Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
2001-03-21
2002-09-24
Oda, Christine K. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
C307S087000
Reexamination Certificate
active
06456056
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-82583, filed Mar. 23, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a system interconnection protective device for a private generator set such as an urban-waste power generation system and a cogeneration system, which interconnects with a system power supply via a circuit breaker.
FIG. 1
shows a conventional system interconnection protective device that has been used in order that a customer can interconnect a system power supply with a private generator set such as an urban-waste power generation system and a cogeneration system.
As
FIG. 1
illustrates, a host substation
4
drops a voltage of a system power supply
1
via a transformer
2
and supplies power to a customer
7
through a circuit breaker
3
.
The customer
7
supplies the power to a load
6
through a circuit breaker
5
.
The private generator set
10
supplies an output voltage of an alternating-current generator
12
to the system power supply
1
via a circuit breaker
11
.
To control the output voltage of the alternating-current generator
12
, an automatic voltage regulator (AVR)
13
controls a voltage of a field winding
14
of the generator
12
.
To control the output frequency of the generator
12
, a governor
15
of an engine
16
for driving the generator
12
controls power of the engine.
A generator fault detection circuit
22
is employed as a failure detection means. The circuit
22
detects an abnormal current based on a relationship between an output current of the generator
12
detected by a current transformer
21
and an output voltage of the generator
12
. The circuit
22
then supplies a detection signal of the abnormal current to a trip circuit
23
to thereby open the circuit breaker
11
.
A current transformer
24
is provided as a protecting means on the output side (the substation side) of the circuit breaker
11
. An overcurrent relay (OC)
25
receives an output of the current transformer
24
to operate the trip circuit
23
.
When the system power supply
1
malfunctions, especially when the power supply
1
is broken or the circuit breaker
3
is opened, various types of protective relays, such as an underfrequency relay (UF)
26
, an overfrequency relay (OF)
27
, an overvoltage relay (OV)
28
, and an undervoltage relay (UV)
29
, detect that an abnormal frequency or voltage is generated from an imbalance between an output power of the generator
12
and a load power of the load
6
.
In response to a detection signal output from the protective relays, the trip circuit
23
gives a trip instruction to the circuit breaker
11
to open the circuit breaker
11
and allow the circuit breaker
3
to be reclosed.
The foregoing system interconnection protective device has the following problems:
If the active and reactive components of the output power of the alternating-current generator
12
and the required power of the load
6
are almost equal to each other when the system power supply
1
malfunctions to open the circuit breaker
3
, the frequency and the voltage hardly vary. For this reason, none of the protective relays
25
to
29
operate, but the private generator set continues to operate. A so-called sole-operation (islanding) phenomenon occurs to prevent the circuit breaker
3
to be reclosed.
Some conventional devices adopt the following method in order to prevent the above islanding-operation phenomenon:
In this method, a transfer breaker
8
is connected to a private line extending from the substation
4
. This device
8
causes the circuit breaker
11
to break the transfer of power.
In other words, the transfer breaker
8
detects a signal indicating that the circuit breaker
3
of the host substation
4
is opened and transmits a break signal to the circuit breaker
11
to open the breaker
11
.
However, the cost of the transfer breaker
8
is very high for the private generator set
10
having a small-to-medium power of several hundreds of kilowatts. There is no practical merit in providing the transfer breaker in the system interconnection protective device.
BRIEF SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a system interconnection protective device for a private generator set, which is capable of reliably and easily detecting and protecting a sole operation of the private generator set, which interconnects with a system, without providing any expensive transfer breaker.
According to a first aspect of the present invention, there is provided a system interconnection protective device for a private generator set, which interconnects with a system power supply via a circuit breaker, the device comprising frequency detection means for detecting a frequency from an output of the private generator set, frequency change rate detection means for detecting a frequency change rate of the frequency detected by the frequency detection means, voltage variation reference determination means for defining a relationship between the frequency change rate and voltage variation reference by a function, the voltage variation reference increasing a leading reactive power of the private generator set when the frequency change rate detected by the frequency change rate detection means is positive increasing a lagging reactive power of the private generator set when the frequency change rate is negative, frequency change rate integration means for integrating the frequency change rate detected by the frequency change rate detection means, frequency variation tendency determination means for determining whether the frequency rises or lowers, based on a frequency change rate integration amount integrated by the frequency change rate integration means, bias means for applying a bias to the voltage variation reference output from the voltage variation reference determination means to decrease the voltage variation reference when the frequency variation tendency determination means determines that the frequency rises and applying a bias to the voltage variation reference output therefrom to increase the voltage variation reference when the frequency variation tendency determination means determines that the frequency lowers, and protection means for detecting a frequency variation promoted due to a voltage variation of the private generator set caused by adding the voltage variation reference to which the bias is applied by the bias means, to voltage reference of an automatic voltage regulator of the private generator set, thereby opening the circuit breaker and disconnecting the private generator set from a system bus.
In addition to the above means, the system interconnection protective device may further comprise reset means for determining whether the frequency change rate detected by the frequency change rate detection means falls within a narrow range close to zero and resetting the frequency change rate integration amount integrated by the frequency change rate integration means to zero when the frequency change rate falls within the small range. The protection means can determine whether the private generator set is to be disconnected from the system line on a first condition that the frequency change rate detected by the frequency change rate detection means exceeds a value, on a second condition that the frequency change rate integration amount integrated by the frequency change rate integration means exceeds a value, or by AND of the first and second conditions. The same is true of the reset means and the protection means of the second aspect described below.
According to the second aspect of the present invention, there is provided a system interconnection protective device for a private generator set, which interconnects with a system power supply via a circuit breaker, the device comprising frequency detection means for detecting a frequency from an output of the private generator set, f
Katoh Toyokuni
Nomiya Shigeo
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Oda Christine K.
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