FIELD OF THE INVENTION:

The present invention generally relates to on-load tap changers (OLTC) for high voltage Transformers. More particularly, the present invention relates to a system for detecting incomplete diverter operation of OLTCs for a high-power Transformer.

BACKGROUND:

Electric power system is a network that includes various electrical components for supplying, transferring and distributing electric power. The components in the power systems have large system reactance and the system also supplies highly reactive loads. The reactive loads are generally characterized by poor voltage regulation. Voltage regulation is the ability of a power system to provide a constant voltage over a wide range of load conditions. Hence, it is necessary to maintain a regulated voltage in a power system. The voltage regulation of a power system is influenced by the nature of components in the power system. Especially, Transformers in power system influence the voltage regulation to a greater extent. To achieve better voltage regulation, a tap changer is generally used in Transformer to regulate the output voltage.

The tap changer is a device used in the Transformer for altering the number of turns in one winding and changing the turns ratio of the Transformer. This function of tap changer regulates the output voltage of the Transformer. There are two types of tap changers namely on-load tap changers (OLTC) and Off-Circuit tap changers. The former is used while the Transformer is connected to load and the latter is used when the Transformer is disconnected from the load. OLTC is one of the most widely used tap changer. OLTC comprises of selector and diverter sections. In OLTC, the selector selects the desired tap position first and then diverter switches the power to the output through the newly selected tap selector contact. The switching of diverter has to be completed within 70ms from the time of initiation which is a desirable switching duration. There are various scenarios where the switching of diverter is not completed within the stipulated time. This leads to catastrophic effect in OLTCs and in Transformer which costs both time and money. Hence, it is essential to closely monitor the correct functioning of the OLTC Transformer to avoid power outages.

There are various techniques and safety devices adopted and provided in OLTCs such as Oil Surge Relay (OSR), Pressure Relief Valve (PRV), PT100 Sensor provision etc for monitoring its condition and operation. But these safety devices depend on their responsivity. For example, OSR activates when there is an abnormal oil discharge in Transformer which is due to overheating of the oil inside OLTC. Similarly, PRVs are operated when there is abnormal pressure built up in OLTC. These devices are operated mechanically and depend on heat and pressure respectively. Sometimes, these protective devices are found to be least effective because of their high response time which leads to the failure of OLTC and Transformer. This has led to the development of systems for monitoring the operation of OLTCs. There are many existing prior art that discuss the regulation of output voltage of Transformers using OLTCs and thereby monitoring the operation of OLTCs.

For example, PCT Publication Number W02013000608 to Henrik Breder and Lars Jonsson entitled “A tap changer and a method of operating a tap changer” relates to a tap changer for controlling the output voltage of a Transformer. The tap changer comprises a diverter switch with two branches, a current detector and a fault detector. Each branch of the diverter comprises a series connection of a main interrupter and a transition interrupter and also a parallel connection of transition resistor with the main interrupter. The current detector detects current flowing through at least one branch of the diverter switch and generates a current indication signal. The fault detector receives this current indication signal and generates a fault indication signal if a fault occurs in the diverter switch. The control system triggers an alarm in response to the fault indication signal.

United States Patent Number 7145760 to Bengt-Olof Stenestam and Gunnar Anderson entitled “Tap changer monitoring” relates to a method and apparatus for diagnosing Transformer tap changers. The tap changer comprises an insulating liquid whose pressure is monitored for a period of time in which switching of the tap changer is carried out. The information related to switching is extracted from the pressure diagram of the insulating liquid. The quantities from pressure peaks are related to specific arcing of the switching sequence. This provides an early error detection and the failure of Transformer is avoided. The measurements are repeated and trend evaluation is followed. An alarm signal is triggered if a trend indicates problems in tap changer.

United States Patent Number 9513654 to Thomas Strof entitled “Method for performing a switching process in an on-load tap changer” relates to a method for performing a switching process in an on-load tap changer between winding taps of a tapped Transformer. The switching contacts of OLTCs in use are monitored during the switching phases. These contacts are completely opened or closed by capacitors in controller in the event of failure of the energy supply.

Though there are various systems for monitoring the operation of OLTCs, none of them are time specific and these existing systems do not monitor and detect the incomplete operation of diverter. Also, the existing systems provide an alert on detecting fault in the operation of OLTC and do not disclose the means for safeguarding the OLTC and Transformer.

Hence, there is a need for a system that monitors/detects the incomplete operation of diverter in OLTC and trip the Circuit Breaker to safeguard the Transformer upon detecting the fault in OLTC.

OBJECT OF THE INVENTION:

The primary object of the present invention is to provide a system for detecting the incomplete operation of diverter in OLTC and safeguard the Transformer by tripping the Circuit Breaker on occurrence of fault. SUMMARY:

The present invention discloses a system for detecting the incomplete operation of diverter in OLTC and safeguarding the Transformer on occurrence of fault.

According to the present invention, the system comprises an OLTC with separate sections for selector and diverter, mechanically actuated Magnetic Reed Switches for ODD and EVEN tap positions of the diverter and a programmed controller. The Magnetic Reed Switches changes its state (From Normally open to closed or Normally closed to open) with respect to the position of the diverter. This change of state of the Magnetic Reed Switches is given as an input to the programmed controller.

