Method and apparatus for correcting the phase error in measurement-control equipment for a power network

Technical Field

The present invention relates to a method and a corresponding apparatus for correcting a phase error in measurement-control equipment for a power network, and in particular to a method and an apparatus for correcting the phase error between the voltage and current generated by the hardware environment of the measurement-control equipment for a power network.

Background Art

In measurement-control equipment for a power network, the requirements relating to the precision of the measurement and control are very high, and normally the error is required to be controlled below 2Vo. However, each of the measured/controlled parameters of an object measured/controlled by the measurement- control equipment for the power network is based on the measurement of two basic values - current and voltage, and it is required that when the current and the voltage are measured there is no error between them introduced by the measurement- control equipment itself, so as to reduce the generation of final error and to improve the precision of the measurement and control; therefore the measurement of the current and voltage is a key element in controlling the precision of the measurement-control system for the power network.

In the measurement-control equipment, before the voltage and current are sampled, the power is transmitted in the hardware circuits of the equipment, and it will undergo a series of treatment processes, for example, said treatment may be amplification by a sampling-and-holding amplifier. These treatments are carried out on the current and voltage separately; therefore the difference in the hardware environment will create a phase error between the current and voltage when performing the measurement and control. Even if the aforementioned difference

is very little, the phase error generated thereby will still exist, and will affect the precision of the subsequent measurement and control severely.

In order to correct said phase error, in the current measurement-control equipment the phase measurement method is adopted, and said phase measurement method is, for example, first-order linear interpolation, quadratic interpolation, correlation analysis method, the fast Fourier transform (FFT) , which is currently the most widely used digital phase measurement method, and so on. By using the aforementioned methods, the phase error between the current and voltage is calculated after the current and voltage have been sampled and measured, and this difference value is transmitted to the subsequent measure- ment and control process and is introduced into the subsequent calculation, measurement and control, so as to correct the phase error between the current and voltage and to improve the precision of the measurement and control.

However, every time the practical measurement and control are performed for the power network, the measurement-control equipment adopting the aforementioned method needs to carry out the above measurement process to calculate the phase error in real time, and to introduce the difference value into the subsequent measurement and control process. However, the measurement and calculation of said phase error under the high precision requirement create a great workload for calculation, and consume a lot of time, thus putting the calculating system under a very heavy burden and increasing the processing time.

Accordingly, when correcting said phase error, the above correction methods can no longer satisfy the demand for further increasing the processing speed and reducing the burden of the calculating system, and have gradually become a bottleneck in the technical development of measurement-control equipment for power networks .

Contents of the Invention

An object of the present invention is to propose a method and an apparatus for correcting the phase error in measurement- control equipment for a power network, and which method and apparatus use a non-computational approach to completely correct the phase error once and for all so that there is no need to introduce the calculation of said phase error again during the subsequent measurement and control, and after this correction there is no need to perform re-correction when the measurement- control equipment is used again, therefore, the method can satisfy the demand for further increasing the processing speed and reducing the burden of the calculating system.

In order to realize the above object, the present invention proposes a method and an apparatus for correcting the phase error in measurement-control equipment for the power network once and for all, and said phase error particularly is a phase error between the current and voltage. The method comprises the steps of: taking a sample of the voltage or the current; delaying for a period of time; after delaying, if the last sampled object is voltage, then taking a sample of the current; if the last sampled object is current, then taking a sample of the voltage; calculating a reactive power with the sampled values of the current and voltage, and comparing the reactive power value with a threshold; when the reactive power is less than said threshold, determining the delay time as the correction time for phase error.

The method and apparatus of the present invention are used before the practical measurement and control, so as to correct the phase error generated by the hardware environment of the measurement-control equipment itself between the current and voltage which are input into the measurement-control equipment when performing the practical measurement and control. During the correction of said phase error, the phase difference of the current and voltage used for correction is zero before they are input into the measurement-control equipment. This is because

that when correcting the phase error generated by the hardware environment of the measurement-control equipment itself, it is required that the reactive power is close to zero in the step of comparing the reactive power, namely it is required that there is basically no phase error between the current and voltage, so as to determine whether said phase error has been corrected or not; if there is phase angle between the current and voltage inputted into the measurement-control equipment when performing the correction, namely the phase error is not zero, then the phase error will be considered as a part of the phase error generated by the hardware circuit, thus causing a deviation of the correction value, and affecting the correction result of the measurement-control equipment.

In the method of the present invention, the initial value of the delay time can be zero, and it also can be set as a very small time value in the case that it is certain that a phase error exists in the hardware circuit, so as to speed up the correction process.

