On Differential Protection Principle Compatible Electronic Transducer

Abstract

The technology of digital interface compatible electronic transducer is studied. The measuring and protective equipment is explored to make a new application of current and voltage signals from the electronic transducer so that electronic transducer differential signal is directly used as a protection input. The new differential protection principle based on the differential input signal is put forward. And the theoretical analysis and simulation shows that the protection principles proposed are feasible.

1 Introduction

Transducers are used to monitor the primary device and provide reliable electric quantities to secondary equipment. The traditional transient electromagnetic transducers have the issues of saturation and low accuracy. The electronic transducer has low output, sufficient bandwidth, good linearity, simple structure, and other advantages. At the same time the electronic transducer does not require direct contact with the measured current circuit. The output of the electronic transducer is a digital signal, which is essentially different from the analog signal output of the traditional transducers and will have a profound impact on secondary equipment.

In the paper, the characteristics of two different interfaces are analyzed and the differential protection principle based on the differential input signals are proposed. Then the simulation tests are made by using PSCAD. The simulation results show that the differential protection principle based on the differential input signals can correctly identify the internal fault and external fault. The electronic transducer differential signals are applied to protection algorithm directly without an integral circuit, which can give full play to the advantages of electronic transducer and improve the reliability and accuracy of protection.

2 Overview of Digital Interface

2.1 Structure of Electronic Transducer

According to IEC60044-8 “Electronic Current Transducer” standards, the electronic transducer includes one or many current sensors and voltage sensors which connect the transmission system to the secondary converter. The measured current and voltage is exported with analog or digital signals and is transmitted proportionally to the protection systems and other secondary measurement and control instruments. As what is shown in Fig. 1, the analog signal of the transducer is supplied directly to the secondary devices, and the digital signal is combined by a merging unit and exported to the secondary devices. Electronic transducers can be divided into two types: active electronic transducers and passive optical transducers, depending on if the transducer require a power supply.

Fig. 1.

Structure of electronic transducer

2.2 Two Modes of Electronic Transducer Interface

Interface with Integral Circuit.

The first interface is shown in Fig. 2. Firstly, the output optical digital signals of the transducer are transported to the low voltage side through optical fibers. Then the signals are carried to the relay protection system after being further processed in the merging unit. Because the outputs of the electronic current transducer based on a Rogowski coil and the resistive-capacitive divider voltage transducer are differential signals. In order to reflect the voltage and current, the outside integral circuit is increased in the sensor system of electronic transducer.

Fig. 2.

Interface with integral circuit

The interface model with integral circuit has many advantages: the interface is simple; the protection system hardware requires few changes; the cost of the protection system change is low; and the protection system software algorithm can be used without adjustment. This interface model has disadvantages too. A digital integrator is achieved entirely by software, so it requires high operation speed and greater hardware cost. In addition, the integral circuit limits the measurement band of the electronic transducer.

Interface Without Integral Circuit.

The second interface is shown in Fig. 3. In this approach, the differential signals from electronic transducer are used in the protection algorithm directly. The transducer integral part is omitted and the traditional protection algorithm is modified.

The interface model without integral circuit has many advantages: the system reliability is increased and takes advantage of the electronic transducer to improve the reliability and accuracy of protection system. On the other hand, the software algorithm of traditional protection system must be adjusted with this interface mode.

Fig. 3.

Interface without integral circuit

3 Differential Protection Basing on Differential Input Signals

Transmission line current differential protection determines whether there is a short circuit fault protection on the protected line by comparing current phase at both ends of the line. Differential protection can cut the fault quickly and is not affected by the power operating mode of single side, mutual inductance in parallel lines, system oscillations, line series capacitor compensation, TV disconnection, etc. Differential protection has become the primary choice for EHV transmission line main protection because of its ability to choose phase. The conventional differential has a big problem. The secondary side current of traditional electromagnetic transducer (CT) is used to make protection to work. At the condition of external short circuit fault, the core may be saturated, which causes the traditional transducer transient current to be distorted and results in a large imbalance current and differential protection malfunction. Electronic current transducer (ECT) has non-magnetic saturation, simple and reliable insulation, wide measuring range, etc.

In electronic current transducer based on a Rogowski coil, after removing the integral link, input signal sent to computer protection system is a current differential signal $\frac{di(t)}{{dt}}$, on the basis of which the differential protection principle and criterion is analyzed.

Assuming line current at both sides are following.

$i_{m} (t) = \sqrt 2 I_{m} \sin (\omega \,t + \varphi_{m} )$, $i_{n} (t) = \sqrt 2 I_{n} \sin (\omega \,t + \varphi_{n} )$.

Then the corresponding current are $\dot{I}_{m} = I_{m} \angle \varphi_{m}$, $\dot{I}_{n} = I_{n} \angle \varphi_{n}$.

