HVDC vs HVAC Power Transmission

1. Need for High Voltage Power Transmission Systems

• Installation of high power renewable energy sources like offshore wind farms are growing continuously due to the continuous increase in power demand. The generated offshore wind power cannot be supplied directly to the customers; instead, it can be integrated with the grid due to non-controllable variability, unpredictable generation, and installed locations (the offshore wind source is located in the sea area that is far away from the AC grids or load Centre).

• To transfer the high power from one region to another because one region may not have enough power generation sources and might have lots of power demand. In such a case, the power can be transmitted from the power generation region to the power demand region.  

2. Possible Options to Transmit High Power for Long-distance

• High Voltage Alternating Current (HVAC) transmission system uses alternating current for electric power transmission. Fig. 1 connects the large-scale offshore wind farm with the AC grid via an HVAC link.

• High Voltage Direct Current (HVDC) transmission system uses direct current for electric power transmission. The HVDC configuration has power electronic converters at both ends like the sending end (called as Rectifier) and receiving end (called as Inverter). Fig. 2 connects the large-scale offshore wind farms with the AC grid via the HVDC link.

2.1. High Voltage Alternating Current (HVAC) Transmission

The HVAC transmission is applicable for short distances such as below 50 km, but it is not suitable for long-distance power transmission due to large charging current since the cable capacitance is significantly high. Further, it is difficult to control the power flow, requires reactive power control, and also has the limitation of high losses. Moreover, it does not have an asynchronous operation and also has an issue with the skin and Ferranti effect. 

2.2. High Voltage Direct Current (HVDC) Transmission

Due to these drawbacks in the HVAC transmission, the application of HVDC has been increased. The HVDC link is the economical option for transmitting bulk power over long distance. It is used for the following purposes:

• Long-distance bulk power transmission: To transfer high power from renewable energy source location to AC grid or one region to another.

• Back-to-back connection: To control the AC & DC systems (to control the power & frequency).

2.3 Comparison of HVDC vs HVAC

The detailed comparison of HVDC vs HVAC transmission system is presented below:

2.3.1. Reactive Power Loss

Reactive power loss is present in the AC system while it does not exist in the DC system. In a DC system, there is only resistive loss and hence it can transfer more active power. However, the reactive power requirement of an LCC-HVDC converter is about 50 to 60 % of active power transferred in a steady state. 

The resistive and reactive power losses can be expressed by,

2.3.2. Stability

In an AC system, the power flow depends on the bus voltages, the reactance of the line, and the phase angle difference between the buses. Usually, the phase angle difference may not exceed 20 to 30 degrees to maintain synchronism and stability. The power flow scenario in AC system is shown in Fig. 3.

In a DC system, the power flow depends on the voltage and resistance, and it does not depend on the phase angle. HVDC system can be loaded up to the thermal limit and no need to consider the stability limit.

2.3.3. Power Flow Control

The power flow control is not possible in AC transmission systems. However, it is possible with the use of Flexible AC Transmission Systems (FACTS) devices. In DC transmission system, the power flow control is possible with the HVDC converter control in both positive and negative direction.

2.3.4. Cost of HVAC & HVDC Transmission

The cost comparison of HVAC and HVDC transmission is shown in Fig. 4. The initial cost for the HVDC system is higher because of converter stations. So, the HVAC is cheaper for short-length lines. As the distance increases, the charging current of AC transmission system will be increased, therefore losses are higher, and it requires compensation devices. So, HVDC transmission system is an economical option for long-distance bulk power transmission. The breakeven distance for cables is 40 – 70 km and for the overhead lines is 600 – 800 km.

2.3.5. Asynchronous Operation

In the AC system, all the frequencies of the interconnected system should be the same in the steady state. In a DC transmission system, the frequency can be varied using HVDC converters, and therefore asynchronous operation is possible in DC transmission system.

2.3.6. Current Carrying Capacity

The current carrying capability of an AC transmission system is very low when compared to a DC transmission system because of the reactive component of the current flowing through the transmission lines or cables.

2.3.7. Skin Effect

When alternating current flows through the conductor, it is not distributed uniformly throughout the conductor cross-section (the inner layer has less current and the outer layer has higher current) due to the frequency of the system. The effective cross-section of the conductor is reduced because the whole cross-section of the conductor is not used. So, the effective resistance will be higher and hence the losses are higher. It is called the skin effect and it is shown in Fig. 5. In the DC transmission system, there is no skin effect because the frequency of the DC system is zero. So, the effective resistance in the AC transmission system is greater than the DC transmission system as given below,

Rac > Rdc

2.3.8. Ferranti Effect

When there is no load or lightly loaded condition in the AC transmission system, the receiving end voltage is higher than the sending end due to the charging current (capacitive current can lead to the generation of reactive power) of the line or cable. This is called as Ferranti effect and it is shown in Fig. 6. The Ferranti effect can lead to the voltage instability. The shunt reactor can be connected at the receiving end to absorb this reactive power generated by the charging of the line. This Ferranti effect is not present in the DC transmission system.  

2.3.9. Requires less space in the DC transmission system compared to the AC system for the same voltage rating and size

For example, consider a 400 kV voltage system, In an AC system, 400 kV is a RMS value, however, the AC transmission system is designed for the peak value of voltage. In the DC system, the transmission system is designed for the same 400 kV. So, the DC system requires less space in terms of Insulators, cross arms, and substation clearance design.

2.3.10. Ground wire as a return conductor in HVDC

In monopolar, bipolar, and homopolar HVDC systems, the ground wires can be used as a return conductor. So, it can reduce the cost of the conductor. However, the ground current can introduce communication interference and erosion problems. However, In HVDC transmission, the ground wire can be used if there is a failure in the conductor. In an AC system, the conductor should be repaired or replaced if any failures.

2.3.11. Corona Loss and radio Interference

Corona loss (P) is proportional to the frequency (f+25). Since the frequency is zero in DC transmission, HVDC may give less corona loss and radio interference.

References

• High voltage DC Transmission, NPTEL Lectures by Prof. S.N. Singh.

• J Arrillaga, High Voltage Direct current Transmission.

• . KR Padiyar, HVDC Power Transmission Systems.