In the method of the present invention, when in the step for comparing the reactive power said reactive power is larger than or equal to said threshold, it further comprises a step for adjusting the delay time, and the adjusted delay time is compared with a sampling period; if it is larger than or equal to the sampling period, then the correction process is ended, and if it is less than the sampling period, then the initial sampling step is re-started.

In the step for sampling the current and voltage, only one measurement parameter is sampled each time, namely only current or voltage is sampled each time, and there is no required order in sampling the current and voltage, namely the voltage can be sampled first, or the current can be sampled first.

In the step for calculating the reactive power, the calculation method for the reactive power can be carried by multiplying the

instantaneous values of the current and the voltage to obtain the reactive power value.

In the step for comparing the obtained reactive power with a threshold, the threshold can be zero, and in the case that it is certain about the existence of a phase error in the hardware circuit it can also be determined according to the required precision, namely setting the product of the maximum value of the reactive power and a factor that is less than or equal to said precision as the threshold.

In the step for adjusting the delay time, the method for adjusting the delay time can adopt a method of increasing by a fixed step-size each time, and it can also adopt a method of increasing by a changed step-size each time. Said step-size is determined according to the sampling period of the measurement- control equipment.

In the step for determining the delay time, the delay time of this cycle is used as the final delay time, namely the correction time for the phase error, and it is recorded in the measurement-control equipment. During any subsequent measurement and control, the correction time of said phase error is not required any more, because there is no phase error between the current and voltage after the correction. Furthermore when the measurement-control equipment is used again, it is not required to perform the re-correction of the phase error, because the phase error generated by the hardware environment of the measurement-control equipment itself will not change any further, whereas the phase error has already been corrected by the correction time for the phase error recorded in the measurement- control equipment, therefore the measurement-control equipment can be used directly.

The present invention also proposes a corresponding apparatus, which comprises a sampling unit, a calculating unit, a comparing unit and a recording unit. Based on the same reasons as the aforementioned method, when the apparatus is used to correct

the phase error, the phase error between the current and voltage inputted into the measurement-control equipment should be zero .

In the apparatus of the present invention, the sampling unit is used to sample, before and after a delay time, the voltage and the current, or the current and the voltage respectively, and to transmit the sampled values of the voltage and the current to a calculating unit; the calculating unit is used to receive said values of the voltage and the current so as to calculate a reactive power value, and to transmit the result to a comparing unit; the comparing unit is used to receive said reactive power value, and to compare said reactive power value with a threshold; and the recording unit is used to record said delay time as the correction time for the phase error when said reactive power is less than said threshold, so as to be retrieved directly when said measurement-control equipment is to be corrected again.

In the apparatus of the present invention, the initial value of the delay time can be zero, and it also can be set as a very small time value in cases where it is certain that a phase error exists in the hardware circuit, so as to speed up the correction process.

The apparatus of the present invention also comprises a time- delay unit, and when said reactive power value in the comparing unit is larger than or equal to said threshold, the comparing unit transmits the delay time to said time-delay unit; the time-delay unit is used to adjust the delay time, and to trans- mit the adjusted delay time to said comparing unit; the comparing unit compares the adjusted delay time with the sampling period, and if it is larger than or equal to the sampling period, then the correction process is ended; if it is less than the sampling period, then the sampling unit restarts sampling. Said threshold is zero, or is determined by the required precision of the measurement-control equipment, namely the product of the maximum value of the reactive power and a factor that is less than or equal to said precision is set as the threshold.

The adjustment to delay time T made by the time-delay unit is by way of increasing it by a fixed step-size or a changed step- size each time, and said step-size is determined according to the sampling period of the measurement-control equipment.

The method and apparatus of the present invention use a non- computational method to correct the phase error between the current and voltage generated by the hardware environment of the measurement-control equipment itself once and for all before the measurement-control equipment carries out any practical measurement and control, which helps to improve the efficiency of phase error correction, and reduce the burden of the calculating system under the conditions of keeping the preci- sion of the measurement and control.

Description of the Drawings

Fig. 1 is a flowchart of a method for correcting the phase error in the measurement-control equipment for a power network according to the present invention; and

Fig. 2 is a schematic diagram of an apparatus for correcting the phase error in the measurement-control equipment for the power network according to the present invention.

Embodiments

When using the method and the apparatus of the present invention to correct the phase error generated by the hardware environment of the measurement-control equipment itself, the phase difference between the current and voltage used should be zero, namely the current and the voltage are in phase. This is for the purpose of avoiding the introduction of such a phase difference into the phase error during the correction, thus affecting the correction result.