If $i_{mj}$ is represented as $i_{mj} = \frac{{di_{m} (t)}}{dt} = \sqrt 2 \omega I_{m} \cos (\omega \,t + \varphi_{m} ) = \sqrt 2 \omega I_{m} \sin (\omega \,t + \frac{\pi }{2} + \varphi_{m} )$ then the corresponding current are as following: $\dot{I}_{mj} = \omega I_{m} \angle \frac{\pi }{2} + \varphi_{m}$.

And if $i_{nj}$ is can be represented as $i_{nj} = \frac{{di_{n} (t)}}{dt} = \sqrt 2 \omega I_{n} \cos (\omega \,t + \varphi_{n} ) = \sqrt 2 \omega I_{n} \sin (\omega \,t + \frac{\pi }{2} + \varphi_{m} )$ then $\dot{I}_{nj} = \omega I_{n} \angle \frac{\pi }{2} + \varphi_{n}$.

Thus we can produce Eq. (1)

$$ \left| {\dot{I}_{mj} + \dot{I}_{nj} } \right| = \omega \left| {\dot{I}_{m} + \dot{I}_{n} } \right| $$

(1)

Compared with conventional phase current differential protection, input signal amplitude at both sides of differential protection based on differential input expands $\omega$ times, phase shifts $\frac{\pi }{2}$, and the current relative relationship on both sides do not change. When line is normal, external fault, internal short circuit fault, current waveform, and phase diagram at both sides are shown in Fig. 4:

Fig. 4.

(a) Principle diagram (b) current waveform and phase of normal operation and external fault (c) current waveforms and phase of internal short fault

It can be concluded that compared with conventional phase current differential protection, protection differential signal as input signal, because the current in line ends has a phase shift at the same time and the relative phase relationship of both sides of the current do not change, the current differential protection principle based on the differential input signals is same as conventional one. At any moment, current phasor summation is zero at both ends of the normal or external fault line. The mathematical formula is expressed as follows: $\sum {\dot{I} = 0}$. When an internal line fault occurs, there is a short circuit current flowing. If current positive direction is from bus to line, current phasor summation at both ends is equal to the current flowing into the fault point without considering the impact of distributed capacitance, namely $\sum {\dot{I} = \dot{I}_{dj} }$.

Using electromagnetic transient simulation software PSCAD to build a double-ended single line power supply system, the paper has simulated the single-phase grounding, two-phase grounding, the two-phase short-circuit, and three-phase short-circuit failures; F1 is set up at the N-terminus of the line as the external fault, F2 serves as the internal fault, and the fault type and fault time can be set flexibly. The simulation system model is shown in Fig. 5.

Fig. 5.

Differential input current differential protection fault simulation model

Typical fault simulation examples are given as follows. $i_{ma}$, $i_{mb}$, $i_{mc}$ express the three phase currents of M side;

$i_{na}$, $i_{nb}$, $i_{nc}$ express the three phase currents of N side;

$dma$, $dmb$, $dmc$ express the three phase currents differential of M side, namely: $\frac{{di_{ma} }}{dt}$, $\frac{{di_{mb} }}{dt}$, $\frac{{di_{mc} }}{dt}$;

$$ {\text{Restraint}}\;{\text{current}} {\left\{ {\begin{array}{*{20}c} {S_{ja} = \left| {\frac{Dma\angle Pma - Dna\angle Pna}{2}} \right|} \\ {S_{jb} = \left| {\frac{Dmb\angle Pmb - Dnb\angle Pnb}{2}} \right|} \\ {S_{jc} = \left| {\frac{Dmc\angle Pmc - Dnc\angle Pnc}{2}} \right|} \\ \end{array} } \right.} $$

Examples: A phase ground short internal fault (F2/AN).

As shown in Fig. 6, three-phase current, differential current, and braking current waveforms are simulated respectively when point A phase ground short circuit fault in the F2 region occurs. Figure 6 shows that when the internal single-phase ground fault occurs, the differential current of the fault phase (A phase) is more than the braking current; the differential current and the braking current of non-fault phase (B, C phase) are small.

Fig. 6.

Three-phase operating current and restraint current waveforms

The fundamental phase is calculated according to the current sample value after fault, and then the differential current and the breaking current are obtained whose trajectory curve operating point is shown as Fig. 7. It can be seen that the operating point of faulty phase (A phase) is in action area and protection work reliably.

Fig. 7.

A phase ground short internal fault operating characteristic curves diagram

4 Conclusions

In this paper, a new differential protection principle is proposed based on the differential input signal of an electronic transducer. The differential signal of the transducer is applied directly to the protection algorithm, which allows the integral part of the transducer to be omitted so that the full potential of an electronic transducer can be realized. It is proved through theoretical analysis and simulation that the protection principles proposed are correct and feasible.