In accordance with the present invention, the programmed controller in the system determines switching event between ODD and EVEN tap positions within set time duration. The programmed controller sends a tripping command to trip Circuit Breaker and isolate the Transformer from incoming supply if the switching event is not completed within set time duration.

The objective and advantages of the present invention will become more evident from the following detailed description when taken in conjunction with the accompanying drawings.

The objective of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 shows a double compartment on-load tap-changer;

FIG. 2 shows the switching sequence of the tap selector present in the OLTC; FIG. 3 shows the switching sequence of the diverter present in the OLTC;

FIGS. 4A and 4B show the proposed system for detecting incomplete tap change in OLTC Transformer;

FIGS. 5(A)-5(F) shows the switching sequence from EVEN tap position to ODD tap position in OLTC; and

FIGS. 6(A)-6(F) shows the switching sequence from ODD tap position to EVEN tap position in OLTC.

REFERENCE NUMERALS:

1 - Transformer

2 - Bushing

3 - OLTC

4 - Programmed Controller

5 - Circuit Breaker

6 - Incoming supply for transformer

7 - Magnetic Reed Switch 1

8 - Magnetic Reed Switch 2

OFT ATT /FT) DESCRIPTION OF THE INVENTION:

The present invention discloses a system for detecting the incomplete operation of diverter in an OLTC and safeguarding OLTC and Transformer on occurrence of fault. OLTCs are used in Transformers to change the transformation ratio on load to regulate the output voltage. The OLTC comprises of selector and diverter sections. There are two types of OLTCs namely single compartment and double compartment. FIG. 1 shows a double compartment OLTC that includes separate selector and diverter sections. The selector and diverter comprise two sets of contact arrangements namely ODD and EVEN. During tap change operation, assuming the OLTC is at EVEN tap position, the tap selector selects the desired tap position first i.e. the selection is at ODD taps shown in FIG. 2. The diverter switches the power to the output through the newly selected tap selector contact i.e. from EVEN to ODD switching contact as shown in FIG. 3. This switching should happen within 70ms which is the desirable switching duration.

FIGS. 4Aand 4B shows the system proposed in the present invention for detecting the tap change operation of OLTC Transformer. The system comprises an OLTC with double compartments for selector and diverter sections, plurality of mechanically actuated Magnetic Reed Switches (7 & 8), that are closed with respect to the diverter operation and a programmed controller as shown in FIG. 4 A. The input power for the Transformer (1) is provided by the incoming supply (6) as shown in FIG. 4B. The Circuit Breaker shown in FIG. 4B initiates tripping command when the diverter operation is not completed within set time duration, preferably within 70ms.

The Magnetic Reed Switches (7 & 8) changes its state from Normally open state (NO) to Normally Closed State (NC) and vice-versa with respect to Tap position (i.e. for ODD to EVEN and EVEN to ODD). The change of state of Magnetic Reed Switches (7 & 8) is given as an input to the programmed controller (4). When the diverter is at ODD tap position, the corresponding Magnetic Reed Switch 1 (7) is in Normally Closed State (NC) and the other switch 2 (8) is in Normally Open state (NO). Similarly, when the diverter is at EVEN tap position, the corresponding Magnetic Reed Switch 1 (7) is in Normally Closed State (NC) and the other switch 2 (8) is in Normally Open State (NO). The overall operation of the proposed system shown in FIGS. 4 A and 4B is explained below:

According to the present invention, if the OLTC (3) is at EVEN tap position and the diverter contact is also at EVEN switching contact as shown in FIG. 5(A). Hence, the Magnetic Reed Switch 1 (7) that is mechanically associated with the EVEN switching contact is in closed state. The Magnetic Reed Switch 2 (8) that is mechanically associated with the ODD switching contact is in open state. During tap change operation, the switching state of the OLTC (3) is changed from EVEN tap position to ODD tap position. The diverter switches its position from EVEN tap position to ODD tap position in a sequence as shown in FIGS. 5(B)-5(F). Magnetic Reed Switch 1 (7) that is mechanically associated with the EVEN switching contact opens as shown in FIGS. 5(B)-5(F). The Magnetic Reed Switch 2 (8) remains in open position till the diverter comes in contact with the ODD tap switching contact as shown in FIGS. 5(B)-5(D). The Magnetic Reed Switch 2 (8) that is mechanically associated with ODD switching contact gets closed when the diverter is in contact with ODD tap switching contact as shown in FIGS. 5(E) & 5(F). The switching of state of the OLTC is changed from ODD tap position to EVEN tap position in a similar manner as shown in FIGS. 6(A)-6(F). The change of switching state of the Magnetic Reed Switches (7 & 8) is given as inputs to the programmed controller (4) that is programmed in such a way that the switching event from EVEN to ODD tap position and vice-versa is completed within set time duration.

The switching event should be completed within 70ms i.e. set time duration and a problem in OLTC (3) is detected if the switching event does not take place within the set time duration. These problems lead to heating of oil and subsequently results in severe damage of both OLTC (3) and Transformer (1). Hence, the programmed controller (4) in the proposed system initiates a tripping command to trip Circuit Breaker (5) and isolate the Transformer(l) from incoming supply (6) if the switching event is not completed within set time duration in order to avoid further damages to OLTC (3) as well for the Transformer (1).

Thus, the proposed system saves OLTC Transformer from catastrophic damage by detecting the tap changing operation especially complete transition of diverter switch within set time. While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.