Generation of High AC, DC
and Impulse Voltages
Introduction
For a transmission line of surge impedance ZL (approx.
250 ohms) at an operating voltage V, the power transfer capability is approximately P= V2/ZL
V (kV) 400 700 1000 1200 1500
P (MW) 640 2000 4000 5800 9000
The rapidly increasing transmission voltage level in recent decades is a result of the growing demand for electrical energy, coupled with the development of large hydroelectric power stations at sites far remote from centers of industrial activity and the need to transmit the energy over long distances to the centers.
Voltage Stresses
• Normal operating voltage does not severely stress the power system’s insulation.
• However, the operating voltage determines the dimensions of the insulation which forms part of the generation, transmission and distribution equipment.
• The voltage stresses on power systems arise from various overvoltages. These may be of external or internal origin.
• External overvoltages are associated with lightning discharges and are not dependent on the voltage of the system. As a result, the importance of stresses produced by lightning decreases as the operating voltage increases.
Voltage Stresses
• Internal overvoltages are generated by changes in the operating conditions of the system such as switching operations, a fault on the system or fluctuations in the load or generations.
• Their magnitude depends on the rated voltage, the instance at which a change in operating conditions occurs, the complexity of the system and so on.
• Since the change in the system’s conditions is usually associated with switching operations, these overvoltages are generally referred to as switching overvoltages.
Voltage Stresses
In designing the system’s insulation the two areas of specific importance are:
• Determination of the voltage stresses which the insulation must withstand, and
• Determination of the response of the
insulation when subjected to these voltage
stresses.
Testing Voltages
• Power systems equipment must withstand not only the rated voltage, which corresponds to the highest voltage of a particular system, but also overvoltages.
• The magnitude and type of test voltage varies with the rated voltage of a particular apparatus.
• The standard methods of measurement of high-voltage and the basic techniques for application to all types of apparatus for alternating voltages, direct voltages, switching impulse voltages and lightning impulse voltages are laid down in the relevant national and international standards.
Testing Voltages
A High voltage laboratory normally should at the very least be able to carry out the following tests:
• HV power frequency AC tests
• HV DC tests
• HV impulse withstand
These tests might be applicable to all components and subassemblies of transmission and distribution networks, such as cables, switchgear, and transformers.
Generation of High Alternating Voltages
• As electric power transmission with high a.c. voltages predominates in the transmission and distribution systems, the most common form of testing h.v.
apparatus is related to high a.c. voltages.
• In every laboratory HVAC supplies are therefore in common use with voltage levels ranging from about 10 kV r.m.s. to more than 1.5MV r.m.s.
• In general, all a.c. voltage tests are made at the nominal power frequency of the test objects.
• Although power transmission systems are mostly of three-phase type, the testing voltages are usually single- phase voltages to ground.
Generation of High Alternating Voltages
• The waveshapes must be nearly pure sinusoidal with both half-cycles closely alike.
• The ratio of peak-to-r.m.s. voltage must be equal to √2 within ±5 per cent.
• The r.m.s. value of the harmonics should not exceed 5 per cent of the r.m.s. value of the fundamental.
• Testing of h.v. apparatus or h.v. insulation always involves an application of high voltages to capacitive loads with low or very low power dissipation only.
Generation of High Alternating Voltages
The capacitance of test equipment Ct may change considerably, depending upon the type of equipment.
Typical values are:
• Simple post or suspension insulators : ~ 10 pF
• Bushings, simple and graded : ~100–1000 pF
• Potential transformers : ~200–500 pF
• Power transformers
<1000 kVA : ~1000 pF
>1000 kVA : ~1000–10 000 pF
• H.V. power cables:
Oil-paper impregnated : ~250–300 pF/m
Gaseous insulated : ~60 pF/m
• Metal clad substation, SF6 insulated : ~1000 – >10 000 pF
Generation of High Alternating Voltages
• If Ct is the capacitance of the equipment or sample under test, and Vn the nominal r.m.s. voltage of the h.v. testing supply, the nominal KVA rating Pn may be calculated from the design formula
Pn = kVn2ωCt
in which the factor k ≥ 1 accounts for additional capacitances within the whole test circuit and some safety factor.
