Figure 1[Marx generator] [1]
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Figure 2[standard impulse waveform] [2]
Figure 3[Proteus simulation of a voltage doubler] [4]
Figure 4[DC waveform after rectification] [5]
Figure 5[Cockcroft – Walton multiplier] [3]
Figure 6[Cockcroft Walton’s waveforms showing that is shifting] [3]
Figure 7[cascaded transformer for HVAC] [8]
Figure 8[sinusoidal waveform] [9]
The aim of this report is to implement various techniques used to generate high voltages for many applications used around the world. This report further analysis the importance of health and safety when dealing with high voltage levels in laboratories or at work. Furthermore, by the end of this report students should have a thorough understanding of how different waveforms are implemented from different high voltage generating methods.
Objectives
Understanding HVAC ,HVDC
Impulse generation
Healthy and safety related to high voltage
Importance of their generated waveforms
Standards on generation
High voltages are used in many applications around the world such as:
Medical imaging
Electrical power distributions
High-power amplifier vacuum tubes
X-rays
Testing laboratories and facilities
3.1 Generating High Impulse Voltage
Single stage generator
Multistage-Marx generator
A Marx generator is used to generate high voltages from a low DC supply which can be used in applications such as an x-rays and large power transformers.
3.2 Operation of the Marx generator
By analysing figure 1 it can be seen that all capacitors are charged up in parallel to
Vin
via the resistors just like an RC circuit charging process. The value of the resistors should typically be reasonably high due to the presence of high voltage. Once the first capacitor is fully charged to Vin the first spark gap breaks down, the voltage on the next stage increases then the spark gap breaks down and passes on to the next stage (like a ladder). Once reached the last stage the low impedance of the ionised air in the sparks effectively links all the capacitors and discharges them in series. By multiplying the supply voltage by the total number of capacitors this results to one large spark at the output of the Marx generator.
Figure 1[Marx generator] [1]
3.3 Impulse waveform
Figure 2[standard impulse waveform] [2]
Figure 2 shows a standard impulse waveform. Impulse tests are carried out on Power Systems Components to ensure that components of high voltage systems are adequately resistant to lightning strikes and other high voltage transients produced by switching events and system faults. [3] By definition an impulse is a unidirectional current or voltage swiftly rising to its peak value.
3.4 Generating High DC Voltage
Full and half wave rectifiers
Doubler
Cockcroft-Walton
3.5 Voltage doubler and Cockcroft-Walton
A voltage doubler works in a very simple manner which consists of semiconductor diodes and capacitor. These capacitors are charged in order to give double the input voltage on the output.
Figure 3[Proteus simulation of a voltage doubler] [4]
Figure 3 illustrates a voltage doubler, multiplying the input voltage by 2 due to the arrangement of the circuit. By analysing this configuration it can be seen that when current flows through D1 (forward biased) and charges up the capacitor C1 when the input voltage is positive, however when input voltage is negative current flows through D2 (reverse biased) charging up the second capacitor C2 and the output voltage as the name implies ‘multiplier’ is doubled at the output due to the capacitors which are implemented in series. The output voltage is then a DC waveform converted from the AC input voltage.
If there is a load on the output, it can be said:
VL<2*Vin
With no load:
VC1+Vc2=2*Vin
3.6 DC waveform
Figure 4[DC waveform after rectification] [5]
As shown in figure 4, the mean value in a DC voltage is defined between the highest and lowest level in a given period, where the duration period depends on the generating system. [6] This DC waveform is obtained from a voltage doublers output, by means of rectification. This process uses diodes, and capacitors in series at the output to double the voltage. The diodes are arranged in a way, so they can conduct at different times during each half cycle while the other is reversed biased resulting to a DC output wave form as shown in figure 4. Also, the capacitors at the output help to smoothen the DC line.
3.7 Ripple voltage
As shown in figure 4 The difference between
Vin
and
Vpk
is the
∆V
which is referred to as ripple voltage, this is due to the incomplete suppression of the AC waveform after rectification within the system. [7] The capacitors charging and discharging process has an effect on this ripple voltage. This could be reduced by increasing the Capacitance value or the frequency.
