FIELD OF THE INVENTION
The present invention relates to a method of diagnosing a fault in a current channel of a poly-phase microcontroller based electrical energy meter. In particular it relates to diagnostics of a very common failure mode, specifically hard to detect otherwise, in a poly-phase microcontroller based electrical energy meter.
BACKGROUND OF THE INVENTION
In a microcontroHer based poly-phase electrical energy meter a current measurement circuit consists of a current transformer (CT) whose secondary windings are connected to the inputs of an analog / digital converter for the respective currents. The current induced in the secondary is linearly proportional to the current in the primary windings and hence by use of a linear scaling factor, the primary current is calculated. The circuit may have the following failure modes:
The CT's secondary windings may get open due to dry soldering, or loose connections.
♦ The CT's core may get damaged, either due to breakages or saturation due to continual usage or high frequency noise or higher than rated currents applied to it.
As explained above, the current circuit can have some failure modes that can go undetected in a poly-phase system for quite some time. If out of the three current channels, one fails, the total energy recorded would be 66%, assuming balanced loads, of the actual energy consumption. The same loss will accrue if the primary winding falls for some reason, but that is easily detected because the load currents are routed through the primary, hence causing load disconnection. However, a secondary failure is nastily difficult to detect. The usual method used for detection is the use of a fourth current measurement element.
The current outputs from the three current measuring elements, CT's are given to the inputs of current measurement ADC which is shorted internally to the output side. The three CT's outputs are shorted to a common point and given as an input to a comparator. The return path from the current comparator then acts as the return path for the CTs secondary.
On the other hand the return currents from the load side are routed through the neutral CT, the outputs of which are fed to the second input channel of the comparator. If the system works correctly, the common output from the CTs secondary windings represents the vector sum of all three currents which must be equal to the return current passing through the neutral CT. This will cause the comparator to give zero outputs which can then be detected by a microcontroller.
In the event of the CT secondary opening or core saturation, it would be apparent that the vector sum of the currents would not match the neutral currents hence causing the comparator to give a finite voltage as output which indicates that one of the current channels is faulty.
This technique though is effective in its operation, suffers from a few shortcomings:-
♦ While it is trivial to determine the failure of a current channel, it cannot be determined exactly which one has failed.
♦ The neutral CT, even if not actively used for measurement purpose, has to be exactly matched and must have the same specifications as the current measuring CTs. In case there is an error in impedance matching, the comparator will give a finite output even if everything is working correctly.
♦ The neutral CT used for the detection suffers from the same failure modes that it is supposed to detect.
♦ The addition of a fourth CT and the associated comparison and amplification circuitry adds substantially to the cost of the energy meter.
♦ There was therefore, a need for detecting the failure of a current channel in a polyphase microcontroller based electrical energy / meter by pin pointing the channel which has failed and also the failure mode.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide an innovative way to detect CT failures which can pinpoint the failed element as well as the failure mode.
In Order to achieve this and other objects of the invention, the current measurement CTs are slightly modified by adding another primary to it. This additional primary is driven by the microcontroller itself. This microcontroller can generate a signal which is measured on the secondary side by the ADC and the ADC's output is given to the microcontroller. If the circuit is working correctly, a specific value of current is read by the microcontroller in response to the generated signal.
In a preferred embodiment the present invention provides a method carried out by a micro controller for diagnosing a fault in a current channel of a polyphase microcontroller based electrical energy meter, comprising the steps of: reading the output of a channel ADC periodically for determining that the load has entered the quiescent state; generating a pulse train in the quiescent state; passing said pulse train through primary of a CT of the current channel, which generates a current in a secondary connected to the ADC for measurement; and detecting a specific value of current in response to the generated pulse train, thereby indicating trouble free operation of the current circuit.
In another preferred embodiment the present invention provides a system for diagnosing a fault in a current channel of a polyphase electrical energy meter each current channel of said system comprising: a current measurement ADC; a current transformer (CT) with a secondary winding connected to each channel of said current measurement ADC for determining the respective currents and its primary connected to a load; wherein said current transformer is provided with a second primary winding driven by a micro controller, said microcontroller for generating a signal measured on the secondary side by the ADC with the ADC output connected to said microconroller to determine if the circuit is working properly by reading a specific value of the current in response to the generated signal.
BREIF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the figures of the accompanying drawings in which
Figure 1 shows block diagram of a poly phase electronic energy meter.
Figure 2 shows use of a fourth CT as a current measurement element.
Figure 3 shows the quiescent state current circuit diagnostics of the present invention.
As illustrated in Figure 3 the quiescent state current circuit diagnostics is a method to effectively isolate and pin-point the faulty current circuitry. The exact modus operandi is as under:
The microcontroller reads the ADC outputs periodically every few milliseconds in course of its normal operation. In case of a load being present, the ADC's digital output is linearty scaled in a preset proportion to calculate the value of primary current. The diagnostic circuit is passive in this state.
Now, if the microcontroller determines that the ADC's digital signal are as low as the estimated noise floor for some time, it concludes that the primary current is absent and the load has entered a quiescent state. In this state, the microcontroller generates a pulse train as an input to the base of the transistor Q1. Q1, being an NPN transistor is triggered at low level of the pulse train and an emitter to collector current limited by the resistance Rl flows through the auxiliary primary winding. The primary current thus generated causes a secondary current which is connected to the ADCs for measurement.
If the microcontroller detects a specific value of current in response to the generated pulse train, the current circuit is deemed to be trouble-free. However if the current is not detected, it concludes that the CT secondary is open. If the current is less than the expected values, the CT might be unusable due to saturation or breakages.
This process is repeated periodically during the no-load conditions to continually monitor the state of the current circuit.
This technique has the following advantages over the use of the fourth element:
♦ The faulty current circuit can be pinpointed and isolated.
♦ Minimal circuitry is required.
♦ The solution is cost effective and light weight.

WE CLAIM
1. A method carried out by a micro controller for diagnosing a fault in a current channel of a polyphase microcontroller based electrical energy meter, comprising the steps of:
- reading the output of a channel ADC periodically for determining that the load has entered the quiescent state;
- generating a pulse train in the quiescent state;
- passing said pulse train through primary of a CT of the current channel, which generates a current in a secondary connected to the ADC for measurement; and
- detecting a specific value of current in response to the generated pulse train, thereby indicating trouble free operation of the current circuit.
2. The method as claimed in claim 1, wherein the outputs of each ADCs is linearly scaled when a load is present, for calculating the value of the primary current.
3. The method as claimed in claims 1 or 2, wherein the city may be found unusable due to saturation or breakage, when a specific value of current in response to the generated pulse train is less than the expected value.
4. The method as claimed in the preceding claims, wherein the sate of the current channel is continuously monitored by repeating the steps of claim 1 periodically during no-load conditions.
5. A system for diagnosing a fault in a current channel of a polyphase electrical energy meter each current channel of said system comprising:

- a current measurement ADC;
- a current transformer (CT) with a secondary winding connected to each channel of said current measurement ADC for determining the respective currents and its primary connected to a load;
- wherein said current transformer is provided with a second primary winding driven by a micro controller, said microcontroller for generating a signal measured on the secondary side by the ADC with the ADC output connected to said microconroller to determine if the circuit is working properly by reading a specific value of the current in response to the generated signal.
6. The system as claimed in claim 5, wherein an NPN transistor is provided which can be triggered at low level of the generated pulse train causing the secondary current to be measured by the ADC.
7. A method carried out by a micro controller for diagnosing a rauh in a current channel of a polyphase microcontroller based electrical energy meter, substantially as herein described and illustrated in the figures of the accompanying drawings.