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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 connec...

Saturday, 11 February 2017

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.


Friday, 3 February 2017

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 14th 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

Monday, 30 January 2017

Role of Simulation Softwares in Solar PV plant Design and Operation

The performance of a Solar PV plant depends on the solar irradiation, the optimum tilt angle, ambient temperature, design parameters, quality of modules, efficiency of inverters etc. Earlier the expected generation output, losses in cables & equipments, voltage drop, lower and upper range of array voltage etc. were manually calculated. Now-a-days this tendency is fast being replaced by advanced simulation software, particularly in the case of solar PV plants with significant capacity.

Function of Simulation Softwares in plant Design and Operation:

Softwares are used as essential tools in the design, construction, operation and maintenance of Solar PV plants. They help to produce solar PV generation assessments, and site-specific production forecast accounting the location latitude and longitude, weather condition, shadow effect etc. These softwares lend a hand to optimize design configurations and system layouts, test the system performance. Economic evaluation and payback period calculation are also facilitated by these softwares. They also make possible the integration of PV plant output to the Utility grid, monitoring and control of the plant health, thus enhancing the operation of the PV plant.

Popular softwares:

There are several softwares available for assisting the solar PV system designing. The popular are:

1.       PVSYST
2.       PVSOL
3.       Solar Advisor Model (SAM)
4.       SMA off-grid Configurator
5.       Solar Pro
6.       PV Design Pro
7.       HELIOS 3D

PVSYST is one of the oldest and trusted PV designing software developed by the University of Geneva. The software offers a wide range of features, a few are –

1.Complete designing of off-grid and grid connected solar PV system.
2.Complete data base of solar modules and inverters.
3.Meteorological data of a large number of stations and sites across the world. Data of new sites can also be imported.
4.Access to PVGIS and NASA database for import of irradiation data.
5.Enables economic evaluation and payback period.
6.Predicts and asses the module behaviour under varying irradiation, ambient temperature and shading scenarios.

PVSYST software has two modes:
1.       Preliminary mode, and
2.       Project design mode.


In the preliminary mode one can get the approximate value of power generation. Project design mode allows user defined values for inverter efficiency, losses and shading effect. The results of these softwares are in the form of report, tables and graphs. The software is available in a trial version with one month access period. Cost of the software depends on its version and the number of design to be carried out per month.     

 Free Softwares:

Some free to download softwares such as PVGIS Estimation Utility, SMA Sunny Design etc are also available which can be used in the design of solar PV system.

PVGIS Estimation Utility is one such software which is freely available. With the help of this software one can calculate the output of a proposed solar PV plant, optimize the design and can have a fair idea about how the PV system is going to perform when completed.

Another free software is “SMA Sunny Design” by renowned solar inverter manufacturer SMA. The software generates detailed reports in “pdf” format of the proposed solar PV plant. The typical input required are the number and type of PV module to be used, location details, roof pitch etc. The software is able to compare the performance of PV panels of different types, makes and models. Although the software has the database of most of the solar PV panels, but one can have the option of feeding his/her own PV module details.

How the Software help out in plant Operation:

After the successful commissioning of a Solar PV plant, the next important activities are monitoring and operation & maintenance of the plant. There are softwares that help in carrying out these works also. The software can indentify under-performing plant and arrays, and thus help in increasing the power production. Incoming data from the PV modules, instruments to monitor the ambient, inverters, transformers and grid interfacing equipments are analyzed. Smart and advanced sensors and data loggers collect this information that affect the PV plant’s performance, and allow the software to create real time performance models. Variances are detected, and notifications and alarm messages are given to the concerned agency/ person.

Thus these softwares can create custom analytics, alerts, contracts and reports. They also helps in streamlining the operation & maintenance activities, and hence reduces Mean Time to Repair (MTTR). They also help in forecasting the power generation of the PV plant based on the weather forecast.  


 One such software is the Qantum Solar Monitoring and O&M software.

Friday, 27 January 2017

Digital Multi-Meters: A very handy tool for Electrical Engineers

An electrical engineer and electrician must be familiar with a variety of test and measurement instruments. They need to know how to use these instruments in a safe and proper way under different working conditions. It is important to use the right tools and instruments when working with electricity. Appropriate equipments can make a hard task easy and at the same time wrong selection can make an easy job difficult and dangerous.

This article looks at the most common type of meter available with an electrical engineer or electrician; i.e. the Multi-meter. 

Another versatile and handy instrument for an electrical engineer, particularly in the Solar PV field, is the digital clamp meter

Multi-meters are hand-held instruments designed to measure more than one electrical value; for example DC and AC voltages, DC and AC current, resistance etc. They are also used to check the continuity of conductors or to check the diode. Figure 1 shows the icon of some of the electrical quantities measured by a multi-meter.



Fig.1: Icon of some of the electrical quantities measured by a multi-meter.

In other words, a multi-meter is a measuring instrument that combines an ammeter, a voltmeter and an ohmmeter to measure the current, voltage and resistance in a circuit; means no need to carry different meters for measuring different values.

Digital Multi-meter:

Both analog and digital multi-meters are available, but the digital version has superseded the analog one. Digital Multi-meters (DMM) has a digital display to show the measured values, a dial or selector switch to change the mode of the meter, and connecting terminals. Figure 2 shows a low cost commonly available DMM.



Fig. 2: Low cost commonly available DMM.

Accessories such as a pair of red and black probes, is also provided with the meters. The black probe is always considered as the negative probe and is connected to the common terminal. Figure 3 shows the probe provided with the DMM.




Fig. 3: Probe provided with the DMM.

System with Harmonics:

When harmonics is present in the system, only true RMS meters can provide accurate measurement   of AC values. Harmonics are frequencies that are multiples of the fundamental frequency (50 Hz in India) and can cause distortion of basic AC waveform. The common loads which causes harmonics in the system are computers, fluorescent luminaires with electronic ballast, inverters etc. These electrical loads are also called Non-linear loads as their impedance are not constant.

Working of Digital Multi-Meter:

The basic circuit of a Digital Multi-Meter is always a DC voltmeter. The AC quantity is rectified into DC with the help of rectifiers and filters. All the quantities are digitised using an Analog to Digital Converter (ADC) and displayed in the digital form on the display.

Figure 4 shows the internal circuit of a commonly available Digital Multi-Meter.
Fig.4: Internal circuit of a commonly available Digital Multi-Meter.

Advantages of Digital Multi-Meter:

Digital Multi-Meters have the following advantages over analog:
1.       Cheap and compact,
2.       Accuracy is very high,
3.       No loading effect,
4.       Reading of values is much easier,
5.       Can be interfaced with external equipment,

Safety:

“Always read and follow the manufacturer’s instructions that are supplied with the Multi-meter. Equipments may be damaged badly and or the user may be seriously injured if proper safety procedures and instructions are not followed.”
Safety is very important when using test and measurement instruments. Many a times, the meter is used to make the measurement while the electrical circuit is energized. Ensure to follow the regular and standard safety procedures when working on live circuits. A safe working practice is always mandatory. The following safety tips are recommended-

1.       Ensure the correct position of selector switch,
2.       Do not be alone when working on live circuits,
3.       Keep your hands dry when taking measurements,

4.  Use rubber gloves and wear shoes when taking measurements on energized circuits.