Please refer to Fig. 1; the present invention comprises the following main steps:

1) taking a sample of the voltage or the current;

2) delaying for a period of time;

3) after the time delay, if the voltage is sampled in step 1), then taking a sample of the current, and if the current is sampled in step 1), then taking a sample of the voltage;

4) calculating a reactive power by using the values of the voltage and current obtained in step 1) and step 3) ;

5) comparing the value of the reactive power calculated in step 4) with a threshold; and

6) when said reactive power is less than said threshold, recording the delay time in step 2) .

In step 1), the measurement-control equipment for the power network first samples the voltage or current in the circuit, so as to obtain the value of the voltage or current at that instant .

In step 2), it is delayed by a delay time T; the initial value of said delay time T is set according to whether or not a phase error exists in the hardware circuit of the measurement-control equipment and the sampling period, and it can be set as zero or as a minimum value close to zero compared with the sampling period, for example, if the sampling period is 312.5 microseconds, the delay time is set as 1 microsecond. In terms of the current manufacturing technology, it is impossible for the hardware circuit to be exactly identical, and there will cer- tainly be a phase error, therefore in order to increase the correction speed, the initial value of the delay time can be set as said minimum value.

In step 3), the measurement-control equipment for the power network again samples the voltage or current in the circuit, to obtain the value of the voltage or the current at this instant, and the object of this sampling is not the same as the previous one. Since it is already past the delay time T of step 2), this time of sampling the current or voltage is later than the previous time of sampling the voltage or current by a delay time T.

In step 4), the reactive power is calculated using the values

of the current and voltage obtained by sampling, and the calculation method is, for example, by representing the voltage and the current respectively as: U = usin(ωt+ψi), (Equation 1), and I = isin(ωt+ψ2), (Equation 2)

in which U and I are the maximum values of the voltage and current respectively, ω is an angular frequency, and cpi and φ2 are the initial phase of the voltage and current respectively.

The apparent power is a sum of active power and reactive power, and it is represented as:

S = P + JQ, (Equation 3)

in which, P is the active power, and Q is the reactive power.

The calculation method of the apparent power is the product of the instantaneous voltage and the instantaneous current, and it is represented as:

S = UI, (Equation 4) .

Therefore, the calculation methods for the active power, reactive power and phase error are represented respectively as:

P = Ulcosφ , (Equation 5)

Q = Ulsinφ , (Equation 6) and φ = φ i -φ 2 , (Equation 7) .

After having calculated the reactive power, it is compared with a threshold, namely step 5) is executed; if the reactive power is less than the threshold, then it enters step 6) , otherwise it enters the step for adjusting the delay time. During this process the threshold can be zero, and it also can be deter- mined according to the required precision of the measurement- control equipment for the power network, for example the maximum value of the reactive power, namely the product of the

maximum value of current and the maximum value of voltage U _max ^χ l _max , is calculated first, and then the value obtained by multiplying the maximum value of the reactive power and a precision value required by the measurement-control equipment for the power network or higher than the required precision of the measurement-control equipment for the power network is set as the threshold; for example when the required precision of the measurement-control equipment is 2Vo, then 2°SΌ of the maximum value of reactive power can be set as the threshold.

In the step for adjusting the delay time, the delay time T is increased according to the sample period. Said increased amount is a minimal time quantity relative to the sampling period, and the increased amount is fixed, for example when the sampling period is 312.5 microseconds, the increased amount is set as 10 microseconds. A new delay time T is obtained by adding the increased amount to the original delay time T. Before entering into a new sampling process, said new delay time is compared with the sampling period, and when said delay time is larger than the sampling period, the correction process is ended, namely the phase error between the current and voltage is beyond the range within which the current hardware can be corrected, and it is needed to add extra hardware circuits; when said delay time is less than the sampling period, then the new delay time T is set as the delay time T in step 2), and it starts from step 1) for sampling the voltage and current, and it enters a new cycle.

In this step, in order to speed up the correction, another ap- proach available for determining the increased amount is to increase said increased amount by a fixed step-size each time. The detailed scheme is as follows: setting the initial increased amount at a minimal time quantity relative to the sampling period, for example when the sampling period is 312.5 mi- croseconds, the initial value of the increased amount is set at 10 microseconds. Each time after entering the step of adjusting the delay time, the increased amount is increased by a fixed step-size, for example 1 microsecond, to make it larger than

the increased amount used in the previous step. That is to say, the increased amount for adjusting the delay time is increased by a fixed step-size, and each time the increased delay time is different to the previous increased amount. Said method of adjusting the delay time can find the value which is less than 2Vo of the maximum value of reactive power more quickly. It may be considered that, in order to further increase the correction speed and to quickly find the value less than 2°SΌ of the maximum value of reactive power, the step-size of the increased amount can also be an unfixed value, or the increased amount can be changed in other ways .

