F O R M 2

THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)

1. Title of the invention : COLD LOAD PICKUP PROTECTION SYSTEM FOR PROTECTION OF FEEDERS AND A METHOD THEREOF

2. Applicant(s):

(a) NAME : LARSEN & TOUBRO LIMITED

(b) NATIONALITY : An Indian Company.

(c) ADDRESS : L & T House, Ballard Estate, Mumbai 400 001, State of Maharashtra, India

3. PREAMBLE TO THE DESCRIPTION

The following specification particularly describes the invention and the manner in which it is to be performed:
TECHNICAL FIELD OF THE INVENTION

This invention relates generally to a system and method for power system protection and more particularly to a cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements and a method thereof.

BACKGROUND AND THE PRIOR ART

In a power system, protection relay plays an important role for proper and uninterrupted functioning of the healthy section of the system. The main objective of the protection relay is detection and tripping/blocking of the breaker, depending upon the application, in order to isolate the faulty section of the system from the healthy part.

A cold load pickup protection system provides protection to the power system in cold load condition. The term “cold load pickup” refers to the increased level of currents due to restoration of power after long interruption. Usually when a distribution circuit is restored after an extended outage, the demand is greater than before the outage and attempting to pick up this load can be problematic because the initial load demand after an outage can exceed the load demand that would have been observed at any time before the outage. This cold load condition can be caused due to a prolonged outage of the load, opening of the circuit breaker, or loss of supply even if the breaker remains closed. After the load is energized the current level shoots up instantly to about 5-6 times the normal current level due to magnetizing inrush currents in transformers, accelerating currents in motors and enduring currents due to loss of normal load diversity.

The factors which determine the magnitude and duration of cold load pickup are outage duration; types of connected load, weather, restoration mode, outage causes, the presence of distributed generation and/or automatic transfer schemes, time of day, and load level. Since in most cases cold load pickup current is greater than pre-outage current; cold load pickup affects fault detection. That is, protective relays can misinterpret the cold load pickup condition as a fault and initiate de-energization of a non faulty circuit. Cold load pickup current can be high enough to cause instantaneous overcurrent and/or time overcurrent relays to operate. So to prevent this cold load condition is set.

Some of the prior arts in the field of present invention are as follows:

US3109961A relates to an automatic circuit recloser and more particularly it relates to cold load pickup means for controlling the operation of the recloser after it has been restored to its closed position following lock-out. It provides an automatic recloser comprising means for opening said recloser in response to an overcurrent, reclosing means for automatically reclosing said recloser following an opening operation, sequencing means advanceable in a step by step manner from a normal position through a series of intermediate positions to a lock-out position in response to closely successive operations of said recloser, said reclosing means being inoperable to automatically reclose said recloser when said sequencing means is in said lock-out position.

WO2009154778 provides methods of controlling activation of electrical appliances include reducing overlapping activation time of different electrical appliances located at a single customer location of an electrical service provider during cold load pickup as a default condition upon determination of restoration of the electrical power to the single customer location subsequent to a failure of the electrical power beyond the single customer. Other embodiment of the present invention provides a computer program product for controlling activation of electrical appliances can include computer readable program code that is configured to reduce overlapping activation time of different electrical appliances located at a single customer location of an electrical service provider as a default condition upon determination of restoration of the electrical power to the single customer location subsequent to a failure of the electrical power beyond the single customer location. In some embodiments according to the invention, a method of controlling activation of electrical appliances can include reducing overlapping activation time of different electrical appliances located at a single customer location of an electrical service provider as a default condition during cold load pickup.

