On Load Tap Changing

1. On Load Tap Changing Transformer Paralleling Simulation and Control

2. OLTC Overview • Transformer Paralleling • The need for control • Current Solutions • Our Plan and System

3. Parallel Transformers • Increase Reliability • Improve Power quality • Prevent voltage sag • Meet increased load requirements

4. Examples • Illustrate the need for control • Present Two Calculation Methods • Superposition Method • Admittance Method

5. Grainger Examples One-Line Diagram Grainger, Example 2.13, pg 78

6. Grainger Examples Per-Phase Reactance Diagram, Grainger pg 78

7. Superposition Method

8. Superposition Method

9. Superposition Method Equivalent Circuit

10. Superposition Method

11. Admittance Method Grainger, Example 9.7

12. Admittance Method

13. Problem Definition • We want to minimize the circulating current. • Why? • Increased total losses of the two transformers • Unable to fully load one transformer without over-loading or under-loading the other • This current is parasitic, serving no benefit • The transformer is not operating at optimum

14. Project Objectives • Build and test an experimental system • Measure the circulating current • Build a mathematical model of the system • Design a control scheme that utilizes SEL technology • Refine the System to minimize circulating current over a variety of conditions

15. Popular Solution Methods. • Master- Follower Method • Power Factor Method • Circulating Current Method • Var Balancing (∆Var) Method TM Source: Advanced Transformer Paralleling Jauch, E. Tom: Manager of Application Engineering, Beckwith Electric Co., Inc.

16. Master-Follower • Desired operation maintains same tap level on all transformers • Consists of one control commanding transformer tap changes to follow

17. Master-Follower • Positives: • Appropriate voltage level via load is maintained • Negatives: • Does nothing to prevent circulating current

18. Power Factor (PF) Method • Desired tap positions provide equal PF • Done by comparing angle of currents • Does not operate controls, Just prevents them from operating in the wrong direction.

19. Power Factor (PF) Method • Positives: • Keeps PF in desired range. • Negatives: • Difficult to apply to more than 2 parallel transformers. • If VAr flow, tap level changed is blocked to minimize PF difference. • If transformers have different impedances, Highest KW loaded transformer is forced to have highest VAr load.

20. Circulating Current Method • Assumes continuous circulating current path • Controls are biased to minimize Icirc. • Higher tap lowered, as lower tap increased the same amount to make equivalent tap level. • Relay used to block operation if tap level variation becomes to great.

21. Circulating Current Method • Positives: • Icirc is put to a minimum • Initial voltage level maintained • Max difference in tap levels maintained • Negatives: • Auxiliary CT’s are required • Flow of KW can not be fixed by changing taps • This causes oscillation of tap levels.

22. Var Balancing (∆Var) Method • Loads transformers by balanced VAr sharing. • Ignores KW loading

23. Var Balancing (∆Var) Method • Positives: • Balanced VArs make Icirc a min or 0 • No auxiliary CT’s are needed • Negatives: • Method is patented by Beckwith Electric Co. INC.

24. Our Plan • SEL 3378 SVP assumes control of system • Provided with phasors from the relay • SVP calculates optimal tap levels • SVP directs tap changers through SEL 487E relay

25. Our Plan • Goals • Appropriate voltage level maintained • Icirc driven to a minimum • Max variation of tap levels met • Avoids tap level oscillation

26. System • Transformers • 487E Relay • 3378 Synchrophasor Vector Processor

27. Transformers • Two Autotransformers will be used to simulate two parallel power transformers • Voltage controlled motors on the tap changers • Transformer secondary will feed an external load from unity to 0.5 lead/lag

28. Transformers • Superior Electric Type 60M21 • Single Phase • Input Voltage: 120V • Output Voltage: 0V-140V • KVA: 0.7 • Toroidal Core • Synchronous Motor • 120VAC, 60Hz, 0.3A, 3.32 RPM

29. Transformers • Short Circuit Tests • The resistance of the tap contact is larger than the reactance of the winding • The MVA imbalance of the parallel combination is expected to be dominantly Watts, rather than Vars • Verified through no-load Paralleling test

30. T1 X and R Vs Secondary Nominal Voltage

31. Transformers • The autotransformers do not exhibit characteristics similar to a typical power transformer • Options • Use these transformers • Different Transformers, 5 kVA Motor driven autotransformers

32. Calculations • The Superposition method will support the real component while the Admittance method will not • The real component will create a negative resistance in the PI equivalent

33. 487E Relay • Uses Lateral Logic • 18 Current Channels • 6 Voltage Channels • Synchrophasor data collected once per cycle, up to 12 Channels

34. 487E Relay • Control transformer tap level • Receives commands from SVP • Displays: voltages, currents, Icirc, apparent power, real power, reactive power.

35. 3378 SVP The SVP time aligns synchrophasor messages, processes them with a programmable logic engine, and sends controls to external devices to perform user defined actions. -SEL 3378 data sheet

36. 3378 SVP • Interface with the 487E Relay via serial connection. • Phasor input to calculate circulating current. • Control output to relay to minimize circulating current. • Display output with real-time circulating current values.

38. Conclusion Proper transformer control results in • reduced losses • increased profits • maximized quality and reliability