Current Transformers – Burden and Classification

Current Transformers occupy a vital part in the measurement and protection scheme of any electric installation.

Hence, it is imperative that the choice of CT is made with full knowledge of the application and the number of relays and meters which are to be connected to it. A current transformer with a wrong burden rating or a wrong accuracy class will seriously compromise the effectiveness of the measuring or protection system.

Calculating the burden of a Current Transformer.

The burden of a current transformer is indicated in its nameplate. The burden is rated in VA. such as 15VA or 25 VA. The rated VA indicates the load the transformer can take.

The current transformer is connected to a measuring instrument or a protective relay by means of wires. The burden on the current transformer is imposed by the connected device and the impedance of the connecting wires which connect it. The VA load of the device can be obtained from the datasheet provided by the manufacturer. The total burden is the sum of the burden of the connected devices and the resistance of the wires. The inductive component of the wire impedance is usually neglected as it is minimal.

The burden of a current transformer can increase over time as the resistance of the connecting wires may slightly increase due to age, change in temperature and loosening of connections. Hence, the current transformer should never be loaded to 100% of its capacity.

Classification of Current Transformers

Depending on their application, current transformers can be classified into measuring and protection current transformers.

Measuring transformers have high accuracy. However, they have a low saturation point. They are deliberately designed this way, so that the measuring instruments are not damaged by the high currents during a fault. During a fault, the measuring transformers get saturated and the output stays within the range of the measuring instruments.

Measuring transformers are classified into 0.1,0.3, 0.5, 1. The values indicate the percentage error at the rated primary current. Thus a 100/5 transformer with 0.3 accuracy will have a maximum error of 0.3 when a current of 100 A passes through the primary.

The current transformers used for protection have lesser accuracy as compared to measuring current transformers. They have a very high saturation limit. This is necessary as they need to continue sensing the current even at high fault values.

Protection Transformers are classified as 5P10,10P10, etc. The first letter in the notation indicates the maximum percentage error. The last number indicates the number of times the rated current. Thus a 5P10 transformer would indicate a maximum error of 5 % at 10 times the rated current.

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Petersen Coils - Principle and Application

Peterson coils are used to in ungrounded 3-phase grounding systems to limit the arcing currents during ground faults. The coil was first developed by W. Petersen in 1916.

Application:

When a phase to earth fault occurs in ungrounded 3 phase systems, the phase voltage of the faulty phase is reduced to the ground potential. This causes the phase voltage in the other two phases to rise by √3 times. This increase in voltage causes a charging current, Ic between the phase-to-earth capacitances. The current Ic, which increases to three times the normal capacitive charging current, needs to complete its circuit. This causes a series of restrikes at the fault locations known as arcing grounds. This can also lead to overvoltages in the system.

A Petersen coil consists of an iron-cored reactor connected at the star point of a three phase system. In the event of a fault, the capacitive charging current is neutralized by the current across the reactor which is equal in magnitude but 180 degrees out of phase. This compensates for the leading current drawn by the line capacitances. The power factor of the fault moves closer to unity. This facilitates the easy extinguishing of the arc as both the voltage and current have a similar zero-crossing.

I_C=3I=3Vp/(1/ωC) =3VpωC

Where IC is the resultant charging current that is three times the charging current of each phase to ground.

Consider a Petersen coil connected between the star-point and the ground with inductive reactance ωL, then

The current flowing through it is given by

I_L =Vp/ωL

To obtain an effective cancellation of the capacitive charging currents, I_L to be equal to I_C.

Therefore,

Vp/ωL=3VpωC

From which we get,

L=1/ (3ω²C)

The value of the inductance in the Petersen coil needs to match the value of the line capacitance which may vary as and when modifications in the transmission lines are carried out. Hence, the Petersen coil comes with a provision to vary the inductance.

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ANSI codes for Protection Functions

The ANSI(American National Standards Institute) has standardized the codes to be used for protection relays. Each protective function is indicated by a specific no. such as 50 for instantaneous overcurrent protection and 59 for overvoltage protection.

Following is the list of the functions. The codes are sometimes followed by an alphabet which gives some additional information for instance, the code 51G may indicate an overcurrent ground relay. 50N may indicate a ground sensitive overcurrent relay based on neutral current measurement. 87T may indicate that a differential relay may be used for Transformer protection.

