Single-Phase Instrument Transformers

Transcription

1 Single-Phase Instrument Transformers S T U D E N T M A N U A L March 10, 2005

2 2 STUDENT TRAINING MANUAL Prerequisites: Single-Phase Transformer Operating Principles module Objectives: Given a Revenue Metering Connection Diagrams manual, you will be able to select and connect a single-phase meter service using voltage and current instrument transformers. Rationale: Some services require the use of special metering using voltage and/or current instrument transformers. Improper connections can easily go unnoticed and cause dramatic errors in customer billing, resulting in loss of revenue or customer relations concerns. Learning Objectives Explain the construction and function of potential transformers. Explain the construction and function of current transformers. Explain the construction and function of single-phase instrument meters. Select and connect all components in a single-phase instrument connected service. Determine the billing multiplier for an instrument connected singlephase service. Calculate the consumption and cost of an instrument connected single-phase service. Learning Methods Self-learning + On-the-job Self-learning + On-the-job Self-learning + On-the-job On-the-Job Demonstration and Practice Self-learning + On-the-job Self-learning + On-the-job EVALUATION METHODS Written test

3 SINGLE-PHASE INSTRUMENT TRANSFORMERS 3 Written test Written test On-the-Job Evaluation Written test Written test STUDENT RESOURCES Learning Steps Electric Service Guide Revenue Metering Connection Diagrams 1. Read the Learning Guide. 2. Follow the steps outlined in the Learning Guide. 3. Clarify any questions or concerns you may have. 4. Complete the Practice and Feedback. 5. Complete the Evaluation. Introduction Instrument transformers are used when the load being metered is greater then the capacity of the self contained meter. They simply change the magnitudes of voltage and current to a level that can be safely metered. Two types of instrument transformers are: Potential transformers Current transformers

4 4 STUDENT TRAINING MANUAL Lesson 1: Potential Transformer Learning Objective:Explain the construction and function of potential transformers. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction A potential transformer is very similar to a standard distribution transformer, both in its construction and operation. It contains a primary and secondary coil and the ratio is such that it produces either 115V or 120V on the secondary winding. As with distribution transformers, the primary coil is connected either line to line or line to ground. The main difference between a distribution transformer and a metering potential transformer is that the latter has a very low capacity (rated in volt-amps) and is extremely accurate in its ratio of transformation.

5 SINGLE-PHASE INSTRUMENT TRANSFORMERS 5 Figure 1. Line to Line Connection

6 6 STUDENT TRAINING MANUAL Ratios Figure 2. Line to Ground Connection As with distribution transformers, the ratio of a potential transformer is obtained by dividing the voltage rating of the secondary winding into that of the primary winding. Example: A transformer with a voltage rating of 480V on the primary winding and 120V on the secondary winding would have the following ratio:

7 SINGLE-PHASE INSTRUMENT TRANSFORMERS 7 Reference For further information, refer to connection diagrams CD-136 and CD-237 in the Revenue Metering manual. Potential Transformer Selection (PT) Determine the conditions that require the use of a potential transformer and if needed, determine the proper size of potential transformer Conditions Requiring a Potential Transformer A potential transformer is required when a high voltage line must be reduced to utilize a low voltage meter. Size Selection of Potential Transformers To determine the proper PT size, the amount of voltage phase-to-phase and/or phase-to-ground must be checked (use of a voltmeter or prior knowledge of the circuit). The amount of voltage determines which metering diagram to select and which potential transformer is required. Check the PT nameplate for sizing and capacity ratings.

8 8 STUDENT TRAINING MANUAL Figure 3. PT Nameplate Figure 4. PT Nameplate

9 SINGLE-PHASE INSTRUMENT TRANSFORMERS 9 Lesson 2: Current Transformers Learning Objective:Explain the construction and function of current transformers. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction Current transformers are designed so the primary winding is in series with the line and is rated as to the amount of current that can be carried in the primary winding. The ratio of CTs are 100/5, 50/5, 20/5, etc. A rating of 100/5 indicates the transformer is fully loaded when 100 amps is flowing in the primary winding. With a full load on the primary, there will be 5 amps flowing on the secondary. Some types of current transformers are: 2-wire current transformer 3-wire current transformer open window type current transformer! CAUTION Current transformers must always be hooked in series. STOP DANGER If a 2-wire, self-contained meter was accidentally installed on a circuit with a current transformer, a dead short would result across the terminals, putting the operator at risk and damaging equipment.

10 10 STUDENT TRAINING MANUAL 2-Wire Current Transformer Figure 5. 2-Wire Current Transformer A 2-wire current transformer consists of one primary winding and one secondary winding. These transformers are normally used on 2-wire, single-phase circuits in conjunction with 2-wire meters. They are also used on all types of threephase circuits.

