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SYSTEM AND METHOD FOR DETECTING UNBALANCED OPERATION OF A CONVERTER

IP.com Disclosure Number: IPCOM000246624D
Publication Date: 2016-Jun-22
Document File: 4 page(s) / 363K

Publishing Venue

The IP.com Prior Art Database

Abstract

The invention proposes a system and method for detecting unbalanced operation of a power converter by installing a key bar. The key bar is implemented on all stators of the synchronous generator and its voltage is measured on the machines. The voltage will be rise in the key bar when there is an occurrence of unbalanced operation. An inverter comprising a silicon controlled rectifiers (SCR) with failed leg could result in unbalanced voltage and a rise in the key bar voltage, which is an indirect indication of unbalanced output. This could be measured by a load commutated inverter (LCI) control system and used as a means to detect SCR failure. The system is configured to set a threshold value for the voltage rise, beyond which it could provide an alarm to the user. The system and method provides effective troubleshooting and enhanced diagnostics on the LCI control system.

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SYSTEM AND METHOD FOR DETECTING UNBALANCED OPERATION OF A CONVERTER

BACKGROUND

The invention is in general related to power controls and in particular to a system and method for detecting unbalanced operation of a power converter. A load commutated inverter (LCI) is a thyristor controlled drive used to start a gas turbine by converting a synchronous generator as motor. It will accelerate the whole shaft line to a rated speed and disconnect after startup of the turbine. Series connected silicon controlled rectifiers (SCRs) are used on the load bridge of an LCI to enhance the voltage levels and to provide redundancy to avoid trip of the machine on a single point of failure. All these SCRs are triggered from a single firing pulse simultaneously. A weak pulse may or may not trigger all the SCRs in the leg. At low load, the LCI would still be able to accelerate the shaft line, but as the speed increases and so the load on the converter, a failed leg, which is not easy to detect, may cause the LCI to bog down and trip. The failure of SCRs could be identified by the voltage drop across them. Sometimes, the SCRs which are not fired or fired partially are difficult to identify. Every SCRs has to be checked for open gate terminals, every firing circuit board has to be checked for output waveforms, and feedbacks have to be checked to confirm or identify the source of the non-conducting leg in the bridge. This could consume between 8 to 16 h for troubleshooting, and could be a major cause of plant outage. Thus, there is a need for a simple method for identifying the SCR condition.

BRIEF DESCRIPTION OF DRAWINGS

The invention discloses a system and method for detecting unbalanced operation of a power converter, wherein:

FIG. 1 shows wave forms measured on the key bar of the generator stator.

FIG. 2 shows a trend recorded on the occurrence of failure in triggering a leg in the bridge.

FIG. 3 shows a distorted firing pulse indicating failure in triggering fourth leg.

DETAILED DESCRIPTION

The invention proposes a system and method for detecting unbalanced operation of a power converter using a key bar installation. The key bar is implemented on all stators of the synchronous generator and its voltage is measured or calculated on every machine. Typical wave forms are measured on the key bar of the generator stator are shown in FIG. 1. There will be rise in the key bar voltages when there is an occurrence of unbalanced operation. An inverter comprising a silicon controlled rectifiers (SCR) with failed leg could result in unbalanced voltage and a rise in the key bar voltage, which is an indirect indication of unbalanced output. This could be measured by a LCI control system and used as a means to detect SCR failure, or

 


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