METHOD FOR ADAPTING A PROTECTIVE RELAY TO THE MAINS FREQUENCY, AND A PROTECTIVE RELAY

Abstract: The invention relates to a method for adapting a protective relay (7) to the mains frequency and a protective relay, the method comprising the steps of determining the mains frequency, and sampling a measured variable of mains frequency in order to provide physical samples. The method further comprises the steps of re-sampling the measured variable, by means of software, on the basis of the physical samples and the determined mains frequency in order to provide virtual samples and performing the further processing of the measured variable on the basis of the virtual samples.

Description: 1 METHOD FOR ADAPTING A PROTECTIVE RELAY TO THE MAINS FREQUENCY, AND A PROTECTIVE RELAY BACKGROUND OF THE INVENTION This invention relates to a method for adapting a protective relay to the mains frequency, which method comprises the steps of determining the mains frequency, and sampling a measured variable of mains frequency to provide physical samples. In general, the mains frequency equals fairly accurately the nominal frequency, which in Europe, for instance, is 50 Hz. That is why relay protection used for protecting electric networks is often mainly designed for a specific frequency. However, the frequency may considerably deviate from the nominal frequency as regards time and location, and hence the operational accuracy of the relay protection may decrease substantially. Frequency deviations are at their greatest in connection with generators supplying the network, during their start-up, when they are not yet connected to the network. The frequencies may then deviate in the order of tens of hertz, but the relay protection should operate accurately, however. Also, when the network power balance fluctuates, it is sometimes necessary to divide the network into sections whose fre- quencies may deviate from nominal. Also in these cases, the frequencies may deviate several hertz. In one known method that is disclosed in EP 325 786, a protective relay is adapted to a changing mains frequency by adjusting the physical sampling frequency of an analog-to-digital converter (A/D converter) used for sampling in such way that the number of samples per AC and voltage cycle remains the same. Then for instance, a discrete Fourier transformation (DTF) gives an accurate value, the number of samples per cycle being assumed constant. The method based on changing the physical sampling frequency requires more electronic components and consequently a more complicated de- vice, and thus the costs of the protective relay increase unnecessarily. All AD converters are not even capable of changing the physical sampling frequency dynamically. Another known method is based on keeping the physical sampling frequency constant and changing the number of samples used in DFT while the mains frequency varies, in the manner disclosed in EP 801 745. Thus, when the mains frequency changes, the number of samples per network cycle changes, the sampling frequency being constant. A disadvantage with the method is that the mains frequency is seldom such that the number of samples per cycle were an integer, which leads to inaccurate measurement results and undesired oscillation. BRIEF DESCRIPTION OF THE INVENTION The object of this invention is to provide a method which avoids the above-mentioned disadvantages, and enables adaptation of a protective relay to the mains frequency in a more reliable manner and with simpler equipment. This object is achieved with the method of the invention, characterized in that the method further comprises the steps of re-sampling a measured variable, by means of software, on the basis of the physical samples and the determined mains frequency in order to provide virtual samples, and performing the further processing of the measured variable on the basis of the virtual samples. The method of the invention is based on the idea that the virtual samples, whose number per network cycle is made to remain constant, are determined on the basis of the physical samples. Consequently, further operations, such as DFT, can be performed on the virtual samples by assuming that the number of samples per cycle is constant. An advantage with the method of the invention is that the number of samples per cycle is made to remain constant aiso when the mains frequency changes, without a more complicated device implementation related to the changing of the physical sampling frequency. Moreover, the constant number of samples in one cycle ensures good operating accuracy of the protective relay. The invention also relates to a frequency-adaptive protective relay comprising interconnected means incorporating a means to determine frequency that is arranged to determine frequency of an electric network, a sam- pling means that is arranged to sample a measured variable, a means to determine the measured variable, and an actuating means that is arranged to perform switching operations on the basis of the determined measured variable, characterized in that the protective further comprises a data processing means that is arranged to determine the virtual samples of the measured vari- able on the basis of the physical samples taken from the measured variable, in such a way that their number per network cycle is constant, and the means to determine the measured variable is arranged to determine the desired property of the measured variable on the basis of the virtual samples and to control the actuating means. By means of this kind of a frequency-adaptive protective relay the advantages of the method of the invention can be achieved with a simple and reliable structure. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail in connection with preferred embodiments with reference to the accompanying drawings, wherein Figure 1 is a block diagram of the method of the invention, and Figure 2 illustrates by way of example a signal that is re-sampled by the method of the invention. DETAILED DESCRIPTION OF THE INVENTION A protective relay that is divided into functional blocks in the manner illustrated in Figure 1 is connected to an electric network 3. The function of the relays is to determine the properties of specific variables in the mains, and if necessary, to perform switching operations. For instance, current or voltage of the mains can be determined. In the manner shown in Figure 1 , the protective relay 7 is connected to the electric network 3, and in accordance with the method of the invention the mains frequency is determined by using a means to determine frequency 2. To determine the mains frequency, any device applicable for frequency determination can be used, given that the device is based on the measured variable or values of the measured variable provided by a sampling means. However, the method of the invention gives the best possible result when the frequency determination is as accurate as possible. In Figure 1 the means to determine frequency 2 determines the mains frequency on the basis of the samples obtained as the output of a sampling means 1. In accordance with the method, the measured variable of mains frequency is further sampled with the sampling means 1 to provide physical samples. The sampling means takes samples with a fixed sampling frequency from the measured variable, which may be current or voltage, of the mains. The sampling means 1 may be, for instance, an A/D converter which converts an analog input signal into a digital output signal of the holding circuit at given instants of time. In accordance with Figure 1 , the output of the means to determine frequency 2 is connected to a processing means 4 which re-samples the measured variable by means of software on the basis of the physical samples received from the sampling means and provides the virtual samples. The number, per one network cycle, of virtual samples to be determined in accordance with the invention is constant. On the basis of the physical samples and the mains frequency, the processing means 4 determines the instants of time at which the virtual samples are determined. In accordance with one embodiment of the invention, the determination of the virtual samples is effected by interpolation on the basis of the physical samples. The simplest interpolation method is linear interpolation, whereby on the basis of two physical sample values and the instant of time, the virtual sample value is determined at a given desired instant of time. Linear interpolation can be performed, for instance, with the following formula yθ) = (ι-% w+(% (^i) wherein x(k) and x(k+1) are consecutive physical samples, between which a virtual sample y(j) is located in time, dts is the time between samples in physical sampling and dt is the time distance of a virtual sample from a sample x(k). It is possible that no virtual sample falls between two consecutive physical samples. On the other hand, a plurality of virtual samples may fall between two physical samples. However, a new virtual sample is preferably calculated from the physical samples between which it will be located in time. Figure 2 illustrates by way of example a signal that has been sampled with a specific fixed sampling frequency. In Figure 2 the physical samples are indicated by circles (o) 8. In order to provide a specific constant number of samples per cycle, the signal is re-sampled, and the new virtual samples are indicated in the figure by crosses (x) 9. The above-described linear interpolation is the simplest, but not the only possibility to determine virtual samples. The virtual samples can also be determined, for instance, by means of polynomic approximations that take into account a plurality of physical samples, whereby more accurate virtual samples can be obtained. The common characteristic of all methods used for re-sampling by means of software is that from the original physical samples they provide new virtual samples which describe the desired properties of the signal with sufficient accuracy for the use. In accordance with the method, the virtual samples having been determined, they are further processed in a means to determine the measured variable 5. Further processing of the virtual samples refers to the operations by which the desired property is obtained from the samples. When voltage is the measured variable to be sampled, the means to determine the measured variable 5 may calculate, for instance, by a DFT transformation the amplitude of the voltage. The measured variable may also be any other property of the electric network, such as current, harmonic content of current or voltage, effective value or phase angle, or actual power, idle power, apparent power or some other derived variable such as resistance, reactance or impedance calculated from current or currents and voltage or voltages. The means to determine the measured variable may also alternatively use other calculating methods than the DFT transformation. The means to determine the measured variable 5 is also arranged to control an actuating means 6. The control of the actuating means is performed when a characteristic, for instance voltage, of the measured variable deviates from the limits set thereto. The actuating means 6 is, for instance, a switch that is arranged to disconnect two sections of the electric network in case of a failure. It is obvious to a person skilled in the art that as technology progresses the basic idea of the invention can be implemented in a variety of ways. Therefore, the invention and its embodiments are not restricted to the above-described examples, but they may vary within the scope of the appended claims.