11 SINGLE-PHASE INSTRUMENT TRANSFORMERS 11 3-Wire Current Transformer Figure 6. 3-Wire Current Transformer A 3-wire current transformer consists of two primary windings and one secondary winding. These transformers are normally used on 3-wire, single-phase circuits in conjunction with 2-wire meters, but were also used as a past standard three-phase 4-wire delta configuration.

12 12 STUDENT TRAINING MANUAL Open Window Type Current Transformer Figure 7. Window Type CT This particular current transformer consists of a circular iron core with many secondary turns wrapped around it. It has no primary winding. This is provided by the phase conductor of the circuit being measured. The figure above illustrates the construction of this particular transformer. The ratio of this current transformer may be altered by varying the number of wires passing through the window of the transformer. The resulting ratio is not affected by the direction of the wires in the window, but the direction of the current in the conductor will affect the metering. Determining the Operating Ratio The operating ratio of a window type current transformer may be determined as follows: If a 200/5 amp current transformer has one conductor through its window, the operating ratio is 200/5 or 40/1, as shown on the nameplate. However, if two conductors are passed through the same 200/5 transformer, the new operating ratio is:

13 SINGLE-PHASE INSTRUMENT TRANSFORMERS 13 This means that with 100 amps in the conductor, there will be 5 amps in the secondary. Figure 8. Window CT and 2-Wire Circuit If the nameplate ratio of this transformer is 200/5 with 100 amps flowing on the primary, there will be 2.5 amps flowing on the secondary and through the meter coil. Example: Assume a 50 amp circuit has a 240 volt load (no 120 volt loads). Assume the connections are made as in the previous figure, and that the JKP_O current transformer has a nameplate ratio of 200/5. What is the secondary current in the meter coil?

14 14 STUDENT TRAINING MANUAL The above diagram shows the same transformer connected for 3-wire operation in a 3-wire, single-phase circuit. This is a special connection since it uses two conductors - one from each phase to make a 3-wire current transformer. Each conductor must pass through in opposite directions to ensure proper registration for this type of metering.

15 SINGLE-PHASE INSTRUMENT TRANSFORMERS 15 Follow the path of the current as it passes through the current transformer window twice in the same direction. Current Transformer Selection (CT) When using a current transformer, first determine the conditions present and determine the proper size of current transformer to use. Some conditions which require a current transformer include: A need to reduce line currents too large for a standard meter. To insulate the meter from high voltages on high voltage lines (above 480V). This is usually the reason why a current transformer with a 1:1 ratio is used. This makes it possible to avoid connecting ammeters or instrument current coils directly in high voltage lines. On circuits where the wires are too large to fit a standard meter terminal. Selecting the Size of a Current Transformer To determine the proper CT size, the amount of current flow in the two phases (ammeter check or prior knowledge) and the amount of voltage (phase-to-phase or phase-to-ground) must be known. For example, a service has 205 amps on one phase and 225 amps on the other phase. If the amperage is between a CT ratio of 200:5, but smaller than the next size of CT ratio 300:5, always select the higher size of CT ratio 300:5. Check the CT nameplate for proper sizing and capacity ratings. This will ensure the coils are not subject to unnecessary stress.

16 16 STUDENT TRAINING MANUAL Figure 9. CT Nameplate

17 SINGLE-PHASE INSTRUMENT TRANSFORMERS 17 Lesson 3: Instrument Meters Learning Objective:Explain the construction and function of single-phase instrument meters. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction Meter Selection The meter used with the instrument transformers on a single-phase service is always a 2-wire meter, regardless of whether the circuit being measured is a 2-wire or 3-wire circuit. These meters are a transformer type meter, with connections and ratings different from that of a standard, self contained, 2-wire meter. Instrument meter selection is determined by the voltage required to measure 120 volts or 240 volts. If a potential transformer is incorporated to reduce the voltage to 120 volts, a 120 volt transformer rated meter is required. On a 120/240V service, with a current transformer installed, a 240 volt 2-wire meter is required. Check the meter nameplate for size and capacity ratings. Connecting Instrument Transformers When connecting instrument transformers, the Revenue Metering manual is used to ensure the proper connections are made. Certain features and procedures unique to the installation of instrument transformers and meters should also be considered. Unique Features Instrument transformers possess built in safety features which include: Polarity markings Shorting links (bars) Wire color coding Polarity Markings Polarity markings identify the path the voltage or current flow takes

