Energy metering system

A. CURRENT TRANSFORMER:

Introduction
A current transformer is a device used to scale down large values of current (for example 100A) to smaller values of 1A or 5A, that can be easily manipulated for the purpose of system protections or metering.
In system protections, the current transformer plays a vital role as it measures current levels under fault conditions and channel these values to the relay for processing. Thus, the CT transformer serves as the eyes of the relay. Depending on the fault current levels, the relay then command the circuit breaker to trip, isolating the faulty portion of the installation from the rest.
In metering systems, the current transformer also plays an important role as it scales down currents of larger magnitudes to smaller magnitude of either 1A or 5A, and then channels this current value to the energy meter for measurements.The energy meter performs an energy calculation base on the current and voltage values it receives within a particular time frame.
With the above analysis, we can conclude that there exist basically two classes of current transformers which are;

The above listed classes of current transformers often poses some confusions as they both literally plays the same principal role (scaling down larger values of currents to smaller values). Perhaps a measurement currrent transformer rated 100/5A will send out a 5A current through it secondary terminal when 100A current passes across its primary, likewise a protection current transformer. The only difference here depends on the loads connected at the receiving ends of the current transformers.The manner in which a relay responses to a signal from a current trasformer is completely different from that of an energy meter. Below we'll discuss some of the reasons why these transformers shouldn't be used for any purpose other from that which they're made for.
  1. Protection Current Transformers
    2. The purpose of a protection class current transformer is to measure current during fault condition, and gives input to the relay. Based on the current input, the relay then decides for the actuation of trip signal to a Circuit Breaker to isolate the healthy section of power system from the faulty section. This simply means that, the main requirement for a protection class CT is that it should measure the current accurately even when the primary current is of the order of 10 or 20 times of the rated current. This fact will be more evident if you look at the accuracy class of protection class CT. It is generally 5P20 or 5P30. This means that the CT will maintain its accuracy within ±5% when the current through it is 20 or 30 times its rated current.
    Another important design feature of a protection class CT is that its core does not get saturated even under the condition of a fault. This necessitates for the higher value of Knee Point Voltage. Therefore, such CTs can measure fault current accurately without getting its core saturated.
    Considering the above unique features, if the protection class CT is connected to a meter then under normal condition, the measurement of meter will not be accurate as desired for a revenue meter. Furthermore, under the condition of fault, the secondary current of the current transformer will be very high, around 20 to 30 times its rated current. This results in a flow of high value of current through the meter. Since meters are not designed for such a high value of current (10 to 20 times of rated current), this will damage the meter.
  2. Measurement Current Transformers
    3. A metering current transformer (CT) is characterized by its ability to measure current accurately (as per its accuracy class) when the primary current is in the range of 20% to 120% of the rated current. Since the secondary of metering CT is connected to various types of meters, an Instrument Safety Factor (the ratio of saturation currrent to the rated current of the CT) is also designated for metering CT. The main purpose of designating ISF is to protect meters connected in the secondary of metering CT during fault condition. Consider a measurement current transformer with an ISF of 2. This means that, if the CT primary current is more than 2 times the rated current, the CT core will saturate to limit the current through the meter connected to its secondary terminals. Any futher increase in primary current doesn't affect the secondary current. Thus, the meter is protected from high value of current under fault condition.
    Let us now assume that this metering CT secondary is connected to protection relay. Under fault condition, the CT core will saturate and hence limit its secondary current. Under such condition, the relay may not operate as the current channeled to it by the CT might still be lower than the threshold value. Thus, the power system stability will be compromised.
    From the above discussion, it is clear that protection class CT should be used for protection purposes and metering CT should be used for metering purposes.
Criteria for the selection of current transformers for measurement

In the selection of measurement CTs, the following criteria should be respected;
  1. Determine the load line current of the system and then use it to determins the primary current of the CT.The load current must be greater than 5A.
  2. The primary nominal rating of the CT should be the closest value to 125% of the nominal current of the load. This is to enable the current transformer measure a current range of 20% to 120% of load nominal current.
  3. The burden VA of the CT should be the closest value to 125% of the burden of the total load. An error burden of 0.5VA (burden of the energy meter) must be added to the overall burden of the load before selection of CTs.
  4. CTs with double terminals labeled 1S1, 1S2 used for measurements and 2S1, 2S2, used for protections, are only applicable for medium voltage systems.
  5. Current Transformers with single terminals are used for low voltage systems.
  6. 5A CT should be used for systems with distances of not more than 10m, while for distances above 10m, 1A CTs should be used. This is done in order to reduce the losses due to the resistance (which strongly depends on the length of the copper cables) of the copper cables leaving the secondary terminals of the CT to the energy meter.
Sizing of current transformers for energy meters
The signal plate of an a.c generator has the following characteristics:
To select suitable CTs for the metering system of this generator, we proceed as follows:
  1. Firstly, we need to determine the maximum current that the generator can generate under normal working conditons. Here, we can use the installed capacity of the generator, or we could use the nominal power of the generator and include a safety factor of 125%. Below, we're using the installed capacity of the generator.
    2. I= S/(√3U)
    =(638×103)/(√3×400)
    I=921A
  2. Secondly, we determine the primary current of our current transformer. This is done by looking at the current transformer ratings and selecting the most closest one so as to improve measurement accuracy. So, we're going to use a current transformer with primary current of 1000A.
  3. Now, we determine the secondary current of our current transformmer. To do that, first we need to know the distance that will exist between our energy meter and the CTs by proceeding as follows:
    4. - Determine a suitable position to install the CTs (they could be installed on the power cable leaving the alternator directly in the generator's carbinate or in a separate carbinate housing the power cables.
    - We the position of the CT determined, identify a suitable position for the mounting of the energy meter and its accessories.
    - Measure the distance between the point where the CTs will be mounted and the point where the energy meter will be mounted and note down the value obtained.
    - If the measured distance above is greater than 10m, then our secondary current should be 1A. And if the reverse is true, then our secondary current should be 5A.
  4. Assuming the measured distance is less than 10m, then the selected current transformers for our generator are rated 1000/5A.
  5. Again, we determing the burden (VA) and accuracy class (Cl) of our CTs while taking into consideration a meter burden of 0.5VA and burden of cables. Most low voltage measurement CTs have burdens ranging from 1,5VA right up to 15VA, and accuracy class ranging from 0,2 right up to 1. The burden and the class varries depending on the primary current of the CT, the material from which the core of the CT is made, the insulation of the CT, and perhaps the size of the CT. Here, we're going to choose a burden of 15VA and a class of 0,5 for our CT.
  6. Finally, the suitable CTs for our generator's metering system are rated 1000/5A, 15VA-cl:0,5.
End