HVDC Circuit Breaker:
In a.c circuit breakers, arc extinction is achieved at the natural current zero of the a.c. waveform used.But in d.c circuit breakers, natural zero of voltage and current is not available as both are continuously available.Thus for extinction of an arc, artificial current zero is required to be introduced.In this, we will discuss HVDC circuit breaker Working and construction.
Such an artificial current zero is possible by connecting LC circuit in parallel with the circuit breaker.The LC circuit is responsible to produce oscillatory arc currents having many artificial current zeros.At one of the artificial current zero arc gets extinguished.
HVDC Circuit Breaker Working Principle:
The below figure shows the principle of HVDC circuit breaker.The HVDC circuit breaker is nothing but a vacuum circuit breaker.The LC circuit along with a switch is connected in parallel with this circuit breaker.
Must Read:
The capacitor C used is pre-charged capacitor with the polarities as shown in the above figure. When the HVDC circuit breaker starts opening, the switch in the LC circuit gets closed. Due to the charged capacitor, the discharging current starts flowing in opposite direction to that of load current carried by the circuit breaker. Due to this, arcing current starts oscillating producing many natural zeros. Thus artificial commutation results and arc extinction is achieved.
The large transient recovery voltage is the main constraint in HVDC circuit breaker and circuit breaker must able to withstand it.For successful operation of HVDC circuit breaker, the switch in LC circuit must be a high-speed switch with a very fast response.Such systems are complex and are costly as they require costly protection system and control systems.
Selection of Circuit Breaker:
The following parameters are required to be known for selecting proper rating circuit breaker at a given location on a power system:
i)The maximum fault current which is to be interrupted by the breaker
ii) The maximum current to be carried momentarily.
As it is discussed previously the fault current consists of both a.c and d.c components and its correct calculation is very complex.A simplified method is recommended by IEEE committee is given below:
1)To determine firstly the required interrupting capacity of circuit breaker the highest value of initial rms alternating current for any type and location of fault. It can be considered as three phase fault as it carries maximum fault current except in some cases.
This current can be obtained by using sub-transient reactance for generators and transient reactance for synchronous motors while induction motors are to be neglected.Following multiplying, factors can be applied to take into account the d.c components of currents and decrements of both a.c. and d.c components.
8 cycle or slow breaker 1.0
5 cycle breaker 1.1
3 cycle breaker 1.2
2 cycle breaker 1.4
Before applying multiplying factor, all the given factors are increased by 0.1 for the breakers on the generator bus where 3 phase short circuit KVA exceed 500,000.
2)To determine the required rated momentary current with time 1 sec or less of a breaker. The calculation of the highest value of initial rms a.c current can be done as given in the step(1) except using sub-transient reactances of all the machines including induction motors. Multiply the value of current obtained by 1.6.Momentary rated r.m.s current of a circuit breaker = 1.5 x V/X″d
Advantages of HVDC Circuit Breakers:
- Reduced Transmission Losses: HVDC systems inherently have lower transmission losses compared to traditional AC systems, making them more efficient for long-distance power transmission.
- Long-Distance Transmission: HVDC technology is particularly well-suited for transmitting electricity over very long distances, such as across continents or under the sea, where AC transmission could suffer from significant losses and reactive power issues.
- Grid Interconnection: HVDC systems can connect asynchronous AC grids, enabling power exchange between regions with differing frequencies and phase angles. This is especially valuable for international power sharing and interconnections.
- Stable Operation: HVDC transmission provides better control over power flow and voltage stability, reducing the risk of cascading failures and blackouts.
- Optimized Power Flow: HVDC systems allow for precise control of power flow, enabling efficient utilization of transmission resources and reducing congestion on overloaded transmission lines.
- Harmonics Mitigation: HVDC transmission reduces harmonic distortions in the power system, leading to improved power quality.
Disadvantages of HVDC Circuit Breakers:
- Complex Technology: HVDC circuit breakers are intricate devices involving complex control and protection systems. This complexity can lead to higher costs in terms of both manufacturing and maintenance.
- High Initial Costs: The installation of HVDC transmission infrastructure, including circuit breakers, can have higher initial costs compared to traditional AC systems. However, these costs may be justified by long-term operational benefits.
- Technological Challenges: Designing and implementing HVDC circuit breakers that can effectively interrupt high DC currents and handle transient recovery voltages remain challenging engineering tasks.
- Maintenance Complexity: Maintenance of HVDC circuit breakers requires specialized expertise and equipment, contributing to higher operational and maintenance costs.
- Limited Availability: The technology for HVDC circuit breakers is still evolving, and there are fewer manufacturers with the capability to produce them, potentially limiting availability and competition.
- Environmental Impact: Like any high-voltage equipment, the manufacturing and operation of HVDC circuit breakers have potential environmental impacts, such as insulation materials and energy consumption.
- Voltage Source Converter (VSC) Limitations: Some HVDC systems use Voltage Source Converters (VSCs) that offer advantages like independent control of active and reactive power. However, VSCs have limitations in terms of power handling capacity compared to Line Commutated Converter (LCC) systems, which could impact the overall performance of the HVDC system.
Conclusion:
HVDC circuit breakers are intricate marvels of engineering that enable efficient long-distance power transmission and grid interconnection. While their advantages, including reduced losses and stable operation, are compelling, their complexity, high costs, and technological challenges pose considerations. As research progresses and technology evolves, the balance between these advantages and challenges will shape the role of HVDC circuit breakers in our quest for a more interconnected and sustainable energy landscape.
Now today we have learnt about HVDC Circuit Breaker Working Principle & Construction. You can download this article as pdf, ppt.
Comment below for any Queries.