18 18 STUDENT TRAINING MANUAL through the primary coil into the instrument transformer s windings and out the secondary bushing or terminal. Instrument transformers are usually subtractive polarity, whereby voltage or current flow enters the H 1 primary bushing and exits the secondary X 1 bushing. A white dot near the primary H 1 bushing and near the secondary X 1 bushing indicates the correct path of voltage or current flow. In order to meter a service properly, it is imperative to adhere to the instrument transformer s polarity markings. Shorting Links (Bars) A current transformer must always have the secondary coil shortcircuited during installation or while in a circuit with the meter removed. CTs must not be energized with the secondary open circuited for the following reasons: The core may become magnetized, thereby altering the characteristics of the transformer. High voltage can occur in an open circuited secondary and may puncture the winding insulation. It can severely shock anyone coming into contact with the winding. Example: E = I x R (R is very high because of the open circuit)

19 SINGLE-PHASE INSTRUMENT TRANSFORMERS 19 Figure 10. Shorting Bar Some types of current transformers are equipped with shorting links (bars) on the secondary terminals and must be closed before the secondary leads are removed. On other types, the secondary is automatically short circuited when the terminal cover is removed. Sockets used on single-phase metering installations where current transformers are required have a built-in short circuiting device on the left hand side of the meter socket. This automatically short circuits the current transformer secondary circuit when the meter is removed. ---Note--- These sockets should not be used as an initial method of short circuiting the CT. They are a precautionary method only. Sockets used on three-phase installations require a test block switch.

20 20 STUDENT TRAINING MANUAL Wire Color Coding The Revenue Meter manual has codes on the wiring diagram with letters representing the color of wire to use (ie: R/BK refers to a red wire with a black stripe). This eliminates confusion when connecting and helps ensure proper metering connections. Installation Procedure When installing instrument transformers to meter a service, correct connections must be made to ensure proper metering. First determine the service voltage and peak load for the service. Meter wiring must be in accordance with the Revenue Metering manual.! CAUTION Always ensure the current transformer shorting links are short circuited until the meter has been installed and the circuit is complete. Once the metering connections have all been made and the meter has been installed, energize the service. Check the disc rotation to ensure the meter is turning in the direction indicated by the arrow. ---Note--- If the disc is rotating backwards, switch either the current leads or potential leads to correct the problem.

21 SINGLE-PHASE INSTRUMENT TRANSFORMERS 21 Lesson 4: Connecting Instrument Transformers Learning Objective:Select and connect all components in a single-phase instrument connected service. Learning Method:On-the-Job Demonstration and Practice Evaluation Method:On-the-Job Evaluation

22 22 STUDENT TRAINING MANUAL Skills Practice 1. Determine: 1. Service voltage size, phase-to-phase and/or phase-toground (prior knowledge or voltmeter reading) 2. Service current flow in each phase (prior knowledge or ammeter reading) 3. Correct revenue metering diagram 2. Select: 1. Correct instrument transformer sizes 2. Meter 3. Correct wire (color code) 3. Power disconnected: 1. Connect instrument transformer and meter according to the metering diagram 2. Open shorting links on CT 3. Install tamper proof seals 4. Energize the service 5. Perform meter check

23 SINGLE-PHASE INSTRUMENT TRANSFORMERS 23 Lesson 5: Billing Multiplier (BM) Learning Objective:Determine the billing multiplier for an instrument connected single-phase service. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction To calculate the cost and consumption of an instrument metered service, an operator must use instrument ratios. Multiply constants when using CTs and PTs. Calculate the cost of an instrument meter service. Reference For further information, refer to the Revenue Metering manual. The billing multiplier calculation is always illustrated at the bottom of the page. Multiplying Constants When current and potential transformers are used in a metering circuit, the consumption is multiplied by the billing multiplier to calculate the total consumption. The BM is calculated as follows: BM = CT ratio x PT ratio x inherent meter multiplier Example: Present reading = 3856 Previous reading = 3428 Potential transformer = 480/120 volt = 4:1 ratio Current transformer = 50/5 amp = 10:1 ratio

24 24 STUDENT TRAINING MANUAL Inherent meter multiplier = 1 BM = 10 x 4 x 1 BM = 40 Consumption = ( ) x BM Consumption = 428 x 40 Consumption = 17,120 kilowatt hours Although the two readings are usually taken one month apart, any interval of time may be used depending upon the rate given to the customer. The factor (CT ratio x PT ratio x inherent meter multiplier) in the previous equation is the multiplying constant in the entire metering installation and will be referred to as the overall multiplying constant. The term billing multiplier is more generally used.

