On the Possibility of Implementing Protection Against Ground Faults for Generators Operating in a Compensated Network Using a Superimposed 25 Hz Current and Phase Current Transformers
Abstract Generated abstract
This paper examines selective ground-fault protection for synchronous generators operating in compensated networks where existing zero-sequence transformer protection may be insensitive to low residual fault currents. It proposes using an externally superimposed 25 Hz monitoring current and investigates whether the responding element can be connected to the neutral of a three phase-current-transformer zero-sequence filter instead of using a special TNPSH transformer. Through magnetization measurements and equivalent-circuit calculations for current transformers under simultaneous 50 Hz and 25 Hz excitation, the authors determine conditions for maximum 25 Hz power transfer and identify an optimum input resistance near 50 ohms. The study concludes that such protection is feasible in principle, that generator no-load operation is the limiting design condition, and that sensitivity should improve under loaded conditions.
Full Text
On the Possibility of Implementing Ground-Fault Protection for Generators Operating in a Compensated Network Using a Superimposed 25 Hz Current and Phase Current Transformers
I. D. Kutyavin, R. A. Vaynshteyn, L. I. Voronova
(Presented by the scientific seminar of the Department of Electric Power Stations)
At present, synchronous generators connected to busbars in most cases supply networks with a significant capacitive ground-fault current, in which arc-suppression coils are installed. Under these conditions, the existing protection of generators against single-phase ground faults, implemented by means of a zero-sequence transformer of the TNPSH type, can operate only in the case when the absolute value of the vector difference between the residual current and the capacitive current of the generator exceeds 5 A.
The possible insensitivity of the protection is considered permissible, since, according to the presently established point of view, it is assumed that a current at the fault location of less than 5 A cannot sustain a stable arc and therefore is not dangerous for the generator. However, the conditions for the burning of a stable arc apparently are determined not only by the absolute value of the current, but also by the nature of the insulation damage and by the voltage across the arc gap; that is, they depend on the number of short-circuited turns. In addition, the presence of a single-phase fault not detected sufficiently rapidly may be a potential source of more severe damage to the generator when a fault appears on another phase. Therefore, the abandonment of selective insulation monitoring of generators operating in a compensated network, under certain operating conditions, appears insufficiently justified.
At the same time, devices have now been developed for selective protection against ground faults of lines in compensated networks, based on superimposing on the network a monitoring current with a frequency different from the industrial frequency.
It is obvious that, if there is a source of monitoring current in the network, the same protection can also be applied to generators.
In this article we consider several questions associated with the development of generator ground-fault protection using a monitoring current with a frequency of 25 Hz.
The source of the monitoring current and the method of its connection, as well as similar protection for cable feeders, are described in [1].
A generator ground-fault protection device that responds to the monitoring current of an external source has a number of fundamental advantages over the existing device, which responds to the natural fault current. These advantages are as follows:
- The device protects 100% of the winding.
- The device operates in the same way when the generator is under load, and also when it is stopped or rotating but not excited. In addition, owing to the difference between the frequency of the monitoring current and the industrial frequency and the frequencies of higher harmonics, it becomes possible to tune out, by frequency, the unbalance currents of the zero-sequence filter.
This circumstance makes it possible to pose the problem of connecting the responding element of the protection into the neutral conductor of a group of three current transformers feeding the measuring instruments and, thereby, to dispense with the installation of a special zero-sequence transformer of the TNPSH type, whose manufacture, installation, and operation are rather complicated. A current transformer of the TNPSH type also requires considerable power (up to 30 VA) from voltage transformers for magnetizing purposes.
In this case, the three-transformer zero-sequence filter will operate under conditions in which a small monitoring current of frequency 25 Hz (up to 1 ampere) flows through the damaged phase, while the cores of all three phases are simultaneously magnetized by a symmetrical operating current, the magnitude of which may vary from 0 to the rated current of the generator.
In order to determine the conditions for transferring power at the monitoring-current frequency into the secondary circuit, we undertook studies of current transformers under simultaneous magnetization by currents of two frequencies—50 and 25 Hz.
The need for a thorough study of this question is dictated by the fact that the expected power at the monitoring-current frequency in the secondary circuits will be very small because of the large transformation ratio of the generator current transformers. Therefore, to facilitate the design conditions for the responding element, it is necessary, in particular, to know its optimum input resistance when various current transformers are used.
To solve the problem posed, simultaneous magnetization curves of electrical steel were taken:
\[ B_{25}=f(H_{25}) \quad \text{at } H_{50}=\text{const}, \]
where
\[ \begin{aligned} B_{25} &\text{ is the component of induction with frequency } 25\ \text{Hz};\\ H_{25} &\text{ is the component of field strength with frequency } 25\ \text{Hz};\\ H_{50} &\text{ is the component of field strength with frequency } 50\ \text{Hz}. \end{aligned} \]
To take the magnetization curves, the cores of TShFD 4000/5 transformers were used.
Sinusoidal currents \(I_{50}\) and \(I_{25}\), with frequencies of 50 and 25 Hz respectively, are passed through two primary windings connected oppositely with respect to one another (Fig. 1, a). The secondary windings of the transformers are connected so that the e.m.f.s of frequency 25 Hz are summed, while the e.m.f.s of frequency 50 Hz are subtracted. Owing to the unavoidable nonidentity of the cores, the secondary e.m.f., in addition to the frequency 25 Hz, contains the frequency 50 Hz and higher harmonics. When \(I_{50}>I_{25}\), the interference many times exceeds the e.m.f. component of frequency 25 Hz to be measured. To make it possible to carry out measurements under such conditions, a special electronic frequency-selective circuit was used.
