Digital Clamp Meter: A more versatile Measuring Instrument

Measurement of Current..

Yeah! the usual meter that comes in mind for current measurement is the ammeter. These meters have to be connected in the circuit to get the reading. Thus, for current measurement using ammeter, we have to disturb the circuit and put the meter in between. Also the prior knowledge of the magnitude of circuit current and it’s nature is necessary. One should also know the proper method and have the means to disconnect and reconnect the circuit.

Clamp-On ammeter is a very handy device which can measure the current flowing through any LT circuit without disturbing the existing connections.

Clamp meter:

A clamp meter is a more versatile instrument that combines a clamp-on ammeter with a multi-meter. It is usually capable of measuring alternating current, AC/DC voltage, and resistance. Many of these meters are able to test the capacitors, measure frequency, temperature, continuity etc. Continuity is a quick test to check an open circuit. When the circuit is closed, the meter emits a “beep” sound. 

The display can be analog or digital; accordingly are called analog or digital clamp meters. Digital meters have several advantages over analog ones and thus are commonly used. Figure 1 shows a digital AC/DC clamp meter.

Fig.1: A digital AC/DC clamp meter

Clamp meters have a movable jaw that can be opened. Open jaw allows the clamp meter to be clamped around a current carrying wire to measure the current flowing through the circuit. Figure 2 shows the digital clamp meter with open jaws.

Fig.2: Digital clamp meter with open jaws.

Working of DC clamp meter:

Conventional clamp meters, capable of measuring alternating current only, do not show the reading when used to measure DC current. DC clamp meters work on the principle of “Hall Effect”. The magnetic field because of DC current produces a small voltage across the Hall Effect sensor. This voltage, which is proportional to the DC current flow, is further amplified for measurement.

DC clamp meters or AC/DC clamp meters are very useful in Solar PV system installation and maintenance, since the output current of a solar PV plant is DC in nature. 

Accessories such as pair of red and black probes for voltage measurement and thermocouple with leads (clamp meters with temperature measurement facility) are provided. Figure 3 shows the thermocople with leads. Figure 4 shows how the thermocouple leads are connected to the clamp meter (also shows the temperature).

Fig.3: Thermocople with leads

Fig.4: Thermocouple leads connected to the clamp meter

The digital AC/DC clamp meter is a battery operated (9V) device. When the battery is low, a “low- battery” indication appears on the display. The “Auto power off” feature automatically turns off the meter when no operation is detected for 15 minutes, thus helps to extend the battery life.   

Advance Features of Clamp Meter:

Several digital clamp meters offer a wide variety of advanced features which ultimately help an electrical engineer to a great extent. These features are:

1.  PC Interface – Clamp meters also come with PC interfacing serial ports to facilitate data transfer to computer for further analysis of data and report generation.

2. Data Logger – Some clamp meters have internal memory capable of holding the measured values to be used later on.

3. Inrush Current measurement – This feature allows the clamp meter to measure the high inrush current usually encountered during motor starting.

4. Autoranging – Automatically sets appropriate measurement range thus, avoids manual selection of range.

Safety Requirements and Standards:

The AC/DC clamp meters are designed accordingly to safety standard IEC61010-1 and IEC 61010-2-032 to meet the safety requirements for electrical measuring instruments and hand-held meters. They also comply the European standards 89/336/EEC for Electromagnetic Compatibility and 73/23/EEC for low voltage  

Precautions:

A few things should be followed so as to ensure that these meters last long in the harsh environment encountered in the field. The checks to be carried out are –

1.  Keep the meters in their safety cover or carry bag after the use to avoid ingress of moisture, dust etc, and shocks and damage.

2.  Do not store these meters in a strong magnetic field to avoid loss of accuracy. Electrical noise, unwanted signals or intense electromagnetic fields in the vicinity may disturb the measuring circuit.

3.  All meters are very fragile and hence should be handled with care.

4. Do not expose the meters to extreme hot or cold temperature and moist atmosphere.

5. Always select the proper range and nature of circuit i.e. AC or DC.

6. Always inspect the meter and test leads for any sign of damage or abnormality, such as broken or cracked, leads before use.

Ancillary services for Indian electric market

In a vertically integrated power system, one organization carried out all the work of electricity generation, transmission and distribution. Example is the erstwhile State Electricity Boards in India. Power industry throughout the world is undergoing the restructuring process and is adopting the deregulated structure for better utilization of resources and for providing quality services to its consumers at an economical price. 

The introduction of deregulation has brought in several new entities and some form of competitive electricity market. The vertical integrated utilities were separated into Generating companies (Gencos), Transmission Companies (Transco’s) and Distribution Companies (Discos). Apart from these three entities,  entities such as System Operators, Market Operators, Regulators etc were also formed. 