25 SINGLE-PHASE INSTRUMENT TRANSFORMERS 25 Lesson 6: Calculating the Cost of an Instrument Meter Service Learning Objective:Calculate the consumption and cost of an instrument connected single-phase service. Learning Method:Self-learning + On-the-job Evaluation Method:Written test Introduction Once the billing multiplier has been determined, it can be applied to the consumption reading. The cost formula can be used to determine the cost (in dollars) for that particular service. Continuing with the previous example, the consumption of the meter is 428kWh and the demand reading is Calculate the cost of the service if the billing multiplier is 40 and the cost per kwh is 7 cents for the first 10,000kWh and 3 cents for the remaining consumption. The demand charge is $0.00 for the first 50kVA and $13.01 per kva on the remaining demand. Consumption = Read x BM Consumption = 428 x 40 Consumption = 17120kWh Demand = Read x BM Demand = 1600 x 40 Demand = 64000VA Demand = 64kVA Cost = Consumption x Rate Cost = (10,000 x.07) + (17,120-10,000) x.03 Cost = Cost = $913.60

26 26 STUDENT TRAINING MANUAL Demand Charge = Consumption x Rate Demand Charge = (64-50) x $13.01 Demand Charge = 14 x $13.01 Demand Charge = $ Total Cost = Consumption + Demand Total Cost = $ $ Total Cost = $

27 SINGLE-PHASE INSTRUMENT TRANSFORMERS 27 Summary To summarize this module, you have learned: The construction and function of potential transformers. The construction and function of current transformers. The construction and function of single-phase instrument meters. To select and connect all components in a single-phase instrument connected service. To determine the overall billing multiplier for an instrument connected single-phase service. To calculate the consumption and cost of an instrument connected single-phase service. Practice Feedback Review the lesson, ask any questions and complete the self-test. Evaluation When you are ready, complete the final test. You are expected to achieve 100%.

28 28 STUDENT TRAINING MANUAL Review Questions 1. A potential transformer is used to reduce: (a) Current. (b) Power. (c) Resistance. (d) Voltage. T / F 2. The primary side of a potential transformer can only be connected phase-to-ground. T / F 3. A potential transformer can be used on a 120/240 volt single-phase service. 4. A potential transformer has a primary winding rating of 2400 volts and a secondary winding rating of 120 volts. The PT ratio is: (a) 20:1. (b) 1:1. (c) 10:1. (d) 2400:1. 5. Types of current transformers are: (a) 2-wire and 3-wire. (b) 3-wire, 2-wire and open window. (c) 3-wire and open window. (d) Open window and 2-wire. 6. The operating ratio of a 600 amp service using an open window type current transformer with two cables running through it is: (a) 100:5. (b) 200:5. (c) 300:5. (d) 600:5.

29 SINGLE-PHASE INSTRUMENT TRANSFORMERS A 2-wire CT with 225 amps flowing in the primary winding has a ratio of 300:5. The amperage at the meter is: (a) 225 amps. (b) 5 amps. (c) 3.75 amps. (d) 4.25 amps. T / F 8. When selecting a meter, an operator should check the meter nameplate. 9. When making instrument transformer connections, an operator should be aware of: (a) Color wire coding, polarity markings and meter box size. (b) Polarity marks, shorting links and wire color coding. (c) Shorting links, size of meter box and polarity marks. (d) None of these 10. A single-phase service is metered using two 400/5 current transformers and a 600/120V potential transformer. The meter inherent multiplier is 1.5. The billing multiplier is: (a) 400. (b) 600. (c) 100. (d) A single-phase service contains a 480/120V PT, a 200/5 2-wire CT and a meter with an inherent multiplier of 2. The previous read was 4,328kWh and the present read is 4,386kWh. The billing multiplier is: (a) 480. (b) 200. (c) 320. (d) 2.

30 30 STUDENT TRAINING MANUAL 12. A single-phase service contains a 480/120V PT, a 200/5 2-wire CT and a meter with an inherent multiplier of 2. The previous read was 4,328kWh and the present read is 4,386kWh. The actual amount of kilowatts consumed is: (a) 18,560kWh. (b) 58kWh. (c) 96,000kWh. (d) 960kWh. 13. A single-phase service contains a 480/120V PT, a 200/5 2-wire CT and a meter with an inherent multiplier of 2. The previous read was 4,328kWh and the present read is 4,386kWh. If the cost per kwh is 3 cents, the kilowatt consumption cost is: (a) $ (b) $ (c) $ (d) $58.00.

31 SINGLE-PHASE INSTRUMENT TRANSFORMERS 31 Review Question Solutions 1. Voltage. 2. F 3. T 4. 20: wire, 2-wire and open window : amps. 8. T 9. Polarity marks, shorting links and wire color coding ,560kWh. 13. $