The measurement results are presented in Fig. 1, c, from which it is evident that for any \(I_{50}\) the dependence \(B_{25}=f(H_{25})\) is expressed by a straight line; that is, the magnetic permeability at frequency 25 Hz,
\[ \mu_{25}=\frac{B_{25}}{H_{25}}, \]
does not depend on \(H_{25}\) at any constant value of the current \(I_{50}\). The resistance—
the magnetizing branch of the transformer at the frequency \(\chi_{25}\) also depends only on \(I_{50}\).
Assigning different values to \(I_{50}\), we obtain the curve
\[ \mu_{25}=f(I_{50}) \]
(Fig. 2), from which it is seen that the minimum magnetic permeability occurs at \(I_{50}=0\).
As indicated above, in order to develop the responding element it is necessary to determine its optimum input resistance. By the optimum resistance we shall mean the resistance at which maximum power transfer at a frequency of 25 Hz is ensured.
The current distribution is calculated according to the equivalent circuit of a three-transformer filter of zero-sequence currents (Fig. 3), where
\(X\) is the inductive resistance of the magnetizing branch of one current transformer at a frequency of 25 Hz;
\(R_{n1}\) is the maximum active resistance of one phase;
\(R_{n.f.}\) is the active resistance of the responding element connected into the neutral conductor of the three-transformer filter;
\(I_{25}\) is the test current with a frequency of 25 Hz;
\(I'_{25}\) is the test current in the circuit of the responding element, referred to the number of turns of the primary winding.
Fig. 1. a) Measurement circuit; b) curves of simultaneous magnetization by currents with frequencies of 50 and 25 Hz
For the possibility of subsequent comparison of experimental and calculated data, the parameters of the equivalent circuit were determined for the available TShFD—4000/5 transformers, although in practice it is assumed that current transformers of accuracy classes 0.5 and 1 will be used.
The parameters of the transformers used are as follows:
\(q\) is the cross section of the core steel, \(20.8\cdot10^{-4}\ \mathrm{m^2}\);
\(w_2\) is the number of turns of the secondary winding, 800;
\(l_{\mathrm{av}}\) is the mean force line, 1.09 m;
\(w_1\) is the number of turns of the primary winding, 1.
Fig. 2. Dependence of magnetic permeability at a frequency of 25 hertz on the biasing current with a frequency of 50 hertz
Taking the resistance of the responding element to be purely active and varying it in the range from 5 to 1000 ohms, we obtain the dependence of the secondary current \(I_{2,25}\) and the secondary power \(P'_{25}\) on the magnitude of the load of the filter \(R_{n.f.}\) (Fig. 3, b).
As the calculated magnetization curve, the curve \(B_{25}=f(H_{25})\) at \(I_{50}=0\) was adopted, since in this case \(\mu_{25}\) is the smallest.
As was to be expected, the dependence obtained has a very sharply pronounced maximum; therefore the choice of an input resistance equal to the optimum will considerably facilitate the design of the responding element.
Fig. 3. a) Equivalent circuit of the three-transformer filter; b) dependence of the secondary power on the load resistance of the three-transformer filter
The variation of the secondary power as a function of the generator load current, within the range from 0 to rated current, at the above-determined optimum resistance is shown in Fig. 4.
Analysis of this dependence shows that, at any load current, the secondary power does not fall below the value corresponding to the most unfavorable condition at \(I_{50}=0\). Moreover, beginning with a current \(I_{50}\) equal to five percent of the rated generator current, the secondary power reaches its maximum value and does not change up to the rated generator current.
Thus, as the calculated condition in which the required sensitivity of the responding element must be ensured, the generator no-load condition should be taken. In all other conditions the sensitivity will be all the more ensured.
Fig. 4. Curve 1—the dependence of the secondary power at the frequency of 25 Hz of the zero-sequence filter on the generator operating current; curve 2—the same for the secondary current of 25 Hz; curve 2—calculation; curve 3—experiment
From the curve in Fig. 3b it is seen that the optimum resistance is a value close to fifty ohms. Connecting such a resistance in the neutral conductor of the group of three transformers of the filter under normal conditions and under phase-to-phase short circuits will not affect the operation of the current transformers. Only in the case of double ground faults does there arise the danger of considerable overvoltages due to strong saturation of one of the current transformers. These overvoltages can be eliminated by connecting, in parallel with the input of the responding element, a surge arrester rated for the corresponding voltage.
Conclusions
-
There is a fundamental possibility of improving generator ground-fault protection by using a superimposed monitoring current with a frequency different from the network frequency.
-
Owing to the use of frequency detuning of the responding element from unbalance currents, it may prove possible to implement the protection on a zero-sequence transformer filter instead of a TNPSH.
-
As the calculation mode for designing the responding element, the generator no-load mode should be adopted. In all other modes, the sensitivity of the responding element will be higher.
References
- R. A. Vainshtein, A. V. Shmoilov, L. I. Voronova. Use of an electromagnetic parametric frequency divider as a source of monitoring current for ground-fault protection. The present collection.