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Problems with highly compensated power systems

This author no longer works for DNV GL.

Major load areas with high levels of shunt capacitors can present special challenges to system planners and operators. Such highly compensated systems are difficult to operate and are subject to extremely rapid voltage collapse, which can occur with little warning, especially during contingencies. Even worse, a complete voltage collapse and total loss of customer load in the area could result. In addition, the urban load loss would likely cause uncontrolled outages in the adjacent system.

In the event of a widespread collapse, it could take many hours to restore customer service. Reconnecting generators, loads, and transmission to restore order in the system is a painstaking process. Generation and load must be balanced, and there must be enough transmission available at each step of the restoration process.

Such a widespread outage could have an enormous impact. For example, it could cripple a major population and commercial center. As a result, highly compensated systems that serve major load centers are rare in the developed world.

Reactive compensation, for this discussion, includes transmission-level (>100 kV) fixed and switched shunt capacitors, static var compensators (SVCs) and similar electronic devices, and synchronous condensers. These devices are typically added to the system to provide reactive support. Unused reactive capacity of generators in the affected area would be an additional source of var support.

Highly compensated systems have four main problems:

  1. Rapid voltage collapse
  2. High normal operating voltages
  3. Additional complexity
  4. Limited operating flexibility

All of these relate to risk. When the load at risk is not a major load center, the risk may be acceptable. But, when a major load center is at risk, other solutions should be considered.

Rapid voltage collapse
The gradual voltage decline of normally compensated systems allows system operators to control the system and other conventional voltage control systems to be effective. Declining voltage acts as a warning signal for system operators. If voltages fall below the accepted voltage range, the operators can change the generation dispatch or take other actions to improve system voltage.

The figure below shows three typical power-voltage (P-V) curves for a major load area with increasing var compensation using capacitors. The “some compensation” curve shows the common shape of a P-V curve—as the power transfer level increases the voltage gradually declines. As the power level gets close to the voltage limit, the voltage begins to fall more quickly until it reaches a point where no more power can flow without the voltage collapsing. This point is called the critical voltage as shown on the figure. It is common to allow for some uncertainty in these limits by using a safety margin (typically 5%), which establishes a safe voltage level.

Example P-V curves with increasing var compensation
Example P-V curves with increasing var compensation

As the amount of compensation increases, three things happen to the curves:

  1. The top portion of the curves becomes flatter and extends farther to the right, which means more power can be imported into the area.
  2. The critical voltage rises, which means there is less difference between normal voltages and critical and safe voltages.
  3. The slope of the P-V curve gets steeper near the point of collapse as the system becomes more highly compensated. This means that a voltage collapse can occur much more quickly with less time for the operators to take corrective action.

Adding capacitors will generally improve the active power margin but will also increase the critical voltage. Therefore, the ability of the system to accommodate capacitors has an upper limit. As the amount of shunt compensation increases, so does the critical voltage. Once the critical voltage reaches the normal operating range, the voltage becomes uncontrollable and small changes in power will result in large changes in voltage, which are unacceptable.

In highly compensated systems, a gradual voltage decline cannot be expected. The voltage remains within the normal band until all reactive resources are exhausted; then, the voltage collapses almost instantaneously. Because the voltage stays in the normal range almost until the end, it can provide a false sense of security to system operators.

A rapid voltage collapse occurs when all reactive sources are exhausted. SVC’s and other controllable reactive sources that act to maintain voltages within the normal range can mask an impending voltage collapse. Once these reactive resources are exhausted, there are no more reserves to support the voltage.

One of the contributing factors to the August 2003 major blackout in the United States was fast voltage collapse. This can happen during periods of heavy load, especially when there is a dominance of motor load (e.g., air conditioning and industrial load). Recovery from the voltage sag during these conditions can be very slow.

High normal operating voltages
The figure above also shows the normal operating range between approximately 1.05 p.u. and 0.95 p.u. of the nominal voltage. System operators use voltage as an indicator for potential problems. If the critical voltage is in the normal range due to very high compensation, then there will be no such warning. The system could be at risk even when the voltages are within the normal range.

Additional complexity
Highly compensated systems can also introduce operating problems due to increased complexity. The multiplicity of devices can work against each other and make a serious situation worse. System operators must handle many combinations of outages, which add further complexity.

The system operators monitor equipment loading, voltages, and other measures that indicate the health of the system. Examples of indicators include import limits (based on planning studies), generation reserve margins, and voltage margins. System operators also use online computer tools to help identify potential problems.

Highly compensated systems can have many combinations of critical events—too many to study in advance. This can create a situation in which operators are unaware that the system is in danger of collapse. Operators and planners do not have a comprehensive computer tool that can handle the complexities and risks associated with highly compensated systems.

Added complexity also comes from each reactive device having its own local control logic based on information at the connection point. These controls react to conditions and changes at their location without regard to overall system needs. The devices are set after careful study of likely system conditions; however, there are many more combinations of events that can be studied.

Having too many independent voltage controllers can risk operation. If the system is critically dependent on these devices, there will be little room for error in the automatic controls. Should the wrong combination of events occur, there will be no warning or way to prevent a voltage collapse.

Limited operating flexibility
While a few switched capacitors and a limited amount of svc’s can provide operating stability, in an extreme situation, these devices actually reduce operating flexibility. The range of acceptable operating conditions becomes narrower. This can be seen in the figure above by comparing the critical voltages. The highly compensated curve has a very high critical voltage. If all the equipment is not operating as expected or if the anticipated reactive power margin is not available, there could be problems. This requires very tight control of generators, synchronous condensers, svcs, and transformer tap set points.

While compensation devices allow a great deal of flexibility in controlling the system voltage, highly compensated systems provide little room for error. An unexpected combination of events can cause voltages to fall below the safe range and result in a surprise voltage collapse.

Jeff Palermo also contributed to this article, which is part 2 of the three-part series, Generation retirements and var planning. Part 3 will discuss solutions to voltage and var problems. Read part 1, “Reactive power: what it is; why it is important.”

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[Image source: DNV GL Energy & Sustainability]

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