Central Electricity Regulatory Commission (CERC) Regulations 2010 defines Ancillary services as those services that are necessary to support the power system operation in maintaining power quality, reliability and security of the system or the grid. Examples are active power support for real time load following, reactive power support, black start, voltage control support etc.

In the deregulated environment, ancillary services are no longer treated as integral part of the electric supply. The system operators have to purchase these services from ancillary service providers. The CERC Regulation, 2010 made operation of ancillary services as an exclusive function of Regional Load Despatch Centre (RLDC). In the Central Advisory Committee meeting of CERC, held on 14^th March 2012, there was a general consensus on the need for introducing ancillary services to enhance the secure and reliable operation of Indian power system.     

Main ancillary services under consideration in India:

Currently three main types of ancillary services are under consideration in India and they are:

1.      Real power support service or Frequency support ancillary service or Load following,

2.      Voltage or Reactive power support services, and

3.      Black Start support service.

Real power support service or Frequency support ancillary service or Load following:

In India, the real power support or the frequency support services are to be provided by generating station or any other authorized entity on behalf of the generating station. The services/capacity of the said station should be available for despatch as required by the nodal agency i.e. RLDC to support the system frequency. 

Considering the Indian power system as a whole there are cases when certain surplus energy, particularly from Captive power plants, lie un-utilized at some point of time at some location. At the same time some portion of the system is facing power shortage and carrying out load shedding to cope up with the situation. 

Therefore, there is an urgent need for service mechanism such as Frequency Support to properly utilize these unused capacities to enhance the Grid security. Initially generators having surplus generation, either because of un-requisitioned quantum by the beneficiaries or quantum of generation which could not find buyers or captive generation capacity, may bid for the frequency support when their services are sought upon by the system operator. 

Frequency support services can also help to mitigate the intermittent nature of renewable energy sources such as wind and solar farms. At this juncture it is worth mentioning that Indian government has set some very ambitious targets towards renewable energy generation particularly from wind and solar.     

Eligibility Criterion and Implementation

All the producers and the regional entities which are members of the scheduling and deviation settlement mechanism for real and reactive power are entitled to participate in the ancillary market, but the condition is that they must have relevant telemetry facilities. 

No other No Objection Certificate (NOC) or clearance, other than that issued by the respective RLDC/SLDC for participation in the Day-ahead Market (DAM), is required. The Frequency Support Ancillary Services (FSAS) are to be implemented through bidding in the power exchange. The participants are free to bid in any of the Power Exchanges of the country for ancillary services. The bids for FSAS to be dispatched on next day are to be opened up after the closure and clearance of the day-ahead market. Time-block-wise bids with the quantum, price and the location (where services are to be given) are to be submitted for the next day despatch.  

Reactive Power Ancillary Services

Unlike the frequency, voltage is a local phenomenon, and hence the requirement for reactive power support from capacitors or reactors at the different sub-stations or generating stations may vary. Thus, reactive power ancillary services are also in the list of ancillary services in India. Although there is a provision in the IEGC regulations and according to which proper voltage profile is to be maintained at all interchange points between the control areas in the Indian power grid.

However this is not sufficient and there is an urgent need for reactive power support to be added in the ancillary services list. Whenever there is persistent low voltage profile at one or more points in the system (this may be frequent during agricultural season), the system operator should have the voltage support ancillary tool in his armory. 

The voltage support ancillary service provider may also bid through the power exchange. Since the reactive power and hence the voltage support is a local phenomenon and their requirement are different at different locations, therefore mobile reactive power supports on trailers etc. can be very useful in the Indian power system. It is supposed that initially the government transmission companies will provide the mobile reactive power support and further opening up of the scenario will depend on their outcome.      

Black Start Ancillary Services

Black start is the process of restoring a power station to operation without relying on the external electric power from the transmission network. Normally the electric power required by a plant for its operation is provided by its own generators. If all the generators of a generating plant are out of service then to restore the generation, the station requires the electric supply externally, drawn from the transmission line connected to the station. In the event of power outage in a wide area, the transmission lines are also out of supply. In the absence of transmission line or grid, the so called Black Start facility is required.   

The Central Electricity Authority (CEA) Regulations 2007 mandates the Hydel power stations in the country to provide the Black Start facility. There is a provision of incentives to such service providers who offer their services (black start services) when asked by the system operator. Black Start Ancillary Services (BSAS) are to be paid when it is required by the SLDC i.e. the nodal agency. The generators capable of providing the start up power are to be paid for one day capacity charges for their services. The quantum of energy supplied during the restoration process is paid (energy prices are paid the BSAS provider) at twice the energy charges as determined by the Commission.  

Ref.: “Introduction of Ancillary services in Indian Electricity Market”, Central Electricity Regulatory Commission, New Delhi, April 2013