• Examples for additional capacitances are h.v. electrodes and connections between test object and voltage source,, or measurement devices as, e.g., capacitor voltage dividers or sphere gaps frequently incorporated within the test circuit.
Generation of High Alternating Voltages
• The nominal current In = Pn / Vn , may range from some 10mA for testing voltages of 100 kV only, up to amperes in the megavolt range.
• Since most of the test voltages are only of short duration, the nominal ratings are, therefore, often related to short time periods of 15 min.
• Due to the relatively large time constants for the thermal temperature rise, no sophisticated cooling systems are in general necessary within the voltage testing supplies.
Testing Transformers
• The power frequency single-phase transformer is the most common form of HVAC testing apparatus.
• They are designed for operation at the same frequency as the normal working frequency of the test objects (i.e., 60 or 50 Hz).
• They may also be used for higher frequencies with rated voltage, or for lower frequencies, if the voltages are varied in accordance to the frequency, to avoid saturation of the core.
• From the considerations of thermal rating, the kVA output and the fundamental design of the iron core and windings there is not a very big difference between a testing and a single-phase power transformer.
Testing Transformers
The differences between a testing and power transformer are related mainly to
• a smaller flux density within the core to avoid unnecessary high magnetizing currents which would produce higher harmonics in the voltage regulator supplying the transformer.
• a very compact and well insulated h.v. winding for the rated voltage.
Testing Transformers
Fig. 1(a) shows the circuit diagram. The primary winding ‘2’
is usually rated for low voltages of ≤1 kV, but might often be split up in two or more windings which can be switched in series or parallel to increase the regulation capabilities. The iron core ‘1’ is fixed at earth potential as well as one terminal of each of the two windings. Simplified cross-section detailing the construction for the unit itself is given in Fig. 1(b)
Testing Transformers
Fig.1Single unit testing transformers. (a) Diagram. (b) construction (1) Iron core. (2) Primary l.v. or exciting winding.(3) Secondary h.v.
winding. (4) Field grading shield. (5) Grounded metal tank and base.
(6) H.V. bushing. (7) H.V. electrode
Testing Transformers
• The sectional view of the windings shows the primary winding close to the iron core and surrounded by the h.v. winding ‘3’.
• This coaxial arrangement reduces the magnetic stray flux and increases, therefore, the coupling of both windings.
• The beginning (grounded end) of the h.v.
winding is located at the side close to the core, and the end close to a sliced metal shield, which prevents too high field intensities at h.v.
potential.
Testing Transformers
• Between both ends the single turns are arranged in layers, which are carefully insulated from each other by solid materials (kraft paper sheets for instance).
• Adjacent layers, therefore, form coaxial capacitors of high values, and if those capacitances are equal – produced by the reduced width of the single layers with increasing diameters – the potential distribution for transient voltages can be kept constant.
• By this procedure, the trapezoidal shape of the
cross-section is originated.
Cascaded Transformers
Fig 2 Cascade transformer connection (schematic) V1 — Input voltage, V2 — Output voltage
aa' — L.V. primary winding, bb' — H.V. secondary winding cc' — Excitation winding, bd — Meter winding (200 to 500 V)
Cascaded Transformers
• For voltages higher than about 300 to 500 kV, the cascading of transformers is a big advantage, as the weight of a whole testing set can be subdivided into single units and therefore transport and erection becomes easier.
• Fig.2 shows the cascade transformer units in which the first transformer is at the ground potential along with its tank.
• The second transformer is kept on insulators and maintained at a potential of V2, the output voltage of the first unit above the ground.
• The high voltage winding of the first unit is connected to the tank of the second unit.
Cascaded Transformers
• The low voltage winding of the second transformer is supplied from the excitation winding of the first transformer, which is in series with the high voltage winding of the first transformer at its high voltage end.
• The rating of the excitation winding is almost identical to that of the primary or the low voltage winding.
• The high voltage connection from the first transformer winding and the excitation winding terminal are taken through a bushing to the second transformer.