In step 6) , the delay time T is determined finally and recorded as the correction time of the phase error. In step 5), if it meets the requirement of the reactive power being less than 2°SΌ of the maximum value of the reactive power, then the delay time T used in step 2) of this cycle is determined as the correction time of the phase error in step 6) . Said correction time of the phase error will be recorded in the measurement-control equipment.

After having recorded the correction time of the phase error, the correction of the phase error between the current and voltage in the measurement-control equipment has been completed. Furthermore, since said delay time has been recorded in step

6) , there is no need to go through the above described correction process again when the measurement-control equipment is used later, and the recorded delay time can be directly retrieved to obtain a current and a voltage without phase error therebetween.

Furthermore, the advantage of the method is that, after having determined the correction time T of the phase error, there is no more phase error between the current and voltage output from the measurement-control system, and therefore there is no need to include the calculation of said phase error during the operation after the correction.

As shown in Fig. 2, the present invention also proposes an apparatus for correcting the phase error in the measurement- control equipment for a power network, and said apparatus comprises: a sampling unit 10, a calculating unit 20, a comparing unit 30 and a recording unit 50, and the apparatus can further comprise a time-delay unit 40.

When the correction of phase error on the measurement-control equipment is performed, the current or the voltage is first sampled by the sampling unit 10; the sampling this time is only aimed at one measurement parameter of the current and voltage, namely, it can sample the voltage first, or it can also sample the current first. Further, after a delay time T, the sampling unit 10 will sample the voltage or the current again, and the sampled measurement parameter this time is different to the one last time.

The values of the voltage and current obtained by the sampling unit 10 are transmitted to the calculation unit 20, and after the calculation unit 20 has received the values of the voltage and current, the reactive power is calculated by multiplying the voltage value and the current value according to equation 6 and equation 7. After having calculated the reactive power, the calculation unit 20 transmits the reactive power value obtained to the comparing unit 30.

After the comparing unit 30 has received said reactive power value, it compares the reactive power value with a threshold, which threshold can be zero, and can also be determined accord- ing to the precision required by the measurement-control equipment in cases where that it is certain that the phase error exists in the measurement-control equipment; the determining process is first to calculate the maximum value of reactive power, namely the product of the maximum value of current and the maximum value of voltage U _max ^χ l _max , and then the value obtained by multiplying the maximum value of the reactive power and a precision value required by the measurement-control equipment for the power network or higher than the required

precision of the measurement-control equipment for the power network is set as the threshold; for example when the required precision of the measurement-control equipment is 2Vo, then the threshold can be set as 2V ₀ of the maximum value of reactive power.

In the comparing unit 30, when the reactive power is less than said threshold, the correction process is ended, and the delay time is transmitted to the recording unit 50 to be recorded, namely the delay time T recorded in the recording unit 50 is set as the correction time T of the phase error; if the reactive power is not less than said threshold, then the comparing unit 30 controls the time-delay unit 40 to adjust the delay time, and after having obtained a new delay time, the time- delay unit 40 transmits said adjusted delay time T to the comparing unit 30 again, and then the comparing unit 30 compares the delay time with the sampling period; if the delay time is larger than or equal to the sampling period, then the correction process is ended, and if the delay time is smaller than the sampling period, then the comparing unit 30 controls the sampling unit 10 to re-start the operation, and the adjusted delay time is set as a new delay time.

When the time-delay unit 40 increases the delay time, it can use a method for increasing it by a fixed step-size each time, and also it can use a method for increasing it by a changed step-size each time. Said step-size is determined by the sampling period of the measurement-control equipment, in which the changed step-size can change in a manner of a fixed increased amount, and it can also be in a manner of an unfixed increased amount .

By using the apparatus of the present invention, it can correct the phase error generated by the hardware environment of the measurement-control equipment itself once and for all, so as to reduce significantly the calculation workload during the correction, to improve the correction efficiency, and lighten the calculation burden on the system. Furthermore, by using the

apparatus to correct the phase error, the current and voltage without a phase error can also be obtained directly during the subsequent measurement and control, thereby avoiding the calculation of phase error during the subsequent measurement and control.