Voltage Regulation

1. VOLTAGE REGULATION 5 June 2014 By : Ernst de Villiers

2. Layout of Presentation Problem Statement Construction of the Voltage Regulator Basic theory of Voltage Regulation Implementation of Voltage Regulators Voltage Regulators Vs. Load Tap Changers Case Study – Commissioning 2

3. 3 Problem Statement An ideal electric power system would supply constant voltage at rated value for every piece of equipment. Power systems cannot deliver a constant voltage level due to line losses caused by the line impedance and an increase in load density. When voltage profile limits are exceeded, either performance or equipment life is sacrificed.

4. How Voltage drops are created MV distribution feeders are designed short enough and/or are loaded so that the voltage profile is kept within specific limits. 4 Medium length line model Properly-designed distribution feeder

5. Problem Statement To minimize the time and expense of serving the new load, the length of the feeder is usually extended. Hence, in most cases, the feeder develops an unacceptable voltage drop as shown in the figure below. 5 Increase of load density and feeder length results in an unacceptable voltage drop.

6. Solution – The Voltage Regulator 6 A Voltage Regulator can continuously monitor the output voltage and automatically adjust itself by changing taps until the desired voltage is obtained .

7. Operational Theory A voltage regulator holds line voltage within predetermined limits and assures the proper operation of connected loads To understand how a regulator operates, one must first understand how a two-winding transformer operates. 7 Transformer Model Ideal Transformer

8. These two independent windings can then be connected so that their voltages may aid or oppose one another. Thus, the output terminal voltage can either be measured as the sum of the two voltages or the difference between them. Therefore; transformer becomes a auto transformer with the ability to raise or lower the primary or system voltage 8 Operation Theory Step-down Trfr Step-up Trfr

9. 9 9 How does a Voltage Regulator work? A single-phase regulator is represented by the autotransformer in the figure below. Series Winding – Low voltage Winding Shunt Winding – High Voltage Winding Voltage Regulator Diagram

10. 10 How does a Voltage Regulator work? By reversing the polarity of the series winding, the autotransformer can boost (increase) or buck (decrease) the output voltage with respect to the input voltage Step-down Trfr Step-up Trfr Voltage Regulator Diagram

11. 11 How does a Voltage Regulator work? Polarity reversing is achieved via a reversing switch. An on-load tap-changer connects a variable number of turns of the series winding into the circuit, thereby allowing small increments of voltage change.

12. 2014/06/04 12 Schematic Diagram – 32 step Voltage Regulator

13. Non Bridging Equalizing Winding and Reactor The preventive autotransformer is required in order to smoothen the transient encountered when switching taps. Thus, decreasing the interrupted current as the tap changer taps. P reventing the regulator from being disconnected from the circuit each time the tap is changed. 13

14. Voltage Regulator Parts 14

15. Bushing Construction 15

16. Bushings The preferred bushing terminals are smooth, hot-dip tinned, copper cylindrical stems provided with bimetallic clamps that are suitable for accepting aluminum or copper conductors of 6 mm to 15 mm diameter . 16 BUSHING SPECIFICATION FOR POLEMOUNTED, SINGLE-PHASE, STEP , AUTOMATIC VOLTAGE REGULATORS

17. MOV Surge Arrester 17 At normal voltage the MOV disk is an insulator and will not conduct current. But at higher voltages caused by surges (faults/ lightning) , it becomes a conductor. Thus, the arrester protects the series winding from surges.

18. MOV Surge Arrester 11 kV Application 22 kV Application MCOV: > 2.5 kV >5 kV Residual voltage: Ures <10.5 kV <20 kV Rated discharge current 10 kA 10 kA Creepage 31mm/kV 31mm/kV 18 According to Specification 34-2110: The surge arrester shall comply with the following parameters:

19. Internal Parts – QD8 Quick Drive tap-changer 19

20. Reactors A bridging reactor is required to maintain continuity during a tap change and to provide impedance for limiting the amount of current to be interrupted by the tap changer. 20 Core Type Reactor Shell Type Reactor Core type is designed that the two coils are interlaced resulting in minimal impedance-to-load current flow and therefore minimal voltage drop.

21. CL6A Control Board 21

22. CL6A Control Board 22

23. Drag-Hand 23 The Yellow tap indicating needle drags minimum and maximum needles.

24. 24 How does a Voltage Regulator work? Type of Regulators Two types of regulators: Type A Type B step-voltage regulators are predominately used for distribution lines outside of the substation and lateral circuits.

25. 25 Type A Regulator The tapped series winding is located on the load side of the shunt winding, and by adjusting those taps the output load voltage changes.

26. 26 Type B Regulator The tapped series winding is located on the source side of the shunt (excitation) winding, and by adjusting those taps the output load voltage changes.

27. Regulator Ratio 27 Typical maximum boosts and bucks are ±10 %. What will a R and n t =N 2 /N 1 be in the maximum boost and buck positions?

