdown in one of the transformers, the loads are transferred to other transformers with the help of Bus Couplers.
The industry is following certain methodologies in APFC panels and the sensing scheme for the APFC controllers, so as to maintain desired Power Factor, in spite of any combination of transformers. This article will brief on the various methods and suggest the most optimum method that simplifies sensing logic and optimizes APFC panel sizes.
A typical configuration of transformer redundancy is shown in below diagram 1. This shows a Bus Coupler (B/C) between two set of loads, incomer circuit breakers (B1 for Transformer-1 & B2 for Transformer-2), CT inputs for APFC 1 under Transformer 1 and APFC 2 under Transformer 2. Let us consider load under each transformer be 1000 kW at load PF 0.8. Desired Power Factor of the entire system in the HT side / metering point shall be maintained in any of the three possible conditions – (i) each transformer supplying to corresponding loads with B/C open, (ii) Transformer 1 supplying to both set of loads with B/C closed or (iii) Transformer 2 supplying to both set of loads with B/C closed.
Method 1:
To achieve PF in the above first condition, 750 kVAr APFC panel would be sufficient under each transformer. But during the conditions (ii) & (iii), either one of the CTs will be sensing and only the corresponding APFC panel would be operational. The sizing of APFC panels need to be made in such a way that each APFC compensates for both set of loads (total 2000 kW at 0.8 PF). Hence 1500 kVAr APFC under each transformer is must against 750 kVAr, as any one APFC panel may need to compensate for total 2000 kW load. This is because, with one transformer OFF, CT sensing to corresponding APFC panel would be unavailable and hence that APFC panel would be non-operational.
The possible scenarios are summarised in below table:
| Method 1 | Method 2 |
|---|---|
| Situation | Description |
| B1 | |
The drawback in this method is size of APFC panels required would be twice the actual required rating. So the user pays more, just for simplifying the sensing & control logic. Utilisation of APFC panels capacity will be poor during normal operating conditions.
Method 2:
There is a possibility of avoiding the higher size of APFC panels if sensing logic is modified. In the below diagram 2, with the addition of new CT-B/C at Bus coupler and summation CTs to each APFC panel will make the possibility of sensing load current separately for each set of loads. In below example - for individual load of 1000 kW at 0.8 PF, each APFC panel of 750 kVAr is installed under each transformer.
By utilising Summation CTs and Bus Coupler CT sensing inputs to both APFC relays are connected as below:
It can be noted in the above wiring diagram that polarity of CT-B/C connection in Summation CT-1 (SCT-1) is reversed (S1 & S2 connections are reversed). Whereas CT-B/C connection in Summation CT-2 (SCT-1) is in normal polarity (+ve). Hence below would be the output of summation CTs:
Sensing current (thro summation CT) to APFC-1 = CT-1 – CT B/C
Sensing current (thro summation CT) to APFC-2 = CT-2 + CT B/C
In the all three scenarios, APFC-1 would compensate for Load-1 and APFC-2 would compensate for Load-2.
The drawback of this method is additional CT at bus coupler and two sets of summation CTs. The connection logic is also relatively complicated and sometimes difficult to troubleshoot later.
The advancement in APFC controller technology can help to avoid the complex CT connections shown in above method to achieve Transformer redundancy, at the same time with optimized APFC panel ratings (750 kVAr each). These latest APFC controllers can communicate between each other and can operate even without summation CTs and Bus coupler CT. Such connection logic is shown in below diagram 3.
Here, APFC controllers have additional terminals (A, B, RT and GRD). Interconnections need to be made between each controller as shown in above diagram. Additionally, Normally Open (NO) contact from Bus Coupler need to be used between Pin A of both APFC controllers. These connections will enable/disable communication between two APFC controllers. Once the communication is enabled (NO close) they will operate in MASTER-SLAVE mode. CT connections are given to corresponding APFC panels only.
Below are the operation logics at different situations:
In this method of using intelligent APFC controllers, it can be noted that APFC panel ratings are optimized (2 x 750 kVAr instead of 2 x 1500 kVAr as against Method-1). At the same time the control & sensing circuit is simplified in a significant way (as against Method-2), in addition the extra B/C CT and Summation CTs can also be avoided. This method of using intelligent APFC controller will help in optimizing initial investment in APFC panels, along with simplified sensing circuit, connections & future troubleshooting will be easier.
These intelligent etaULTRA APFC controllers also come with loads of other advanced features like cascading of multiple APFC panels for capacity enhancement & maximize step numbers (up to 64 nos), Dual PF setting for EB & DG, HT/MV side sensing for Transformer compensation, alarm for many sets of parameters (harmonics, temperature, PF, etc). Maintaining PF is very important now-a-days as many states have moved to kVAh billing and some states do not give incentive to leading PF. It is wise to make use of such intelligent APFC controllers in all critical systems to maintain healthy PF in-spite of complicated scenario like transformer redundancy.
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