Major black-outs in recent years have shown how relatively minor malfunctions in interconnected grids can have repercussions over wide areas
Major black-outs in recent years have shown how relatively minor malfunctions in interconnected grids can have repercussions over wide areas. As one link overloads it is tripped, increasing the strain on neighboring links which in turn disconnect, cascading black-outs over vast areas and causing huge productivity losses for the economy.
The solution is a ‘firewall’ permitting the interchange of power but preventing the spread of disturbances. This can be accomplished using an HVDC link since it can fully control transmission but does not overload or propagate fault currents.
PERFORMANCE UNDER AC SYSTEM FAULTS
When a temporary fault occurs in the AC system connected to the rectifier (AC to DC), the HVDC transmission may suffer a power loss. Even in the case of close single-phase faults, the link may transmit up to 30 per cent of the pre-fault power. As soon as the fault is cleared, power is restored to the pre-fault value. When a temporary fault occurs in the AC system connected to the inverter (DC to AC), a commutation failure can occur interrupting power flow. Power is restored as soon as the fault is cleared. A distant fault with little effect on the converter station voltage (less than around 10 per cent) does not normally lead to a commutation failure. A CCC (Capacitor Commutated Converter) HVDC converter can tolerate about twice this voltage drop before there is a risk of commutation failure.
HVDC Light® is even more fault-tolerant. Since the converter can control the reactive power and the filters are small, the loss of active power has no impact on the AC voltage.
Another advantage of both HVDC and HVDC Light® transmissions is that they do not contribute to the fault current: the impact on the fault-free side of the DC transmission is smaller, and on the side with the fault, the fault current is lower than it would be with an AC link. The fault-free network experiences an interruption of power flow in the DC transmission but no fault current.
HOW HVDC CAN HELP DURING CONTINGENCIES
The main reason why a fault condition spreads to a wide area is often that AC transmission links become overloaded. This leads to their disconnection which in turn overloads other lines and so on.
An HVDC transmission link is easily engineered to take specific remedial actions in case of a disturbance. Furthermore, such actions are often smooth and continuous – in contrast to the hard switching of AC links. The most important feature of HVDC is that it can never become overloaded.
EMERGENCY POWER CONTROL
When HVDC transmission connects two asynchronous networks and there is a sudden outage of generation in one of the networks leading to an abnormal frequency and/or voltage, the link can be made to automatically adapt its power flow to support the troubled grid. The power flow is limited so as not to jeopardize the integrity of the sending network. When HVDC transmission is connected inside an AC grid with AC lines parallel to the DC link, the power in these lines can be monitored and the DC power can be automatically adapted to protect the AC lines from being overloaded.
VOLTAGE CONTROL
In a disturbed network, voltage depressions or oscillations often occur. In many cases the reactive power capability of a classic HVDC station can help reduce this by modulating its reactive power consumption. An HVDC Light® converter has an even greater ability to generate or consume reactive power within a wide range.
DEPENDENCE ON SHORT CIRCUIT POWER FROM THE CONNECTED AC GRID
Classic, thyristor-based, HVDC depends on the correct functioning of the AC system. The AC/DC converter requires a minimum short circuit power from the connected AC grid. Classic HVDC cannot feed power into a network which lacks generation completely or which has little or very remote generation. HVDC Light® does not rely on short circuit power to function because the inverter does not require the help of external generators. It can thus energize a ‘dead’ network with its ‘black start’ capability.
REACTIVE POWER
A great advantage of HVDC is that it does not transmit reactive power. The classic HVDC converter consumes reactive power; it is therefore common practice to include a reactive power supply in the converter station. This is normally done in harmonic filters and shunt capacitor banks. The CCC HVDC converter consumes less reactive power as the converter includes a series capacitor. A classic thyristorbased HVDC station can help to stabilize the AC voltage by modulating its reactive power consumption.
An HVDC Light® converter has the ability to generate or consume reactive power within a wide range and can therefore take an even more prominent roll in stabilizing the AC voltage.