ISP’s Different Operating Modes and How to Test Them

The ISP is Dairyland’s original solid-state AC conducting/DC blocking device designed for the corrosion prevention industry in 1990. Of all Dairyland’s decouplers, the ISP—which stands for Isolator/Surge Protector—provides the widest range of features and ratings. This includes the highest levels of AC fault-handling capability – up to the highest typical transmission system faults, which makes the ISP the preferred product for power utility underground cable decoupling/bonding requirements.

The ISP is typically connected between the casing of a power utility pipe-type transmission cable system and the substation grounding system. Here, the ISP provides DC isolation and AC coupling to the grounding system of the substation.

This article will cover what an ISP is and how an ISP functions during different field conditions that include steady-state, AC faults and lightning faults. In addition, a brief explanation on how to test ISP’s in-circuit and the benefits of testing ISP’s using Dairyland’s automated ISP tester.

What is an ISP?

Dairyland’s ISP includes the following components: a capacitor circuit for steady-state operation, thyristors T1 and T2 rated for high fault current handling, a logic control circuit for switching operation, and a surge protector circuit for lightning and fast transient events. The two-terminal device has a blocking threshold voltage of 12.5V or 20V, in which it will block DC current if the voltage threshold is not exceeded. Once the threshold voltage is exceeded, the ISP transitions into a low-impedance state within 4ms, effectively becoming a virtual short circuit that safely conducts AC and DC fault or surge currents to ground, limiting system voltage rise. Reference simplified ISP circuit below in Figure 1.

Figure 1: Simplified Circuit of an ISP

Functions of an ISP

Steady-State AC Operation

As seen in Figure 2 below, under normal operating conditions, when AC voltage is applied to the ISP, current flows through the inductor (L) and capacitor (C), supporting continuous conduction up to the selected steady-state AC current rating. The switch (S) and surge protection circuit remain open, and no current flows through them.

Figure 2: Steady-State AC Current Flow

AC fault current operation

The ISP is designed to withstand currents above its steady-state AC current rating, which means that during AC faults, it is protected against failure, however it no longer blocks DC. Whenever the device operates beyond its steady-state AC current limit or the DC voltage exceeds the switching threshold, a red indicator on the ISP cover flashes intermittently as a warning.

As shown in Figure 3, during an AC fault, the predominant current flow path moves through switch S1 (S) with minimal current flow through the capacitor (C). During this time, the capacitor (C) is protected from excessive current and voltage. There is no current flow through the surge protection path.

Dairyland ISPs are rated for 35kA, 68kA or 118kA, 1 cycle at 60Hz. Some older models remain in operation as well in 7kA and 24kA ratings.

Figure 3: Current Path During AC Faults

Lightning and transient event operation

Under lightning (or switching transient) surge conditions, where the input current has a very fast rise time, the conduction path through the ISP is different than for AC conditions.

Please note Figure 4 below. When the ISP is exposed to a lightning surge, the voltage developed across the inductor (L) instantly rises to a value that places the surge protector (metal oxide varistor) into conduction, enabling the surge protector to carry the bulk of the current. Since the peak voltage across the thyristor also rapidly exceeds the threshold level, switch S is also put into conduction. However, the impedance of the current path through the thyristor is now much higher because the inductor is in series with this path. The impedance of the inductor allows only a small fraction of the surge current through the ISP to be carried by the thyristors, thereby preventing failure of the thyristors due to excessive rate of change of current. Most of the current will travel along the lower impedance path through the surge protector.

ISPs are rated for 75kA (standard) or 100kA (optional) 8x20ms waveform.

Figure 4: Current Path During Lightning Faults

Testing an ISP in-circuit

This test is completed with the device connected in-circuit — no terminal disconnections needed. Please note: this test only checks for internal shorts and not logic control circuit functions.

Refer to and follow company safety procedures prior to testing.

Equipment Required

• A multimeter with AC/DC amperes, volts, ohms.
• A clamp-on AC/DC ammeter

The ISP should be completely silent (noise-free). If an audible clicking sound is heard, the ISP is being subject to an AC current that is above its rating for the DC voltage applied. This sound is caused by a magnetic effect of certain solid-state components and is not any form of electrical arcing. In addition, a red indicator on the cover of the ISP will flash intermittently.

To verify that the ISP is functioning properly, first measure the AC voltage across the terminals using the DMM. Second, measure the AC current flowing through the negative terminal. Finally, calculate the impedance, Vac/Iac. These values should be approximately equal. Refer to the impedance table in this article and the nameplate of the ISP to determine the correct value to compare to.

Example1:

• Vac measured across terminals = 1V
• Iac measured with clamp-on = 10A
• Impedance Calculated = 1V / 10A = 0.1Ω
• For ISP with 90A rating, Published Impedance = 0.09Ω
• Values agree = Unit passes

Example 2:

• Vac measured across terminals = 0.1V
• Iac measured with clamp-on = 10A
• Impedance Calculated = 0.1V / 10A = 0.01 Ω
• For ISP with 90A rating, Published Impedance = 0.09Ω
• Values disagree = Unit fails test
  • Unit is actively conducting or failed shorted.
  • LED flashing if conducting (if present)

 

Testing with ISP Tester

The ISP Tester is an automated test device for performing in-circuit analysis of an ISP. The ISP is typically connected between the casing of a power utility pipe-type transmission cable system and the substation grounding system. Performing the functions of DC isolation and AC grounding for the transmission casing/sheath, an ISP is not easily removed from the system for the purpose of testing, therefore the ISP Tester performs an important role in analyzing an in-service ISP to determine its health.

The ISP Tester automatically evaluates the ISPs as-found condition and applies simulated AC fault signals to verify proper operation. It confirms that the ISP transitions into the low-impedance (ON) state under fault conditions, then reliably returns to its normal mode, blocking DC (from stray sources or cathodic protection systems) while allowing induced or coupled AC to pass to ground.

The ISP tester is initially configured via a simple touch panel interface. Following configuration, an automated test sequence is initiated by the operator. During the automated portion of the testing, the ISP is subjected to a multi-step test sequence controlled entirely by an on-board microprocessor. The sequence consists of various observational phases followed by two injections of a high current, short duration pulse which creates a temporary “fault” condition. During the test, the ISP is observed to verify that it properly triggers at either the 12.5V or 20V settings as initially selected on the touch panel and that the ISP properly recovers from a triggered condition. The brief high energy pulse causes the ISP to trigger only for a matter of several milliseconds, and a shorting system is applied to the external terminals to limit any influence on the external system. Operators of the ISP Tester should avoid contact with the exposed terminals of the ISP under test. Once initiated, the full test sequence requires approximately three to five minutes to complete. At the conclusion of the automated test sequence, informational data is combined with the actual test results and logged to a USB data drive.

If the ISP tester detects a problem during testing it will report the results in the home screen status box and abort any additional testing. There are fourteen possible failures and three successful results reported to the operator.

The ISP tester is available for purchase or rental. Please contact Dairyland for additional information.

Additional information may be found in the ISP’s technical literature,  installation instructions and application guide. If you have any questions on the operation or testing of ISP’s, please contact Dairyland’s Technical Support team for guidance.

ISP Frequently Asked Questions