We all know that to build an extended cable line, we have to replace HVAC technology with HVDC technology. However, where does one get the value of the length of the line, after which the transition to HVDC can no longer be avoided? It turns out that this length can be determined by calculation, and the most primitive one, which is done using a pocket calculator.

It is believed that long HVAC lines have the following problems, the presence of which pushes us to more complex HVDC technology:
1️⃣An increase in AC 50 Hz voltage at the end of the line when the line is powered from its one side.
2️⃣Significant reactive power generated by the capacitance of the cables, which requires compensation using shunt reactors.
3️⃣Significant charging current (capacitive no-load current), which heats the cable and creates problems for the circuit breakers when switching the line.
4️⃣Significant losses of active power in the cores, including due to the passage of the reactive power along the line.
5️⃣Other issues.

If we consider all 5 issues together, it turns out that for cable lines up to 220 kV, the maximum HVAC length is about 100 km. If you need to build longer 6-220 kV lines, you are guaranteed to need one of two measures:
✅Equipping the line at the ends with shunt reactors, preferably controlled.
✅Transition from HVAC technology to HVDC technology.

Where did 100 km come from? This is shown in part 8 of the book. Well, today we will consider only point 1 – check the voltage at the end of the line. Usually, the single-side power supply mode of the line is limited in time. We will assume that it is no more than 20 minutes, and then the insulation strength of the main equipment (including the characteristics of the surge arresters) is such that it is allowed to increase the voltage by 15% above the maximum operating voltage of the equipment/network.

How much voltage increases at the end of the line depends on two factors:
➡️The inductive impedance of the network (Xn), i.e. the current of the three-phase short circuit of the network, which is usually greater than 20 kA, and therefore usually Xn < 5 Ω.
➡️The wave impedance of the line (Zw), which is usually Zw > 12.5 Ω.

Then calculations based on formulae show that HVAC lines, from the point of view of voltage increase, can have a length of up to 100 km, even without reactors at the ends. Similarly, reactive power and other issues can be easily calculated, and it will also be shown that 6-220 kV cable lines up to 50-100 km do not require either reactors or HVDC.

MILESTONE
Everyone knows what role CIGRE plays for the global energy sector. For example, we all know dozens of perfect CIGRE technical brochures on completely different energy-related topics. So, it is with great pleasure that I must say that this week the CIGRE President joined us – Konstantin O. PAPAILIOU. I must admit that this gave me the extra strength I needed to continue working on the preparation of new materials. And, of course, great thanks to all LinkedIn group members as well.