US4362986(A) provides a method of monitoring an electric power system and recording faults for subsequent analysis comprising the steps of (a) monitoring voltage and current at a selected point in said system, (b) identifying voltage and current above preselected values, (c) recording voltage and current values at a plurality of sampling rates for a predetermined period of time subsequent to voltage and current exceeding said preselected values, (d) recording voltage and current values at a plurality of sampling rates for a predetermined period of time subsequent to the re-establishment of voltage and current after a fault interruption thereof, and (d) recording voltage and current values at a plurality of sampling rates and for predetermined periods of times subsequent to current exceeding a zero level following a preselected period of zero level current. In this selected point in an electric power system is monitored for subsequent fault analysis. The voltage and current at the point is continuously sampled with samples for a predetermined period of time being temporarily stored in memory. In response to a Fault Event, voltage and current samples at differing sample rates and for differing periods of time are recorded. Voltage and current samples after a Cold Load Pickup are recorded, also the recorded samples allow later analysis of the fault and identification of the cause of the fault.

The prior arts fail to provide a simple and accurate method for pickup protection. The present invention therefore provides a simple and accurate method of protection. It provides a cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements.

OBJECTS OF THE INVENTION

A basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.

Another object of the present invention is to provide a cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements.

Another object of the present invention is to provide a method of cold load pickup protection for protection of feeders after load restoration by changing the operation of overcurrent elements.

Yet another object of the present invention is to provide cold load pickup protection using a numerical relay.

These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

There is provided a system for power system protection.

According to one embodiment of the present invention, there is provided cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements , said system comprising a numerical relay for identifying cold load pick up condition; a HMI unit for providing cold load pickup settings and timer settings; a means for detecting status of a circuit breaker contacts and further sending to the digital input of said numerical relay; and a flex logic screen for adding separate flex logic operands.

Other embodiment of the present invention provides a numerical relay comprising a CPU controller having a protection algorithm logic installed therein, an analog/digital converter for capturing current analog samples from a current transformer secondary and converting said analog signals into digitized form; a data acquisition and metering section taking said digitized inputs from said analog/digital converter and advancing said digitized signal to said CPU controllers.

Yet other embodiment of the present invention provides a method of cold load pickup protection for protection of feeders after load restoration by changing the operation of overcurrent elements, said method comprising the steps of identifying cold load pick up condition using a numerical relay; providing cold load pickup settings and timer settings using a HMI unit; detecting status of a circuit breaker contacts and further sending to the digital input of said numerical relay; and adding separate flex logic operands through a flex logic screen.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig 1 illustrates the cold load pickup protection system.

Fig 2 illustrates the method steps followed in cold load protection.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.

Reference is first invited to Fig 1 where the cold load protection system is shown in detail. It shows the different components of numerical relay used in cold load protection.

Fig 2 shows the method steps used in cold load protection. It shows the activation of different timers at different point of time.

The invented system is thus a cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly the present invention elucidates the application space and concept of Cold Load pickup and illustrates the HMI settings and the firmware logic to be incorporated to include this feature in New Feeder Protection relay.

Figure 1 depicts the flow of the inputs /commands to the corresponding hardware of the numerical relay. There are numerous components in terms of embedded systems (hardware) in the numerical relay of which Data acquisition & metering and CPU controller are the two main sub sections of the relay. Initially the current transformer secondary inputs i.e. phase currents which are analog inputs are fed to the Analog/Digital (A/D) converter located inside the relay. The A/D converter captures the current analog samples per cycle and converts into digitized form. 64 samples / cycle are captured by the A/D converter. These digitized samples from A/D converter go into the Data acquisition and metering section of the numerical relay and further to the protection algorithm logic.

The protection algorithm logic is installed in the CPU controller, in a coded form, which is embedded inside the relay. Through Data acquisition and metering section, the current inputs advance to CPU controller where the protection algorithm logic is processed.

Here in this case for cold load pickup protection, the status of breaker contacts whether open or close is asked by this feature from breaker and the input is sent to the digital input of relay. If the breaker status is open, then after a cold load pickup pre-timer expires the conditions are checked as mentioned further in the algorithm flow logic shown in Figure 2. If all conditions are met, then after the cold load pickup timer expires the setting groups are changed.

The cold load pickup feature is initiated if the circuit breaker remains open for a time more than a pre timer value. Afterwards the breaker closing or load restoring operation is checked by monitoring the phase current above 5% of nominal. The settings group is changed to Cold load pickup settings group for the set timer which is the Cold load pickup timer, after the timer expires, the settings group will be returned to the normal over-current settings group.