1 - Master Element
2 - Time Delay Starting or Closing Relay
3 - Checking or Interlocking Relay
4 - Master Contactor
5 - Stopping Device
6 - Starting Circuit Breaker
7 - Anode Circuit Breaker
8 - Control Power Disconnecting Device
9 - Reversing Device
10 - Unit Sequence Switch
11 - Reserved for future application
12 - Overspeed Device
13 - Synchronous-speed Device
14 - Underspeed Device
15 - Speed - or Frequency, Matching Device
16 - Reserved for future application
17 - Shunting or Discharge Switch
18 - Accelerating or Decelerating Device
19 - Starting to Running Transition Contactor
20 - Electrically Operated Valve
21 - Distance Relay
22 - Equalizer Circuit Breaker
23 - Temperature Control Device
24 - Over-Excitation Relay (V/Hz)
25 - Synchronizing or Synchronism-Check Device
26 - Apparatus Thermal Device
27 - Undervoltage Relay
28 - Flame Detector
29 - Isolating Contactor
30 - Annunciator Relay
31 - Separate Excitation Device
32 - Directional Power Relay
33 - Position Switch
34 - Master Sequence Device
35 - Brush-Operating or Slip-Ring Short-Circuiting, Device
36 - Polarity or Polarizing Voltage Devices
37 - Undercurrent or Underpower Relay
38 - Bearing Protective Device
39 - Mechanical Conduction Monitor
40 - Field Relay
41 - Field Circuit Breaker
42 - Running Circuit Breaker
43 - Manual Transfer or Selector Device
44 - Unit Sequence Starting Relay
45 - Atmospheric Condition Monitor
46 - Reverse-phase or Phase-Balance Current Relay
47 - Phase-Sequence Voltage Relay
48 - Incomplete Sequence Relay
49 - Machine or Transformer, Thermal Relay
50 - Instantaneous Overcurrent or Rate of Rise, Relay 51 - AC Time Overcurrent Relay 52 - AC Circuit Breaker 53 - Exciter or DC Generator Relay 54 - High-Speed DC Circuit Breaker 55 - Power Factor Relay 56 - Field Application Relay 57 - Short-Circuiting or Grounding (Earthing) Device 58 - Rectification Failure Relay 59 - Overvoltage Relay 60 - Voltage or Current Balance Relay 61 - Machine Split Phase Current Balance 62 - Time-Delay Stopping or Opening Relay 63 - Pressure Switch 64 - Ground (Earth) Detector Relay 65 - Governor 66 - Notching or Jogging Device 67 - AC Directional Overcurrent Relay 68 - Blocking Relay 69 - Permissive Control Device 70 - Rheostat 71 - Level Switch 72 - DC Circuit Breaker 73 - Load-Resistor Contactor 74 - Alarm Relay 75 - Position Changing Mechanism 76 - DC Overcurrent Relay 77 - Pulse Transmitter 78 - Phase-Angle Measuring or Out-of-Step Protective Relay 79 - AC Reclosing Relay 80 - Flow Switch 81 - Frequency Relay 82 - DC Reclosing Relay 83 - Automatic Selective Control or Transfer Relay 84 - Operating Mechanism 85 - Carrier or Pilot-Wire Receiver Relay 86 - Lockout Relay 87 - Differential Protective Relay 88 - Auxiliary Motor or Motor Generator 89 - Line Switch 90 - Regulating Device 91 - Voltage Directional Relay 92 - Voltage and Power Directional Relay 93 - Field Changing Contactor 94 - Tripping or Trip-Free Relay 95 - Reluctance Torque Synchrocheck 96 - Autoloading Relay

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Fluke launches 430 Series Three-Phase Power Quality Analyzers

Fluke has launched their 430 series of three-phase power quality meters to help you locate, predict, prevent and troubleshoot problems in power distribution systems. The new IEC Class A standards for flicker and power quality are built in to enable precise monitoring

Applications

* Frontline troubleshooting – quickly diagnose problems on-screen to get your operation back online
* Predictive maintenance – detect and prevent power quality issues before they cause downtime
* Quality of service compliance – validate incoming power quality at the service entrance
* Long-term analysis – uncover hard-to-find or intermittent issues
* Load studies – verify electrical system capacity before adding loads
* Energy assessments – quantify energy consumption before and after improvements to justify energy saving devices

image and text courtesy: fluke.com

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