Claims: CLAIMS 1. A method for adapting a protective relay (7) to a mains frequency, which method comprises the steps of determining the mains frequency, and sampling a measured variable of mains frequency to provide physical samples, characterized in that the method further comprises the steps of re-sampling the measured variable, by means of software, on the basis of the physical samples and the determined mains frequency in order to provide virtual samples, and performing further processing of the measured variable on the basis of the virtual samples. 2. A method as claimed in claim ^characterized in that the sampling of the measured variable of mains frequency comprises the step of determining the value of the measured variable at predetermined intervals. 3. A method as claimed in claim 1 or 2, characterized in that the re-sampling of the measured variable comprises the steps of calculating mains-frequency-dependent instants of time at which the virtual samples are determined, and determining the value of the measured variable at said instants of time. 4. A method as claimed in claim 3, characterized in that the determination of the value of the measured variable comprises the step of interpolating the virtual samples from the physical samples. 5. A method as claimed in any one of preceding claims 1 to 4, characterized in that the further processing of the virtual samples comprises the step of calculating the desired property of the measured variable from the virtual samples. 6. A frequency-adaptive protective relay (7) comprising intercon- nected means incorporating a means to determine frequency (2) that is arranged to determine frequency of an electric network (3), a sampling means (1) that is arranged to sample a measured variable, a means to determine the measured variable (5) and an actuating means (6) that is arranged to perform switching operations on the basis of the determined measured variable, characterized in that the protective relay (7) further comprises a data processing means (4) that is arranged to determine virtual samples on the ba- sis of the physical samples taken from the measured variable, in such a way that their number per network cycle is constant, and the means to determine the measured variable (5) is arranged to determine the desired property of the measured variable on the basis of the virtual samples and to control the actu- ating means (6). 7. A protective relay as claimed in claim 6, characterized in that the desired property of the measured variable is the amplitude of current or voltage. 8. A protective relay as claimed in claim 6, characterized in that the desired property of the measured variable is the effective value of current or voltage. 9. A protective relay as claimed in claim 6, characterized in that the desired property of the measured variable is the phase angle of current or voltage. 10. A protective relay as claimed in claim 6, characterized in that the desired property is the amplitude of any harmonic wave of current or voltage. 11. A protective relay as claimed in claim 6, characterized in that the desired property of the measured variable is the actual power, idle power or apparent power or some other derived variable such as resistance, reactance or impedance calculated from current or currents and voltage or voltages.

Inventors: KUISTI HARRI, LEHTINEN REIJO, KUISTI, HARRI, LEHTINEN, REIJO
Applicants: ABB SUBSTATION AUTOMATION OY, KUISTI HARRI, LEHTINEN REIJO, KUISTI, HARRI, LEHTINEN, REIJO
Application year: 1999
Main classification: H02H3
IPC codes: G01R19, H02H1, H02H3