The primary objective of this feature is to block the operation of the overcurrent elements under the above stated conditions as this is not a fault and hence not a desired condition for tripping.

Settings:-
The following settings are required on the HMI:-
Cold load pickup feature:-Enabled/Disabled
Cold load pickup settings:- 0.1 to 20 times of In
Cold load pretimer:-0 to 1500 seconds in steps of 10 sec.
Cold load pickup timer:- 10 to 15000 seconds in steps of 10 secSeparate Flex logic element has to be added in Flex logic screen

Inputs
Status of the cold load pickup feature (enabled/disabled)
Three Phase currents (Ia, Ib, Ic)
Breaker status (From 52a Status DI)

Cold load settings group (CLD settings group are required on all stages of non directional and directional overcurrent element)

Outputs
The operand bit of the Output block (Cold load block becomes high or low depending upon the system conditions)

Abbreviations used:
HMI: Human machine interface
CPU: central processing unit
A/D: analog to digital
CLD: cold load
Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM:

1. A cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements , said system comprising:
a numerical relay for identifying cold load pick up condition;
a HMI/controller unit for providing cold load pickup settings and timer settings;
a means for detecting status of a circuit breaker contacts and further sending to the digital input of said numerical relay;
a flex logic screen for adding separate flex logic operands;
a pre timer; and
a data acquisition and metering means;
wherein the said controller unit comprising a controller logic checking status of circuit breaker contacts (open/close) and the cold load pickup is enabled through the controller unit (ON/OFF); thereby starting the pre timer and after the expiration of pre timer, the controller checks the status of breaker(Close/Open) and takes the input of three phase currents from Data Acquisition and metering and sends the signal to start the cold load pick up timer based on the set time value to replace the overcurrent settings with the cold load pickup settings.
2. System as claimed in claim 1 wherein said numerical relay comprising a CPU controller having a protection algorithm logic installed therein, an analog/digital converter for capturing current analog samples from a current transformer secondary and converting said analog signals into digitized form; a data acquisition and metering section taking said digitized inputs from said analog/digital converter and advancing said digitized signal to said CPU controllers.

3. System as claimed in claim 1 where said numerical relay further comprising a cold load pre timer and a cold load pickup timer.

4. A method of cold load pickup protection for protection of feeders after load restoration by changing the operation of overcurrent elements, said method comprising the steps of:
identifying cold load pick up condition using a numerical relay;
providing cold load pickup settings and timer settings using a HMI unit;
detecting status of a circuit breaker contacts and further sending to the digital input of said numerical relay; and
adding separate flex logic operands through a flex logic screen.
5. Method as claimed in claim 4 comprising the steps of capturing current analog samples per cycle and converting into digitized form using an analog/digital converter; said digitized samples from said analog/digital converter fed to a data acquisition and metering section; output from said data acquisition and metering section further fed to a CPU controller where the protection algorithm logic is processed; said CPU controller further generating the flex logic operand to block the over-current elements.

6. Method as claimed in claim 4 wherein said numeric relay initiates a cold load pre timer when breaker status is open and cold load pickup is high, on expiration of said cold load pre timer rechecking said breaker status, initiating cold load pickup if said breaker status is closed.

7. Method as claimed in claim 6 further comprising the step wherein phase current of each phase compared to a prefixed value and on detecting increase in said phase current initiating cold load pickup timer.

8. Method as claimed in claim 7 further comprising the step wherein group settings are changed to group cold load setting and said group setting is changed to original group setting after the expiry of said cold load pickup timer.

9. A cold load pickup protection system for protection of feeders after load restoration by changing the operation of overcurrent elements as herein described and illustrated with respect to the accompanying drawings.

10. A method of cold load pickup protection for protection of feeders after load restoration by changing the operation of overcurrent elements as herein described and illustrated with respect to the accompanying drawings.

Dated this 23rd day of February 2012

Abhishek Sen
Of S. Majumdar & Co.
(Applicant’s Agent)