Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of ground fault indication
Reexamination Certificate
2001-01-30
2003-02-25
Oda, Christine (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of ground fault indication
C324S076410
Reexamination Certificate
active
06525542
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improvements in protection and control systems for use in power generation, transmission and distribution, and in particular to an improved design of status acquisition apparatus for monitoring the status of devices forming part of such systems, e.g., relays, circuit breakers, interrupters, isolators and similar protective switching devices.
BACKGROUND OF THE INVENTION
It is well known, for example, that a protective relay includes appropriate circuitry that monitors the condition of a power system circuit in order to decide when to trip the circuit breaker of that circuit. The output contact of the protective relay can either be used for driving the trip coil of the circuit breaker directly, or can be used to signal another protective relay, via a status acquisition apparatus associated with that relay, that it has detected an event. The circuit connecting the output contacts of the protective relay (or any other external apparatus) to the status acquisition apparatus of another protective relay is known as a status circuit. The status circuits require an external power supply for them to operate.
The protection relay circuitry is driven by a power supply that is independent of the electrical circuit that is being protected. In many cases the protection relays will be powered from an independent battery supply. This is usually 24/27V, 30/34V, 48/54V, 110/125V or 220/250 volts depending on the type of installation. This supply will also be used to power the status circuits. More than one battery voltage may be present in any given installation.
Protective relays are usually housed in non-ventilated enclosures to improve reliability. Low power dissipation of any electrical circuitry provided in the enclosure is therefore essential.
A protective relay will typically incorporate status acquisition apparatus which allows interaction between a number of protective relays and other associated external apparatus. The status acquisition apparatus may be an integral part of the protection relay circuitry located within the non-ventilated enclosure.
The signals fed into the status acquisition apparatus will be used by the protective relay, in conjunction with data obtained from its own monitoring circuitry, to determine when to initiate a trip via its output contacts. The status acquisition apparatus has two defined states: One state indicates ‘valid signal’ where the voltage present at its input is above a defined threshold and the other indicates ‘no signal’ where no voltage or a voltage less than the threshold is present.
Of course, a status acquisition apparatus may be adapted to monitor the status of the output of other devices as well as or instead of relays. They may be incorporated in a variety of systems for control or monitoring or any other application where fault monitoring or general status monitoring is needed.
The status acquisition apparatus design must meet several criteria. It must tolerate noise on the battery voltage without producing a false signal yet provide a rapid response if a true signal is detected. It must consume a minimum amount of power if it is located within a sealed housing and ideally be easily reconfigured to operate with different battery voltages.
Several prior-art status acquisition apparatus designs have been developed which can cope with a range of different supply voltage ratings. In one arrangement it is known to provide a constant current circuit which is fed from the battery voltage. The circuit draws a constant current from the battery supply that is used to drive an opto-isolator. The opto-isolator produces a ‘valid signal’ output as long as the current drawn exceeds the turn-on threshold of the opto-isolator. This depends on the current transfer ratio of the opto-isolator. If the battery voltage is not present or drops too low then the circuit cannot draw current, the opto-isolator produces no output, indicating that no signal is present.
A problem with the use of the constant current circuit is that the circuit will consume more power at a higher supply voltage than at lower supply voltage (since power=current multiplied by voltage). Thus, a constant current based circuit that is sufficiently sensitive to operate at the lowest expected battery voltage (say 24/27 volts) will have excessive power dissipation at the largest expected input voltage (say 220/250 volts).
In an alternative design, it has been proposed periodically to sample the input voltage for a fixed period. Current is able to flow through the opto-isolator for the duration of the sampling interval. Appropriately valued linear components, such as resistors and capacitors, are used in the circuit to set the current level. By getting a customer to specify the battery voltage of their system, a variant of the circuit using the most suitable linear components can be offered to meet their requirements. In this way power drawn is kept to a minimum. Unfortunately, this limits the use of the circuit to specific supply voltages, and if the incorrect variant of the circuit is used then it may be damaged.
It is an object of the present invention to overcome at least partially the disadvantages present in prior art systems.
SUMMARY OF THE INVENTION
In accordance with a first aspect the invention provides a protection and control system for use in power generation, transmission and distribution, the system including status acquisition apparatus for monitoring the status of at least one device in the system. The apparatus comprises:
conversion means adapted to receive an input signal representative of the status of the device and convert the input signal into a pulse width modulated signal, the mark-space ratio of the pulse width modulated signal decreasing with the magnitude of the input signal; and
comparison means adapted to compare the mark-space ratio of the pulse width modulated signal with a reference value and produce a status indication signal in the event that the result of the comparison meets a predetermined criterion.
The invention thus provides apparatus in which the input signal is converted to a pulse width modulated waveform which changes in mark-space ratio as the input voltage increases. By making the mark-space ratio decrease for increases in the magnitude of the input signal, such as an increasing input voltage, it is possible to ensure that at low voltages the power dissipated is similar to that dissipated at high voltages.
The pulse width modulated signal may comprise a single on-period (mark) and a single off-period (space) within each cycle. Of course, other pulse width modulated waveforms are envisaged.
The device that produces the status signal may comprise any device, apparatus or circuit from which a signal indicative of an event such as the status of a switch or of a voltage level can be obtained. This may in one application comprise a protection relay with the status signal comprising a signal representative of the switching state of the relay. The status signal may therefore be indicative of the status of the relay contact, i.e., open or closed.
Alternatively, it is envisaged that the input to the status acquisition circuit may be a signal taken from alternative devices. It may, for example, be a voltage taken from a point in a circuit to be monitored. It may be a direct or indirect measure of a battery voltage, for example, of a supply battery associated with a relay or other device. In this case the status acquisition apparatus would monitor the battery voltage.
The input signal to the status acquisition apparatus will typically comprise a DC voltage. Alternatively, it may comprise an alternating voltage. In the latter case, the conversion means may be associated with a rectifier that converts the alternating voltage to a DC voltage signal prior to inputting it to the conversion means. In both cases, the mark-space ratio of the pulse width modulated waveform will decrease with increasing voltage magnitude.
Preferably the apparatus includes means for providing a user-definable pre-set reference value
Alston
Kirschstein et al.
Oda Christine
Teresinski John
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