• In a similar manner, the third transformer is kept on insulators above the ground at a potential of 2V2 and is supplied likewise from the second transformer.
• The number of stages in this type of arrangement are usually two to four, but very often, three stages are adopted to facilitate a three-phase operation so that
√3V2 can be obtained between the lines.
Cascaded Transformers
• Supply to the units can be obtained from a motor- generator set or through an induction regulator for variation of the output voltage.
• The rating of the primary or the low voltage winding is usually 230 or 400 V for small units up to 100 kVA. For larger outputs the rating of the low voltage winding may be 3.3kV, 6.6 kV or 11 kV.
• Cascaded transformers are the dominating HVAC testing units in all large testing laboratories.
• The world’s largest a.c. testing station at WEI Istra near Moscow, Russia, is equipped with a cascaded testing transformer rated for 3MV, 12MVA.
• The disadvantage of transformer cascading is the heavy loading of primary windings for the lower stages.
Resonant Transformers
• The equivalent circuit of a high voltage testing transformer consists of the leakage reactances of the windings, the winding resistances, the magnetizing reactance, and the shunt capacitance across the output terminal due to the bushing of the high voltage terminal and also that of the test object. This is shown in Fig. 3.
Fig.3 Transformer Equivalent Circuit
L0 — Magnetizing inductance, R0 — Resistance due to core loss L1, L2— Leakage inductances of the transformer
r1, r2— Resistances of the windings
Resonant Transformers
• It may be seen that it is possible to have series resonance at power frequency ω, if (L1 + L2)= 1/ ωC. With this condition, the current in the test object is very large and is limited only by the resistance of the circuit. The waveform of the voltage across the test object will be purely sinusoidal. The magnitude of the voltage across the capacitance C of the test object will be
where R is the total series resistance of the circuit.
•If resonance occurs accidentally, then at supply frequency the effect can be extremely dangerous, as the instantaneous voltage application can be of the order of 20 times the intended high voltage.
Resonant Transformers
• This has given rise to some vicious explosions during cable testing. Resonance of a harmonic can similarly occur, as harmonic currents are present due to the transformer iron core.
• With the series resonant set, however, the resonance is controlled at fundamental frequency and no unwanted resonance can therefore occur.
• The tuned series resonant h.v. testing circuit,
thus arose as a means of overcoming the
accidental and unwanted resonance to which
the more conventional test sets are more prone.
Resonant Transformers
• The factor Xc/R = 1/ ωCR is the Q factor of the circuit and gives the magnitude of the voltage multiplication across the test object under resonance conditions.
Therefore, the input voltage required for excitation is reduced by a factor 1/ Q, and the output kVA required is also reduced by a factor 1/Q. The secondary power factor of the circuit is unity.
• In a series resonant circuit, a transformer with 50 to 100 kV voltage rating and a relatively large current rating is connected together with an additional choke, if necessary as shown in figure 4.
Resonant Transformers
Fig. 4 Series resonant a.c. test system
Resonant Transformers
Fig. 5 Parallel resonant a.c. test system
Resonant Transformers
• A voltage regulator of either the auto-transformer type or the induction regulator type is connected to the supply mains and the secondary winding of the exciter transformer is connected across the H.V. reactor, L, and the capacitive load C.
• The inductance of the reactor L is varied by varying its air gap and operating range is set in the ratio 10 : 1.
Capacitance C comprises of the capacitance of the test object, capacitance of the measuring voltage divider, capacitance of the high voltage bushing etc.
Resonant Transformers
The chief advantages of the resonant transformer are:
(a) it gives an output of pure sine wave,
(b) power requirements are less (5 to 10% of total kVA required), (c) no high-power arcing and heavy current surges occur if the
test object fails, as resonance ceases at the failure of the test object,
(d) cascading is also possible for very high voltages, (e) simple and compact test arrangement, and
(f) no repeated flashovers occur in case of partial failures of the test object and insulation recovery.
The disadvantages are the requirements of additional variable chokes capable of withstanding the full test voltage and the full current rating.