The equation given for ripple voltage is:
V=I*n2fc(1+n)
3.8 Cockcroft-Walton
In order to obtain very high voltages at the output, the voltage doubler technique is cascaded on top of each other which becomes the Cockcroft-Walton multiplier. Where it enables you to add n amount of stages required to get the desire output voltage, however there is a limit. As illustrated in figure 5 this design is a three-stage multiplier (n=3). For example, in this figure the voltage is 3 times the voltage at stage 1 and it does not necessarily double the voltage but multiplies.
n can be found by using the equation:
n=Vin*fCI
Figure 5[Cockcroft – Walton multiplier] [3]
When voltage is first induced, and its polarity is
–Vpk
C1 is charged from D1 (forward biased), once polarity has switched the current from C1 flows through D2 and not D1(reverse biased), charging up C2. The voltage at point F is than
2*Vin
relative to ground. When the polarity switches again current flows from C2 through D3 charging up C3. When it changes once more current from C3 goes through D4 charging up C4, now point G is
4*vin
. This process happens once more as is 3 stages, once finished point H will be
6*Vin
relative to ground.
Figure 6[waveform shifting at each stage] [3]
This is the input and output waveform at each stage which shows it is multiplying the input by 2 at stage one then the waveform shifts again by two and then by another 2. Resulting to
6*Vmax
(the reference point keeps shifting).
Table 1[voltage levels]
Tesla coil
Resonant transformer
Cascaded transformer
5.1 Cascading Transformers
If voltages more than 400kV are required than it is essential to cascade two or more transformers in order to get the desired AC output voltage. By analysing figure 4 it can be seen that at the initial stage the transformers primary winding which is connected to a low voltage AC source has the same amount of turns as and tertiary winding. This is than fed to the primary transformer of the second stage. During the second stage the transformers secondary winding is connected in series with the secondary winding of the first stage transformer. Resulting to 2*V across the secondary winding at stage 2. For stage three the same process repeats resulting to 3*V at the output.
Figure 7[cascaded transformer for HVAC] [8]
High AC voltages are frequently used in transmitting and distribution lines due to being relatively easier to transport over long distances, at high voltages less energy is lost in the transmission. They are very useful as the voltage levels can be stepped up or down by using transformers in the means of induction. The sinusoidal waveform for HVAC is given by the equation:
Vt=Vpeak*sin(2πf(t)+∅)
Vpeak
is the amplitude of the peak voltage
Sin indicates that the waveform will be a sinusoidal
The frequency of the waveform is defined as
f=1t
2f
which is equal to
ω
defines how many radians per second the phasor will move.
∅
represents the phase of the sinewave which is measured in degrees
Figure 8[sinusoidal waveform] [9]
The root means squared value which is an important parameter of a sinusoidal wave is given by:
Vrms=Vpk2
Advantages and disadvantages of the cascaded transformer:
Disadvantage
Requires space for installation and it can be costly
Advantages
It can be available indoors and outdoors
For three units, three phase connection in star or delta is possible
Better cooling [10]
When dealing with high voltage levels extra precautions must be considered such as:
The correct PPE should be worn when present in high voltage labs or vising high voltage sites such as Bolney super-grid switching station.
If the controls system faults, the system response may lead to a dangerous working environment if it is closely coupled with human interaction. To prevent this from happening the generators must have an external emergency shut down switch which can switch off the high voltage source
All labs must be proper insulated to stop the high voltage from reaching out
High voltage capacitor must be shorted out to earth
The operator must watch out for short circuiting and live wires to avoid electrocutions
A proper risk assessment must be issued by a senior person in charge
Specified distance to the high voltage equipment should be monitored, all objects to ground potential must be placed away from all exposed high voltage points at a minimum distance of 1 inch for every 7500 volts [11]
All cables must be tucked well away to avoid trips
All HVDC Conductors shall be considered to be operating at their nominal voltage regardless of DC Operating Configuration [11]
All high-voltage generating equipment should have an indicator which signals that the high-voltage output is enabled [11]
All high voltage equipment must have an interlock [11]
Safety inspection on the equipment
Table 2[high-voltage standards] [12]
Table 2 shows the standard requirements used for high voltage generators, this is designed so the operator can analyse to see whether the test equipment used can handle the operation standards which is the recommended high voltage standards for testing as illustrated above.
With reference to the aims and objectives to this report, the following criteria’s were achieved:
Three methods of implementing high voltages were analysed. Using DC, AC and impulse to generate high voltages for different purposes and applications.
When dealing with high voltages it is important to understand the health and safety aspects and further precautions must be taken, as this is an important feature when dealing with high voltage applications, failure to do so it can result to serious injuries and death.
Furthermore, the significance of the given waveforms for various generating methods along with their standards were described and analysed.
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