28. Voltage Regulation P er S tep According to AVR Spec 34-2110 – Regulators will provide 10% regulation with 32 step (16 buck and 16 boost). Since 16 steps provides 10% boost (or buck), then a single step will result in a voltage change of 1/16 of 10%, which is :   Since “% voltage” is Per Unit voltage x 100, the Per Unit voltage change per step is So we obtain 0.00625 Per Unit voltage change per step . 28

29. The ratio (N 2 /N 1 ) effective is then given by the pu voltage change per step times the number of steps (Tap ): Note that if Tap is 0, then the effective turns ratio is 0. In this case, there is no boost, and V L =V S . 29 Voltage Regulation P er S tep

30. Line Drop Compensation (LDC) Allows a constant voltage to be maintained at a load centre remote from the regulator. 30

31. 2014/06/04 31 Wiring Diagram – CT Circuit Protection

32. How is it implemented? Two or three single-phase regulators banked together make it possible to regulate the voltage of a three-phase system. 32 Closed-Delta Open-Delta 3 Voltage Regulators 2 Voltage Regulators 15% Regulation of Input Voltage 10% Regulation of Input Voltage Mostly used in ECOU Cost effective

33. Connection Configuration Two or three single-phase regulators banked together make it possible to regulate the voltage of a three-phase system . Closed Delta Implementation Open Delta Implementation 33

34. Configuration 34 Open-Delta connection Closed-Delta connection

35. How ratings are selected Two parameters are necessary to elect a voltage regulator: 1) Rated Voltage 2) Rated Current Rated Voltage: It must be equal to or higher than the system nominal voltage. Rated Current: It must be equal to or higher than the maximum load current at the place that regulator is going to be installed. 35

36. Regulator Tests The AVR shall have been type tested in accordance with, and found to comply with, the following requirements of ANSI/IEEE C57.15. Impulse test; Temperature rise; Short-circuit tests – KA rating-3 seconds amps; Insulation testing – Megger 36

37. Where to Apply 1) On existing feeders with voltage drop problems This is a common application of voltage regulators. They are installed before the point that the voltage drop problem starts under heavy load conditions. 37

38. 38 Where to Apply 2) Important laterals from a main feeder can be effectively controlled with regulators.

39. 39 3) To serve a remotely located load These types of loads can be economically and quickly served by extending the existing feeder and installing voltage regulators to correct for the voltage drop in the extension. Where to Apply

40. Operating sequence to put a regulator bank on bypass – Spec 34-1436 Apply all regulators on neutral tap Check regulator incoming and outgoing links to be closed. Close regulator bypass links. Open regulator outgoing links. Open regulator incoming links. Test regulator to be dead. Apply relevant earths 40

41. Operating sequence to put a regulator bank back in service - Spec 34-1436 Remove all earths. Check regulator bypass links to be closed. Close regulator incoming links. Ensure that regulators is on neutral tap. Close regulator bank outgoing links. Open regulator bank bypass links. Put regulator on auto 41

42. 42 Voltage Regulators vs. Load Tap Changers The basic function of an electrical utility is to supply power to customers. With system losses and voltage sags that must be corrected by regulation in order to stay within the voltage limit. Even in a real world of ongoing maintenance and occasional component failure, objectives of minimizing outage time and limiting failures can be realized.

43. 3 Single phase Regulators Transformer + LTC One phase offline- replacement of only one regulator unit. Failure of one phase of the TC, entire unit removed – all phase effected. 43 Voltage Regulators vs. Load Tap Changers One spare regulator unit can be utilized on any of the phases if maintenance is required. Full maintenance can be completed without diminishing system service. Each Single-phase unit has an independent control to react to the voltage variances of the phase.

44. 44 Voltage Regulators vs. Load Tap Changers

45. Unbalanced phase voltages – single phase units can respond individually to these independent feeder models. By separating each phase into its own unit, every unit is made smaller and easier to handle. Bypass switching provides easy installation and the ability to remove a regulator without dropping the entire bus or feeder. 45 Voltage Regulators vs. Load Tap Changers

46. Cost Comparison 46 There are some choices to be made in how to regulate voltage; some of these choices can require significant capital expenditures. Study done by Cooper Power Systems Ref. Number R225-90-21

47. Prospect - Kaysers Beach V oltage Regulator 11kV Pine conductor line that interconnects with Prospect - Gately line via a normally open point situated at structure PRO-KBH-79. Approximately 2078 number of customers – 83km line. This feeder is at its voltage limit due to the normal growth from supply upgrades and new developments in the area. The acceptable voltage limit for this feeder is 95% of nominal. Currently, the minimum voltage is 95%, hence the initiation of this project. 47

48. Commissioning Test Sheet Procedure 34 – 1218 : Presents the Commissioning Check Sheet 48

49. Evaluation of losses in step-voltage regulators is increasingly a major consideration when evaluating competitive alternatives. . Some of the areas that should be evaluated in addition to the cost : Maintenance, Installation and Operation, Technical support, Dielectric strength, Short circuit strength, Losses 49 Regulator Characteristics

50. Thank you Open for Discussion