Functions of Instrument Transformers (CTs and PTs)

AC type protective relays are actuated by the current and voltage supplied by the current and potential (voltage) transformers which are generally classified as instrument transformers. Generally instrument transformers are used for mainly two purposes. For metering purpose which steps down and displays voltage and current levels from kV to (0-110 volts in case of PT) and few kilo amps to (0-5 amps in case of CT) . Second purpose is to supply current and voltage magnitudes to the relays to carryout protection functions.

The main functions of instrument transformers are:

• Instrument transformers (current and potential transformers) provide insulation against the high voltages of the power circuit and to protect the apparatus and the operating personnel from contact with the high voltages of the power circuits
• Instrument transformers (CTs and PTs) supply protective relays with current and voltages of magnitude proportional to those of the the power circuits. These current and voltage magnitudes supply by the instrument transformers are sufficiently reduced such that the relays can be made relatively small and inexpensive
• Instrument transformers helps in attaining different types of secondary connections to obtain the required current and voltages

For proper applications of CTs and PTs required considerations are:

Mechanical construction , type of insulation (dry or liquid), ratio in terms of primary and secondary currents or voltages, continuous thermal rating, short time thermal and mechanical ratings, insulation class, impulse level, service conditions, accuracy and connections

For the safety purpose, the secondaries of the current and potential transformers (CTs and PTs) are grounded.

Why Instrument Transformers Used and Advantages

Voltmeters and Ammeters are used to measure the voltage and current in the circuits. Using instrument transformers ranges of these devices to measure can be increased. A current transformer is bascially a step down transformer hence it steps down the current. When used in conjunction with the low range ammeter, a current transformer (CT) increases the range of the ammeter. Thus a 0-5A ammeter can be used to measure several hundreds or thousand amperes of current. Similarly a potential transformer (PT), which is basically a step down transformer can increase the range of the low voltage voltmeter. Thus a voltmeter designed to measure voltage upto 110V can measure a much higher voltages (several thousnd volts) when used in conjnction with suitable potential transformer

Instrument transformers have many advantages. Some of the advantages are given below:

Advantages:

• Single range ammeters and voltmeters can measure a wide range of currents and voltages, if used in conjunction with suitable Current Transformers (CTs) and Potential Transformers (PTs)
• The measuring instruments like ammeter, voltmeter and wattmeters etc are incorporated in the secondary circuit and hence they are totally segregated from the high voltage, thereby ensuring safety for the operator and observer
• The meter need not be insulated for high voltages which would be the case if they are directly included in a high voltage circuit
• Using current transformer with suitable split and hinged core, it easy to measure heavy currents in the busbarwithout having to break the conductor carrying current. The core of the Current Transformer (CT) is opened at the hinge, the current carrying conductor is introduced in the center of the core through a opening made and the core is tightly closed again. The conductor itself acts as a single turn primary winding of the current transformer

WHAT IS MBUS? AND HOW IS IT USED IN METERING SYSTEMS?

August 23, 2021

Necessity is the mother of invention, they say but in the case of MBus, it was frustration that drove Professor Dr. Horst Ziegler of the University of Paderborn, who, in cooperation with Texas Instruments Deutschland GmbH and Techem GmbH. Developed the Meter-Bus or M-Bus protocol. 

WHAT IS AN M-BUS OR METER BUS?

The M-Bus concept was based on the ISO-OSI Reference Model, in order to realize an open system that could utilize almost any desired protocol. Today, M-Bus is ideal for sub-metering because it simplifies the communication between smart metering devices and the gateway or Data Collection Unit in the building allowing for the frequent collection of meter readings for energy management, or utility billing. This meter reading technology created in Europe is gaining popularity in North America with a wide variety of applications when it comes to submetering.

Let us take a closer look at what M-Bus is all about.

It is a serial communication system that uses a single pair of conductors that can tie up to 250 slaves, in this case, meters: to a single master, gateway, or in our case a Data Collection Unit. Every meter is then connected to the bus. The M-Bus string is exceptionally insensitive to external interference that may be caused by capacitive or inductive coupling. It can power the communication module on the meter, while electrically isolating it from other external sources, and it is a very cost-effective method for wiring in buildings, particularly when compared to Pulse-Output meters.
In addition to the savings in the initial installation costs, servicing the system is also easier, as any addition of a meter can be done without the need for reconfiguration of a daisy chain and there is further simplification because M-Bus incorporates protection against reversed polarity.

ROLE OF MBUS IN THE METERING SYSTEM

Metering systems, in particular, those that meter flow, like water or Thermal Energy (heat)meters, use M-Bus to transmit energy consumption via a field bus (the Meter Bus) to a Master.
The communication is simple because the size of the information to transmit is small it can be transmitted up to one kilometer between the meter and the Data Collection Unit, provided the M-Bus voltage is maintained, once in operation; more than one meter can simultaneously and accessible on one common platform, making data available for energy-efficiency purposes.

HOW MBUS IMPROVES METERING SYSTEM PERFORMANCE

M-bus systems have come a long way from manual reading methods where an employee from the metering company would have to read the data in hard-to-reach areas of the building - A previous time-consuming method that was often measured at long intervals. With M-Bus one, many or all the meters can be interrogated (polled) at any one time, or periodically, this is advantageous when tenants or occupants move in the middle of a billing period, and a partial period reading of the meter is required.
M-Bus can be programmed for multiple Baud rates from 300, to 38,400 Bauds, although most operators will settle with 9,600 Bauds.

MBUS PROVIDES FAST AND ACCURATE REMOTE DATA

Mbus reads measurements electronically at shorter intervals, allowing landlords to have a more accurate view of tenant consumptions. The ability of MBus technology to read data faster from virtually anywhere eliminates manual reading errors made by field personnel.

OFFERS THE POSSIBILITY OF EXPANSION

While each master has a maximum of 250 slaves, there is no theoretical limit on the number of masters that can be installed this provides added benefits for future troubleshooting, for instance, in a large apartment building the water metering for Domestic Cold Water can be segmented from the Domestic Hot Water meters, allowing for additions to each of these systems at a later date. Another option is to retrofit and add another bus for another utility, like for Thermal Energy metering without disturbing the existing buses, and integrating all into a single reporting platform.

GET RELIABLE ELECTRONIC READINGS

Compared to pulse-output meters, M-Bus is more accurate because it transmits the value of the actual read from the meter, and not the pulse count, avoiding using multipliers for conversion, which ensures the availability precise data. Additionally, the M-Bus wire-runs present higher immunity to noise and electromagnetic interference that may affect pulse transmission through non-shielded wiring.

SAVES COSTS

Unlike pulse output meters the MBus wiring is simpler and uses less cable.  

* Above document is sourced from our supplier Intellimeter

December 07, 2018